Sex and the Rotifer

Bethany: I'm sorry. Sex is a joke in heaven?
The Metatron: The way I understand it, it generally is down here as well.

--Scene from the movie Dogma

Gladyshev, E. A., M. Meselson & I. R. Arkhipova. 2008. Massive horizontal gene transfer in bdelloid rotifers. Science 320 (5880): 1210-1213.

Nature has an annoying, almost pathological, tendency to break her own rules (I think it may have been Terry Pratchett who commented, "There's a reason why nature is called a mother"). Just when we biologists think we've got it all sorted out, something new comes along to mess up the theory. And, of course, nothing gets more complicated than sex. Sex, one would think, is a good thing - as well as its obvious immediate attractions, it serves to mix up the gene pool and increase variety in the population, increasing the chances for survival in a changing world. And yet many organisms do without it. How?

Most authorities who have given thought to the subject have concluded that asexually reproducing organisms survive on an "if it ain't broke, don't fix it" principle. After all, sexual reproduction can be a complicated, energy-sapping buiness, and if the individual is already well-suited to its environment, then the best option is to bypass the issue entirely. The resulting hypothesis is that asexual reproduction works better in the short term by preserving the parent's advantages, but sexual reproduction works better in the long term, as changing circumstances increase the chance of prior advantages becoming less so. Indeed, many organisms capable of asexual reproduction, such as aphids, support this hypothesis by acting in a manner that seems geared to extract the best from both options - reproducing asexually so long as conditions remain good, then switching to sexual reproduction when conditions deteriorate.

But remember what I said about nature breaking its own rules? Bdelloid rotifers are the prime exception in this case. Despite being estimated to have diverged from other animals some 80 million years ago, bdelloids are entirely asexual. How, the question runs, have they been able to survive so long without some form of genetic recombination? A paper in today's Science suggests how - by incorporating genes from other organisms. In a study of transposable elements (TEs - pieces of DNA that are able to move about in the genome) in the bdelloid Adineta vaga, Gladyshev et al. unexpectedly found that many of the TEs actually contained protein-coding sequences. What is more, analysis of these coding sequences found that many of them were not similar to genes found in other animals. Instead, the bdelloid genes clustered with bacteria, fungi or even plants.


A bdelloid rotifer, probably Philodina acuticornis. Photo by Aydin Örstan.

Horizontal gene transfer (HGT) is the transfer of genetic material from one organism to another by non-reproductive means. This may occur through genetic material being carried by viruses, for instance, or by direct transfer. The occurrence of HGT in bacteria has been established beyond a doubt, and most researchers regard it as a significant factor in bacterial evolution. Whether (or to what degree) it occurs in eukaryotes has been a far more contentious subject*. A certain degree of HGT has been demonstrated in flowering plants (Bergthorsson et al., 2003; Nickrent et al., 2004 - one of my earlier posts touched on a probable case of HGT to a parasitic plant from its host). Animals, however, are regarded as much less prone to HGT, but bdelloids seem to be an exception once again.

*There is one notable class of exceptions. Many of the eukaryote organelles (such as mitochondria and chloroplasts) have been derived from endosymbiotic bacteria, and one component of their conversion from independent organisms capable of living freely to obligate endosymbionts has been large-scale HGT from the endosymbiotic bacterium to the nucleus of the host eukaryote. Let it suffice to say for now that the candidate HGT-derived genes in bdelloids do not appear to have been derived from this route.

The reason why animals are so resistent to HGT, and the main problem with recognising its occurrence in bdelloids, is that there are less apparent methods for foreign genetic material to be transferred into animal cells (there is also the separation in most animals between the somatic and reproductive cells). Bacteria are able to transfer genetic material between each other by the productive of pili, tubular structures that latch onto other cells. Plants lack pili, but they do possess plasmodesmata, openings in the cell wall that allow for the transport of materials between adjoining cells, and it is possible that HGT can occur via the plasmodesmata when plants of two different species grow in contact with each other (this may be how the host-parasite transfer mentioned earlier occurred, for instance). Animals, on the other hand, lack both pili and plasmodesmata. If the HGT-candidate genes in bdelloids really are such, their means of entry remains entirely hypothetical. Viral transfer is one possibility, but would require that bdelloids be somehow more prone to viral infection than other animals. Gladyshev et al. point tentatively at the unusual life history traits of bdelloids as a possible solution. Bdelloids are able to survive extreme dessication, and Gladyshev et al. suggest that damage to cellular membranes in the course of dessication might increase their chance of taking up foreign genetic material. Also, the authors found no cases where the specific source of an HGT-candidate was identifiable, though this could merely represent evolutionary change in the time since assimilation.

What is also interesting is that the HGT-candidate genes were not randomly distributed in the bdelloid genome. Most were concentrated in parts of the genome separate from more standard animal genes, closer to telomeres in areas rich in TEs. The authors suggest (quite reasonably, I think) that this results from the greater potential for interference with pre-existing genetic processes were horizontally transferred genes to insert in functional sectors of the genome. (Note that this is not necessarily to say that HGT products don't become inserted in these sectors, but that most of those cells that did experience such an insertion would not remain viable.) As already referred to, many of the HGT-candidate genes seem to have undergone significant change since their insertion, and a few of the genes that appear to have been derived from bacteria have themselves actually had introns (characteristic of animals, but generally absent from bacteria) inserted into them.

If bdelloids are indeed so amoenable to HGT, this could go some way to explaining their ability to cope without sexual reproduction, as HGT supplies another potential method for genetic recombination. It would be of great interest to see whether other microscopic animals that often undergo dessication cycles, such as tardigrades, also show elevated HGT rates, as this may be informative in testing whether it is the bdelloids' life cycle that has made them so accepting.

REFERENCES

Bergthorsson, U., K. L. Adams, B. Thomason & J. D. Palmer. 2003. Widespread horizontal transfer of mitochondrial genes in flowering plants. Nature 424: 197-201.

Nickrent, D. L., A. Blarer, Y.-L. Qiu & R. Vidal-Russell. 2004. Phylogenetic inference in Rafflesiales: The influence of rate heterogeneity and horizontal gene transfer. BMC Evolutionary Biology 4: 40.

Odds and Sods

The International Institute for Species Exploration, a cybertaxonomy project based at Arizona State University in the US, has released their State of Observed Species (SOS) report tallying the number of new species described in 2006, and a list of the Top Ten Species described in 2007. The report found some 16,969 new species of organism described in 2006 (which is almost certainly a little short, because many species get named in small circulation journals and such that may not have been checked). That's nearly fifty species a day, with no sign of slowing down any time soon. The Top Ten list has garnered a bit of criticism, but ultimately, yes, it is a piece of publicity. My personal favourite on the list, because it highlights just how little we know about the planet's biodiversity, has to be the new mushroom described from a university campus in London.

Another notable recent event has been the that the Society of Vertebrate Paleontology has released its judgement on the allegations of unethical conduct involving Spencer Lucas (see Mike Taylor's webpage for the background and other comments on this sordid little affair). The SVP's decision does seem ultimately to have been a political one - the committee felt that unethical conduct on the part of Lucas et al. could not be unequivocally proven, but they do lay down guidelines for avoiding such 'misunderstandings' in the future. I feel it is worth stressing that, similar to what I said earlier about the ICZN in relation to this incident, the SVP is not a legal body, and their concern is more with promotion and facilitation of comunication between researchers than with judging guilt. Mickey Rowe has written a detailed blow-by-blow critique of the decision (and for the most part I'd just like to say "What Mickey said"), and you can also read Kevin Padian's response.

Where To Next?

As some of you may have noticed over the course of the past year, I have an interest in phylogenetically problematic taxa. I think anyone with even a passing interest in evolutionary matters does: few things appeal to the human spirit more than a good mystery. So I've decided to give you my own completely-biased, not-in-the-least-bit-impartial list of the ten taxa that I think currently spark the most phylogenetic questions, based on nothing more than my own subjective judgement.


The mitrate Rhenocystis latipedunculata. Photo from The Virtual Fossil Museum.
1 - Stylophora: These featured here a couple of weeks ago. I think a case could well be made that this is the single most confusing group of organisms in all of palaeontology. What is one to make of a group of animals for which researchers cannot even agree on which end is the front and which the back, which way up they go, and which may occupy a critical position in our own evolution, or may have nothing to do with it? Nearly three years ago I wrote a page for Palaeos.com on these animals. When I started writing the page I favoured one reconstruction, by the time I finished I was feeling a lot more uncertain. There was a lot more going on in my head than those ten short paragraphs let on.


Photo by Linda de Volder.
2 - Opisthocomus hoazin: The hoatzin, the prime exemplar of all things uncertain in avian systematics. Arguably not as problematic as some other things on this list (at least we can be sure that it's a bird), but it still well deserves its place here for the sheer volume of time and effort that has gone into the seemingly almost futile attempt to pin it down among its feathered associates. The hoatzin is famed for its vile odour and loud call, as it laughs derisively and farts insolently at the unfortunate systematists it has reduced to tears through its intransigence.


The pycnogonid Nymphon gracile. Photo by Asbjørn Hansen.
3 - Pycnogonida: The sea-spiders, bizarre marine organisms that appear all legs and only barely arthropod. The segmentation is almost invisible, the abdomen reduced to a mere stump, and the head occupied by a tubular gigantic proboscis that they use to suck the juices out of sponges and corals. Some authors think them the sister group to all other living arthropods, others think them related to the arachnids. The combined morphological/molecular analysis of Giribet et al. (2002) actually placed them within the arachnids, but as sister to the Palpigradi, which brings us to:


The palpigrade Eukoenenia mirabilis. Photo from here.
4 - Palpigradi: When you first see one of these tiny arachnids, you might think they look much like a miniature whip scorpion. Then you look a bit closer, and discover that it appears to be a whip scorpion constructed out of bits of mite. After that, it gets confusing. Palpigrades are widespread in tropical and subtropical regions of the world - there's even a couple of marine species living amongst the meiofauna - but they're not letting anything on about their affinities without a fight.


The flying duck orchid (Caleana major). Photo by Katrina Leel.
5 - Angiospermae: They're all around us. Look out almost any window and you'll see hundreds of them - the grass, the trees, the scraggly little weeds that force their way up between pavers. Our lives are shaped by them. In fact, we would never exist without them. They feed us, clothe us, inspire us. But where did they come from? At one point, researchers thought they had a pretty good idea of the origins of flowering plants, but in the past two decades molecular data have cruelly conspired to make our understanding far more shaky than it once was. They may rule the planet now, but they might as well have come from outer space.


Galapagos tortoises. Photo by Nathan Farb.
6 - Testudines: And speaking of organisms that might as well have come from another planet... The earliest known fossil turtle comes from more than 200 million years ago. It lacks a few of the features of modern turtles, but it's still quite unmistakeably a turtle. Since then, turtles have thrived and spread to all corners of the planet, but in all their diversity of forms, they're still unmistakeably turtles. The lack of intermediary fossils and the all-encompassing carapace (complete with limbs moved inside the rib-cage) do an exemplary job of disguising which other group(s) of reptiles the turtles call their nearest and dearest. Unless, of course, the Germans are on the right track with their "schildkröten", and they're really just frogs out to fool us.


The Carboniferous problematicum Tullimonstrum gregarium, reconstructed alongside Escumasia roryi (the y-shaped organisms) and Etacystis communis (the ones that look like Wu-Tang Clan symbols). Painting by Stanton Fink.
7 - Tullimonstrum gregarium: The so-called "Tully monster" is known from one place and time only - the Carboniferous Mazon Creek lagerstätte of Illinois. As indicated by the name "gregarium", it occurs there in considerable numbers, probably in the thousands. And yet nothing else is known anywhere that looks anything like it. In the years since its description, Tullimonstrum has been a gastropod, a nemertean, even a super-derived descendant of the Cambrian stem arthropod Opabinia. It has also been suggested that a giant descendant of Tullimonstrum yet lives, and is making its living by calling itself "the Loch Ness monster" and posing for tourists. This theory remains unproven, and rumours of an impending lawsuit for copyright infringement being brought by the plesiosaurs are entirely speculative.


The unplaceable eukaryote Stephanopogon. Photo by NEON ja.
8 - Stephanopogon: Stephanopogon looks like a ciliate. For many years, it sat cosily amongst its fellow ciliates, sometimes even given pride of place for its features as a link between the ciliates and their flagellate relatives. But the closer it was scrutinised, the more "flagellate" and less "ciliate" it appeared, until eventually the horrified ciliates realised that it was not one of theirs at all, but a horrid interloper! Now poor Stephanopogon floats aimlessly among the unicellular eukaryotes, looking for a place to call its own. There are rumours that it may yet be accepted among the discicristates, but will the fair Euglena and shy Naegleria be able to forgive their new confamilial's shady past? Only time will tell.


The zorapteran Zorotypus hubbardi. Photo by David Maddison.
9 - Zoraptera: Zorapterans, or "angel insects" (if angels are fond of living among decay) have been featured before on this site. Despite the small number of known species, this is the most phylogenetically problematic of all insect orders. Relatives of cockroaches, crickets or the outgroup to Holometabola? You decide.

And to round things off:


Lobatocerebrum psammicola. br = brain. Photo from Rieger (1980).
10 - Lobatocerebrum psammicola: When first described, Lobatocerebrum was included with the Annelida, the well-known phylum of coelomate, segmented, chaetae-bearing worms, and that is where it has tended to stay. The problem is, this minuscule member of the meiofauna is acoelomate, non-segmented, and lacks chaetae. It is the supposed annelid that has lost almost the entire complement of annelid features. Lobatocerebrum is an annelid that is doing its best to be a flatworm, or a flatworm that is doing its best to be an annelid, or something else entirely. Or maybe its just one of those things that exist simply to give systematists headaches.

REFERENCES

Rieger, R. M. 1980. A new group of interstitial worms, Lobatocerebridae nov. fam. (Annelida) and its significance for metazoan phylogeny. Zoomorphologie 95 (1): 41-84.

And So Much Yet to Do!

It seems hard to believe, but on this day a year has passed since I put up my very first post on Catalogue of Organisms (for the record, it was on dinoflagellates). A whole year of time spent writing posts for the blog when I should have been doing my research work (hope my supervisor doesn't read that). So what have people liked in that time? Through the magic of Google Analytics, I can tell you.

No. 1 – Gulper Eels: People really like gulper eels. In fact, with 1800 or so page-views over the year this has scored nearly double the attention of any other page. With their gigantic jaws and missing rib cages, who could fail to love them? The Cosmic Gulper will be pleased.

No. 2 – Sex Determination in Frogs: Because nothing ups the page-views like putting "sex" in the title. "Frog sex" also seems to the most commonly used search term to get to this site. I still don't know why.

No. 3 – What is a Daddy-Longlegs?: I have a confession to make. The success of this post actually kind of annoyed me. It was my first experience of the effect whereby you can spend hours working on posts and raising nary a blink, then spend five minutes putting together a couple of pictures and about five lines of text and have the thing take off on you.



No. 4 – 'Sophophora' melanogaster: The potential renaming of everyone's favourite lab animal got a lot of attention. It also sparked what I think may be Catalogue of Organisms' longest comments thread so far.

No. 5 – Development of Buddenbrockia: Parasitic amoeboids come together to form parasitic worms. How cool is that? A large part of the popularity for this post came over the course of three days, as I witnessed the power of a Pharyngulation.

No. 6 – Nettles: But as powerful as Pharyngula is, it must still take second place to the power of LiveJournal. I guess the idea of a five-metre tall deadly tree appealed to people. To quote Drhoz, "First it stings you, and then you fall into the hole and die. And if you somehow miss the hole, you die anyway".

No. 7 – Giant Fossil Cephalopods: To be honest, I'm not sure why this short little post has been so popular. I'm guessing that it may have something to do with the proximity of the phrases "buxom wench" and "tentacles".

No. 8 – Bipedal Anoplotherium: The comparison between a fossil relative of the camels and the modern gerenuk went down pretty well.



No. 9 – Arachnid Phylogeny: I still need to edit this one to make it a little easier to read. But trust me, there's a lot more to arachnids than spiders!

No. 10 – Receptaculites: In a post written to tie in with a reworking of a song by They Might Be Giants, I presented this attractive if confusing organism. Some receptaculitids do look a bit like pineapples.

Mind you, if you asked me to highlight my own favourites of the past year, I might not necessarily point at the above. I might prefer the cannibal algae, or the ostrich feet. Then there were the slime-nets, or the Strepsiptera, or the turacos.

I've learnt some things myself in the course of researching posts. I fully expected chancelloriids to be some sort of sponge - in the end I wasn't so sure. I thought the relationships within the herons were fairly well sorted. It turns out they haven't been looked at since the days of DNA-DNA hybridisation. And I learnt that Rajah took seven months to prepare for display.

In a bunch, in a bunch!

Cytisus scoparius, a widespread broom species, and the most widespread as an invasive species. Photo by Paul Slichter.

After the dummy-spit of the last post, let's get on to something a little more comforting. This is, I think, the perfect time for a botany post to calm the nerves. Which is lucky, because the Taxon of the Week post this week is on Genisteae.

Genisteae is the tribe of about 450 species of mostly Holarctic leguminous plants that includes brooms*, gorse and lupins**. The first of these are the Palaearctic brooms - in an earlier post, I wrote about the New Zealand brooms, which belong to a different legume tribe, the Galegeae, and whose broom-like appearance is convergent with that of the Genisteae brooms. However, a number of species of Genisteae have been transported by humans to many temperate regions around the world (including New Zealand), and many are familiar weed species outside their native ranges.

*The shrubs, obviously, not the things you sweep with. Though brooms the plants were often used for the making of brooms the implements, which is probably the origin of the name of one or the other.

**Monty Python fans are permitted to start humming now.


Argyrolobium zanonii, a Mediterranean species of the possibly polyphyletic genus Argyrolobium that is one of best contenders for inclusion in the Genisteae. Photo from here.

Phylogenetically, Genisteae can be divided into three well-divided groups (Ainouche et al., 2003). The probably paraphyletic Argyrolobium group includes five genera of mostly southern African and South American plants that lie outside the clade formed by the Palaearctic members of Genisteae. Previously members of this group were included in the sister tribe Crotalarieae, and their position remains unsettled. In particular, it has been suggested that Argyrolobium itself may be polyphyletic, with some species belonging to Crotalarieae and others to Genisteae. With the Argyrolobium group included, the Genisteae are characterised by a basically two-lipped calyx with a trifid lower lip, and the presence of quinolizidine alkaloids of a-pyridone type (chemical characters have proven to be very useful in plant systematics, with many groups characterised by the production of particular secondary metabolites).


Lupinus polyphyllus, a lupin species native to western North America. Photo from Wikimedia.

The genus Lupinus (the lupins) form a distinct group from the remainder of the Palaearctic Genisteae. Lupinus is by far the largest genus of Genisteae, including about half the species and the only genus to have made it to the New World, where it is found in both North and South America. Most lupins are readily distinguished from other Genisteae by their palmate, divided leaves. While the flowers are not particularly edible (despite the rumoured possibilities of lupin soup, roast lupin, steamed lupin, braised lupin in lupin sauce, lupin in the basket with sauted lupins, lupin meringue pie, lupin sorbet...), the beans of some species are eaten pickled (raw beans generally contain toxic alkaloids) in Mediterranean areas or Latin America. Lupins have been widely grown as stock feed and as ornamentals, and include many of the aforementioned weed species.


Valley in Victoria (Australia) overrun by the vile Ulex europaeus. Photo by Kate Blood.

The remaining genera of Genisteae form the subtribe Genistinae. Diversity of Genistinae is centred around the Mediterranean, with the three best-known genera being the brooms in Genista and Cytisus, and gorse in Ulex (Ulex and the closely related, possibly synonymous, genus Stauracanthus have been placed in a separate subtribe Ulicinae, but Ainouche et al., 2003, demonstrated that these genera fell within Genistinae). A number of other broom genera are recognised, but classifications may differ on which genera are recognised as including which species. The Genistinae also includes the tree genus Laburnum (Käss & Wink, 1997). Members of the Genistinae are characterised by adaptations for arid habitats such as very small or absent leaves and photosynthetic stems. Ulex species have the leaves modified into long spines. The species Ulex europaeus (commonly called simply "gorse") was introduced into New Zealand for use in hedges, a role which it apparently fulfils admirably in Britain. Unfortunately, the imported gorse plants found the New Zealand climate much more to their liking than that of their native Britain, and are currently one of New Zealand's most widespread and visible weed species. Yours truly has many unwelcome memories of hot summer days spent grubbing up gorse plants.

REFERENCES

Ainouche, A., R. J. Bayer, P. Cubas & M.-T. Misset. 2003. Phylogenetic relationships within tribe Genisteae (Papilionoideae) with special reference to genus Ulex. In Advances in Legume Systematics part 10, Higher Level Systematics (B. B. Klitgaard & A. Bruneau, eds.) pp. 239-252. Royal Botanic Gardens: Kew.

Käss, E., & M. Wink. 1997. Phylogenetic relationships in the Papilionoideae (family Leguminosae) based on nucleotide sequences of cpDNA (rbcL) and ncDNA (ITS 1 and 2). Molecular Phylogenetics and Evolution 8 (1): 65-88.

Poor Taxonomic Practice takes some F****ing Liberties!

Bortolus, A. 2008. Error cascades in the biological sciences: the unwanted consequences of using bad taxonomy in ecology. Ambio 37 (2): 114-118.

As you may have guessed from the title of this post, I'm kind of pissed. I got forwarded a copy of this paper earlier today, and was horrified by the results presented in it. Bortolus presents the results of a paper survey of 80 ecological papers published from 2005 to 2007 in ten top-ranked peer-reviewed ecology journals. The subjects of this survey were community ecology studies dealing with more than one species of organism, and for each study Bortolus took note of the sources for the identifications of the taxa surveyed. I hardly need point out how significant such identifications would be for the papers in question - after all, if the subjects of an ecological study are not reliably identified then the usefulness and significance of the entire study comes into question. So you would think that it would be pretty important for the authors to demonstrate the firmness of their identifications, no? Well, let me show you the graph of the results:



Note that the numbers do not necessarily add up to 100%. A single paper might haved used more than one form of identification. I also disagree slightly with Bortolus' treatment of "grey" (not or not significantly peer-reviewed) literature - lumping the citation of field guides in with unpublished theses does somewhat overlook that excellent field guides do exist for some taxa (most notably birds), and even amateurs without formal taxonomic experience may through these field guides hold a significant amount of expertise in identifying the taxa covered (also, of course, field guides are widely available and consultable while theses are not). Nevertheless, let me point out the part of the results that has incensed me the most. It's the second bar from the left. Fully a third of the papers surveyed gave no authority whatsoever for the identity of the species surveyed! It's not as if the bar was set particularly high for citing an authority, either. Acknowledging the input of a taxonomist who identified specimens (or having a taxonomist as one of the authors) counted. Noting that one of the authors of the paper had personally identified the taxa counted, or the use of a cited identification key, counted. All it would have taken in most cases would have been a couple of sentences. And yet, it seems that this is a couple of sentences too many in a lot of cases.

Of course, the availability of voucher specimens would do a lot to alleviate these concerns. I've written before about the importance of voucher specimens that allow other researchers to check specimen identifications. so how many of the papers surveyed noted the availability of voucher specimens?

2.5%.

Yes, you read right. Two point fricking five percent. What is more, those papers that did record voucher specimens were invariably among those that acknowledged the input of a professional taxonomist in some way, and so were probably not among the more suspect class in the first place.

As is de rigeur in this kind of discussion, Bortolus gives us examples of the kind of errors that can result from poor taxonomic practice. Some errors are fairly innocuous, but others can have fairly serious consequences. There's the case of a restoration programme in San Francisco that included the propagation and replanting of the native littoral grass Spartina foliosa. Unfortunately, the actual specimens propagated were not S. foliosa, but the non-native S. densiflora, and as a result of the programme S. densiflora was transformed from a minor adventive to a significant invasive. There's the case of three morphologically similar but ecologically distinct Mytilus mussel species, the decline of one of which due to the invasive spread of another went unnoticed for many years. There's the case of a failure to distinguish Anopheles mosquito species in Burma resulting in the long-term targeting of a non-malaria-carrying species by control programmes instead of the actual malaria vector. Bad taxonomy does matter.

Some of my readers may protest that it seems a little harsh to expect identification sources from authors who may be very familiar with the species involved and hence might be trusted to identify them reliably. To which I reply, bullshit. One of the main principles of science is that it is not enough to simply assert that one "just knows". And besides, is it really asking that much? Biomolecular studies, for instance, are expected to include full details of methods used, and it would not be considered unusual for an author to cite Saiki et al. (1988), no matter how many thousands of PCR reactions that author might have conducted during their career. Also, there are significant ethical considerations involved. In failing to cite or acknowledge their identification sources, authors are doing a significant disservice to the taxonomists on whose work their own relies. It is exactly this assumption that the reliability of taxon identifications can be taken for granted that has resulted in the undervaluing and loss of taxonomic expertise in many parts of the world. Is a passing mention in the acknowledgements really that much to ask?

West Indian Raccoons: From Endangered Endemics to Invasive Introductions

Raccoons on Guadeloupe. Photo from Wikipedia.

In 1788, the journals of one Johann David Schöpf were published in Germany describing his travels in North America. Upon arriving at New Providence Island in the Bahamas, Schöpf noted the presence there of a healthy population of raccoons, which he noted had become established there as the result of "one or more tame pairs of these droll beasts, brought by the curious from the mainland". An English translation of Schöpf's work was not to appear until 1911, but had it appeared earlier this post may have been somewhat different.

The raccoons of the genus Procyon are generalist omnivorous carnivorans* widespread in North and South America. The two most widespread species of the genus are the North and Central American P. lotor and the South American P. cancrivorus. Four further species have been described from isolated populations on Caribbean islands. All these island populations have restricted ranges, and the Barbados population is most likely extinct. However, recent research has shown this might not be the conservation tragedy it seems to be, as it turns out that the majority of the four "species" are not valid species after all, but human introductions of the North American P. lotor.

*I just love how confusing that sounds.

Helgen et al. (2008) recently published the results of their genetic investigation into three of the four Caribbean "species" - 'Procyon maynardi' from the Bahamas, 'P. minor' from Guadeloupe and 'P. gloveralleni' from Barbados. All three were firmly nested within representatives of raccoons from the eastern and central United States (the fourth species, P. pygmaeus from Cozumel off the coast of southern Mexico, is still regarded as a valid taxon). This finding corroborates earlier studies that found no significant morphological differences between the three island populations and continental populations. What is more, Helgen et al. were able to locate historical records relating to the introductions of two of the three populations. The Bahamas raccoons were the very ones referred to by Schöpf in 1788 (some time before their description as a distinct species in 1898), while the Barbados population was established some time between 1650 and 1680. The origins of the Guadeloupe population are still unknown. Helgen et al. (2008) found that their sampled continental specimens fell into two haplotype clusters, with the Bahamas raccoons among one cluster and the Guadeloupe and Barbados raccoons within another, but there was no clear geographical division between the two clusters.

All three of the original descriptions of the Caribbean populations as distinct species were based on juvenile specimens, and this may have been a factor in their being mistaken for new species. It may have also been the origin of the mistaken belief that specimens of the island populations were smaller than their mainland counterparts. Rather than being conservation targets in their own right, the introduced raccoon populations are a potential cause for concern. This is no small change - in Guadeloupe, the identification of the raccoon population as an endemic species has led to its touting as a flagship species, and the symbol of the Parc National de la Guadeloupe. The removal of its protected status could be an act fraught with political and emotional consequences.

REFERENCES

Helgen, K. M., J. E. Maldonado, D. E. Wilson & S. D. Buckner. 2008. Molecular confirmation of the origin and invasive status of West Indian raccoons. Journal of Mammalogy 89 (2): 282-291.

Forcing Out the Secret

SEM of a y-cypris (Hansenocaris, Facetotecta), from Høeg & Kolbasov (2002).

A few months ago, I wrote a post on the mysterious y-larvae or Hansenocaris (Facetotecta), distinctive crustaceans known only from their larval form and of unknown adult morphology. I've just been informed of a significant step that has been taken towards solving this mystery (Glenner et al., 2008).

One way to discover the adult form of Hansenocaris would be to rear larvae through to adulthood. However, so far it has not been possible to rear y-larvae past the cypris stage (y-larvae belong to a group of crustaceans called Thecostraca, also including barnacles, that hatch out as a nauplius larva, which eventually transforms into a cypris larva, followed by other larval stages or adulthood). Glenner et al. have broken that barrier by exposing cypris y-larvae to the moulting hormone 20-hydroxyecdysone (20-HE). Exposure to this hormone induced the cyprids to moult through to the next stage in the life cycle.

The appearance of this next stage certainly goes some way to explaining why adult facetotectans have not yet been recognised. Gone are the swimming appendages and arthropod segmentation of the cypris. Instead, the y-larvae moults into a limbless, worm-like organism that wriggles vigorously. This worm-like creature is without a properly developed digestive system or other such extravagances, and about the only feature suggestive of its arthropodan nature are the disorganised and degenerate remnants of a pair of compound eyes. Overall, the emerged organism (dubbed an "ypsigon" by Glenner et al.) bears a significant resemblance to the vermigon larva described for some members of another group of the Thecostraca, the parasitic Rhizocephala. Ypsigons kept in culture for 24 hours underwent another moult (remaining as an ypsigon), but no specimens were kept alive beyond that stage. I am inclined to wonder whether the laboratory-induced form is truly the same as what would emerge in the wild or whether the growth hormone adversely affected the larvae's development, but Glenner et al. do indicate that rhizocephalan vermigons induced in the lab are comparable to those occurring naturally.


Emerged ypsigon next to its moulted cypris cuticle. From Glenner et al. (2008).

Loss of derived arthropod characters is not uncommon among endoparasitic crustaceans - the Rhizocephala and Pentastomida are two particularly extreme examples, and certain features of the y-larvae cypris had already lead researchers to suspect that the adult might be parasitic. Unfortunately, it is still unknown what the host of that parasitic adult might be. It is also worth stressing that, contrary to what has been implied on news releases, the ypsigon is not the adult, but an additional larval stage, probably (by analogy with the rhizocephalan vermigon) representing the stage at which the y-larva enters its host. Identification of the full adult form (which may or may not resemble the vermigon) will probably require the identification of that host. Despite the similarities between the facetotectan ypsigon and the rhizocephalan vermigon, the two groups are not believed to be each other's closest relatives within the Thecostraca (Facetotecta are a basal branch, while Rhizocephala are closer to barnacles - Høeg & Kolbasov, 2002), suggesting that this derived parasitic form has developed independently in the two groups. We eagerly await any discovery that will reveal the final clue to this mystery and present with a fully adult facetotectan in all its slimy glory.

REFERENCES

Glenner, H., J. T. Hoeg, M. J. Grygier & Y. Fujita. 2008. Induced metamorphosis in crustacean y-larvae: Towards a solution to a 100-year-old riddle. BMC Biology 6: 21.

Høeg, J. T., & G. A. Kolbasov. 2002. Lattice organs in y-cyprids of the Facetotecta and their significance in the phylogeny of the Crustacea Thecostraca. Acta Zoologica 83: 67-79.

The Boneyard XX

The newest edition of the palaeontology carnival the Boneyard is up at Laelaps. This is a special edition collecting palaeo-fiction or palaeo-experience, and you're asked to vote on your favourite contribution. So go support your favourite!

Return of the Slime-nets

Another milestone has been reached in the Catalogue of Organisms - this will be the first Taxon of the Week post where the taxon in question has already been covered in the series. So what group of organisms is illustrious enough to be worth introducing twice? In this case, that honour goes to the Labyrinthulea, the slime-nets, and I don't think you could find a more deserving target. The biggest challenge, I suspect, will be finding enough good images online to illustrate to post - for some reason, good labyrinthulean images seem to be a little thin on the ground.


Vegetative cells of Labyrinthula terrestris within epidermis of Poa trivialis. Scale bar = 10 µm. Photo from Bigelow et al. (2005).

In my previous post on labyrinthuleans, I introduced you all to the group and gave a brief rundown of their division into three morphological if not necessarily phylogenetic groups, the labyrinthulids, thraustochytrids and the Diplophrys group. I recommend reading that post before this one, but this time round I'd like to focus more on the ecological role of Labyrinthulea. As alluded to in the previous post, the vast majority of known labyrinthuleans are marine, a habitat in which they appear to be pretty much ubiquitous (Raghukumar, 2002). It is thought that within the marine habitat labyrinthuleans mostly act as decomposers, living by breaking down and extracting nutrients from dead plant, algal and animal matter. They are very effective in this role - for instance, thraustochytrids are capable of breaking down sporopollenin, the extraordinarily resistant polymer that coats pollen grains. However, because labyrinthuleans in such a role have little direct effect on humans (though their significance to nutrient cycles vital to other organisms that are significant to humans is probably considerable), this is not how they have been most studied. Instead, much more attention has been given to their interactions with living organisms as commensals or pathogens.

Despite numerous records of labyrinthuleans as pathogens, it seems likely that in the majority of cases they are not primarily so, but merely facultative. Despite their rapid growth on necrotic algal tissue, growth of thraustochytrids on live algae is minimal or non-existent, even when said algae have been deliberately innoculated with thraustochytrid spores. Raghukumar (2002) postulated that antimicrobial substances produced by healthy plants might effectively keep thraustochytrid growth down. Labyrinthulids are somewhat more aggressive in their relationships with marine plants and algae, but are not necessarily harmful - the species Aplanochytrium minutum, for instance, has been recorded living within the tissues of brown algae without any noticeable adverse effects on the host. Thraustochytrids are also known as pathogens of animals - in many cases, the effects of thraustochytrid infection are not severe except in young animals, but the QPX organism that has caused mass mortality in the clam Mercenaria mercenaria has been shown using molecular means to belong to the thraustochytrids.


Lottia alveus, the seagrass-inhabiting limpet driven to extinction by the seagrass wasting disease epidemic of the 1930s. Reconstruction from here.

A dramatic exception to the generally low-key nature of labyrinthulean pathogenicity reared its ectoplasm in the 1930s. In the early part of that decade, a wasting disease of then-unknown cause devastated populations of the seagrass Zostera marina in the North Atlantic. More than 90% of the seagrass beds on both sides of the Atlantic were wiped out. In some places, the effect was so severe that the ecology of the area affected was completely altered and the seagrass never returned (Short et al., 1987). The indirect effects of this devastation on other organisms, needless to say, were also severe. Migratory waterfowl populations declined, while several commercial fisheries were hard hit - in particular, the scallop fishery on the American eastern seaboard collapsed entirely. At least one species dependent on the seagrass, the limpet Lottia alveus, became extinct as a result of the epidemic. By the 1940s, the epidemic had run its course, and seagrass populations began to recover by the 1950s. The reasons for the wasting disease remained unknown, though it was suggested that abnormally high sea temperatures may have been a factor. It wasn't until a smaller scale outbreak of seagrass wasting disease in 1986 that Short et al. (1987) were able to show that it was caused by a Labyrinthula species. While the cataclysmic levels of the 1930s have never, thankfully, returned, wasting disease remains a concern for the maintenance of seagrass populations.


Grass infected with rapid blight. Photo by D. Bigelow.

It is a pathogenic species that also provides the sole exception yet known to the otherwise entirely aquatic lifestyle of Labyrinthulea. Rapid blight of cool season lawn turf was first described in California as recently as 1995, and has since been recorded from multiple locations across the southern United States. Once again, the identity of the causative organism was difficult to resolve, but it was eventually described as a new species of Labyrinthula in 2005. How a representative of an otherwise exclusively marine genus came to be parasitising grass in a terrestrial environment remains a complete unknown, though Bigelow et al. (2005) suggested that Labyrinthula may be more common in soil than previously thought, but overlooked due to the difficulty in culturing it.

REFERENCES

Bigelow, D. M., M. W. Olsen & R. L. Gilbertson. 2005. Labyrinthula terrestris sp. nov., a new pathogen of turf grass. Mycologia 97 (1): 185-190.

Raghukumar, S. 2002. Ecology of the marine protists, the Labyrinthulomycetes (thraustochytrids and labyrinthulids). European Journal of Protistology 38 (2): 127-145.

Short, F. T., L. K. Muehlstein & D. Porter. 1987. Eelgrass wasting disease: cause and recurrence of a marine epidemic. Biological Bulletin 173 (3): 557-562.

Awwww!

Yesterday afternoon, I got a call from my mother that I'd been waiting on for about eight months - my sister yesterday gave birth to a healthy baby boy, Andrew. My first nephew, my parents' first grandchild, my maternal grandparents' first great-grandchild (my sister did pull a surprising piece of mental cruelty by telling myself and my parents that she was expecting some two or three months before she told my grandparents, forcing us into the torturous position of having to keep our mouths shut about the one thing we knew they would be estatic to learn). So, in honour of this happy event, I am going somewhat out of character giving this post entirely over to cute baby pictures!


Polyphemus moth caterpillar. Photo by Mick Phillips.


Baby walrus (Odobenus rosmarus). Photo from Gothamist.


Porcelain crab larva. Photo by Jean-Marie Cavanihac.


Sloth mother and baby. Photo from Green Tracks.


Fungia (mushroom coral) larva. The brown spots are symbiotic dinoflagellates. Photo from the Weis Lab, Oregon State University.


Mouthbrooder and young. Photo from Deeble & Stone Productions.


Ponderosa pine (Pinus ponderosa) seedling. Photo from here.


Swallows. Photo from A Nature Observer's Scrapbook.


Amblypygid carrying babies. Photo by Laurel Symes.

Conversations with Cothurnocystis

Fossil of Cothurnocystis elizae. Photo from Ayrshire History.

I first encountered the Cothurnocystis whilst wandering through the Ordovician of the mid-1910s. I had been feeling rather peckish, as it was then Tuesday, so I was collecting brachiopods from the sea floor. I had been assured by an assistant of mine that a collection of these roasted over an open fire made a most filling and nutritious snack, though I must regret the use of the bullwhip that the results of this assertion later required of me. While picking up a large specimen of (as it turned out) more than usual toughness and toxicity, I espied a diminutive object like an elfin boot attached to the shell of my accursed brachiopod by a slender filament from the sole of the boot. I immediately recognised it as a specimen of the Cothurnocystis recently described by Mr Bather, and considered my luck at finding this rare individual. The specimen was much too small to be worthy of consideration for the pot, so instead I lost no time in enquiring after its nature. Together we soon established that the Coturnocystis was intimately familiar with the same works of Bather and Jaekel as I was, as shown by its firm attachment with its delicate stalk, so like the stem of its fellow pelmatozoans. Nevertheless, at that point in time the Cothurnocystis was little willing to speak with me - I do not think I had yet gained its trust, and I suspect my zeal in baking its brachiopod home may have further inclined it against me. I therefore took my leave, promising to return anon.

As luck would have it, commitments elsewhere prevented me from returning to the Ordovician until the 1960s. It was then Thursday, and a satisfying repast of jellyfish and cycads before I left had put me in a far more affable mood than before (though the cycads were to give me a rough time later that evening). I immediately sought out my prior acquaintance, scrupulously checking every brachiopod shell I could find. I was beginning to despair of finding the Cothurnocystis again, when I heard its voice politely enquiring as to what, exactly, I thought I was doing? I sought out the source of this speech and discovered the Cothurnocystis no longer affixed to any brachiopod but lying freely on the muddy floor. Indeed, I could barely recognise my former friend - in place of the stem it bore a long feeding arm with a fine ambulacrum, which it was projecting into the water column in search of tasty micro-organisms. I asked what had brought about this change, and it informed me that it had become inspired by the works of a Mr Ubaghs, who had wholeheartedly convinced it that this was a much better way to live for a stylophoran. I asked how its fellow pelmatozoans had taken this change, and it solemnly informed me that there had been a parting of the ways - most of the other pelmatozoans were crinozoans now, while the Cothurnocytis and its closest associates now regarded themselves as homalozoans. It also invited me to regard the fine hydropores and anal pyramid that Mr Ubaghs had given it, but the mention of the latter reminded me of the cycads, and I had to make for home with great haste.


The mitrate Rhenocystis latipedunculata. Photo from The Virtual Fossil Museum.

When next I travelled to the Ordovician, it was the 1980s. As it was then Friday, all I had been able to find in the cupboard for dinner was some tinned scorpions and a small bag of sequoias, but I had made the most of my frugal meal and was feeling quite refreshed and invigorated. It turned out that I was not the only one so invigorated, for when I found the Cothurnocystis it seemed surprisingly active and to have thrown off the sedentary habits of its past. The feeding arm had been drawn down from the water column, the ambulacrum had vanished, and the Cothyrnocystis was putting it to great use in drawing itself around the sea floor. I apologised for my hasty leave of two decades earlier, and indicated that I was now more able to examine the hydropores and anal pyramid it had mentioned earlier. To my astonishment, however, the creature only regarded me with a look of bafflement. Was I not, it asked me, familiar with the work of Mr Jefferies? It was quite obvious that I was not, for otherwise I would have been aware that it had no such structures. What I had taken for hydropores and anal pyramid was nothing less than its gills and mouth, and it was astonished at how I could confuse such dissimilar structures. Naturally, I protested that it had been the Cothurnocystis itself that had informed me of their natures, at which it relented and admitted that that had been before Mr Jefferies had improved its understanding, and that its ire had arisen from being reminded of past mistakes. At least it had not been so mired in error as its relatives the mitrates, who had been astonished to discover that what they had believed their dorsal surface was, in fact, their ventral, and that they had spent all this time lying on their backs! I enquired after the loss of the ambulacrum, and the Cothurnocystis showed me that it had a mobile tail instead, complete with notochord. I remarked that a notochord and gills were unusual features for an echinoderm to have. Unfortunately, this merely raised the creature's ire once more, and it snootily pointed out that it was no longer a mere echinoderm, but a proud chordate - indeed, a respectable ancestral form whose primal position was far more elevated than my own debased and degraded lineage! Unable to withstand the slurs of the Cothurnocystis' temper any longer, I made my way back home in a state of much vexation.

I have recently returned to the Ordovician once more, on a Sunday after a suitable meal of ecumenical sponges and St. John's wort. Despite the unpleasantness of our last parting, enough time had passed that I once again felt inclined to seek out the Cothurnocystis and see if it had made any further changes. When I did find it, I asked if much had happened in the last twenty years. "Has it ever!" the creature replied. "No sooner had I accepted the word of Mr Jefferies than a Mr Parsley came along to try and change my mind again. He insisted that I did have an anal pyramid after all, and that it was foolish of me to forsake my fine echinoderm brethren for those glitzy chordates. Mr Parsley even turned the mitrates back over. But every time he did so, Mr Jefferies and his associates came back to invert them once more, and now they don't have a clue which way is up! There's a Mr David and Mr Mooi who not only think I should go back to the echinoderms, but think they can get me close to the crinoids once more, even though we hadn't been talking for years!" The Cothurnocystis stretched out its feeding arm and opened its plates to expose its ambulacrum, but then brought it back in and started crawling around with tightly fused plates over its tail. Anal pyramid became mouth, then back to anal pyramid again, at a rate that left me feeling decidedly dizzy. The Cothurnocystis, meanwhile, was continuing its tirade. "There's a Mr Shu who's introduced us stylophorans to a creature by the name of Vetulocystis, and want us to get close to it, but to be honest we don't yet know what to make of it. Mr Hotchkiss could probably handle the idea, but it really doesn't fit with Mr Sumrall's plans for us." At this point I took my leave once more, but the Cothurnocystis left me with one final rejoinder, "I've really come to dread each coming issue of Journal of Paleontology, because I hate to think what they're going to say about us next!"

REFERENCES

Gregory, W. K. 1935. Reduplication in evolution. Quarterly Review of Biology 10 (3): 272-290.

Lefebvre, B. 2003. Functional morphology of stylophoran echinoderms. Palaeontology 46 (3): 511-555.

Ruta, M. 1999. Brief review of the stylophoran debate. Evolution and Development 1 (2): 123-135.

A Whole New Twist on Things, or Just Shifting Back and Forth?

Once again, the Taxon of the Week here at Catalogue of Organisms is probably not worthy of the title. Rectiplanes Bartsch, 1944 was recognised by Powell (1966) as a separate subgenus of Antiplanes Dall, 1902, a genus of turrid shells found in cold waters of the North Pacific. Rectiplanes species, at 18-36 mm in length, were slightly smaller than Antiplanes (Antiplanes) species that reached up to 55 mm. However, a search on Google Scholar suggests that Rectiplanes disappears off the radar in about the mid-1970s. Why should this be?


Good pictures of 'Rectiplanes' seem to be unavailable on line. This photo of Antiplanes vinosa from the Jacksonville Shell Club is labelled Rectiplanes vinosa at the site I found it on, but is obviously sinistral rather than dextral.

The distinguishing feature of the two subgenera was that Antiplanes had sinistral coiling, while Rectiplanes had dextral coiling. The two terms refer to the direction in which the shell coils (its chirality). If you hold a gastropod shell with the tip pointed upwards and the opening at the bottom facing towards you, then in a dextral shell the opening will be on the right-hand side while a sinistral shell will have the opening on the left. The two forms of coiling are mirror images of each other. This has more than merely aesthetic consequences for the individual snails - because snail anatomy is asymmetrical, these differences in orientation are also reflected in the position of the genital openings. Snails with different orientations have the genital openings on different sides of the head, and in species that mate face-to-face the genital openings cannot be brought into contact between snails of different orientations. Usually the orientation of the body matches that of the shell, but there are some gastropods in which the two are opposite - as pointed out by Gould et al. (1985), this condition (called hyperstrophy) is actually a kind of developmental illusion caused by a reversal in direction of growth between the protoconch (the larval shell) and the teleoconch (the adult shell).


A rare mating between dextral and sinistral individuals of the edible snail (Helix pomatia). The awkwardness of such an attempt is readily apparent. Photo by Peter Leonhardt.

Dextral coiling is far more common in gastropods than sinistral coiling. Gould et al. (1985) noted that from hundreds of thousands of examined specimens of the West Indian snail Cerion, only six sinistral examples had been recorded*. When Powell (1966) compiled his catalogue of turrids, Antiplanes contained eleven dextral species compared to four sinistral species. The reasons for this imbalance remain entirely unknown. In many cases that have been investigated, a single gene appears to be involved in determining gastropod chirality. Differences between dextral and sinistral forms other than mere chirality are generally minor - Gould et al. (1985) did note morphometric differences in e.g. size of the aperture between sinistral and dextral Cerion, but Dietl & Hendricks (2006) found that sinistral individuals of Planaxis were more resistant to predation by crabs than dextral individuals. Some of the few clades of predominantly sinistral taxa have been perfectly successful, thank you very much, while a few gastropod taxa even maintain populations that are stably polymorphic for shell chirality.

*In the typical Stephen Jay Gould style that people either love or loathe, the paper opens with, "Vishnu, the great Hindu god of preservation, holds in one of his several hands a shell of the genus Turbinella".

To get back to Rectiplanes, the obvious question is whether chirality can be regarded as a significant factor in taxonomy. Does it indicate a close exclusive relationship between members of one or the other subgenus, or has chirality changed back and forth multiple times in the history of the genus? Unfortunately, no-one seems to have looked at the interspecific phylogeny of Antiplanes itself, but the general indication from other taxa seems to be that chirality alone is not a reliable indicator of phylogeny. Because only a single gene is generally involved in determining chirality*, mutations might be expected to happen fairly recently, and even if selected against somehow, recessive mutations might still persist in the population via heterozygote carriers. What is more, chirality seems to be determined not by the genotype of the individual, but the genotype of its mother (Gittenberger, 1988; Anonymous, 2005). In other words, a genetically sinistral snail born of a dextral mother will be phenetically dextral, but all of its offspring (whether genetically sinistral or heterozygotic) will be born sinistral. This means that even though sinistral and dextral individuals may be unable to mate directly, gene flow can still occur between sinistral and dextral morphs in the population. It also means that rather than appearing as isolated individuals doomed to pass away without finding a suitable mate, reversed-chirality individuals are more likely to appear in numbers, allowing for a greater chance of establishment.

*A big caveat needs to be slapped across what I'm saying here. Probably because of matters of pragmatism, genetic determination of chirality has been almost exclusively studied in members of the Pulmonata, the clade of gastropods that includes the vast majority of terrestrial snails and a fair proportion of freshwater ones. Therefore, caution should probably be exercised in extrapolating what we know about chirality in Pulmonata to gastropods that lie outside Pulmonata - such as Antiplanes.

Such factors mean that we might reasonably expect reversals in chirality to occur multiple times within a clade that includes examples of both chiralities, and that monophyly of one or the other chirality cannot be assumed. For instance, Ueshima & Asami (2003) found that while sinistral species of the land snail genus Euhadra had derived from a single ancestor, reversals to dextrality had occurred multiple times within the sinistral clade. Indeed, the dextral E. aomoriensis had potentially arisen polyphyletically from its sinistral ancestor E. quaesita. Therefore, there is little justification in maintaining Antiplanes and Rectiplanes as separate taxa without anything to separate them other than chirality.

REFERENCES

Anonymous. 2005. Speciation begins, but doesn't end, with the twist of a shell. PLoS Biology 3 (9): e330.

Dietl, G. P., & J. R. Hendricks. 2006. Crab scars reveal survival advantage of left-handed snails. Biology Letters 2 (3): 439-442.

Gittenberger, E. 1988. Sympatric speciation in snails; a largely neglected model. Evolution 42 (4): 826-828.

Gould, S. J., N. D. Young & B. Kasson. 1985. The consequences of being different: sinistral coiling in Cerion. Evolution 39 (6): 1364-1379.

Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: An evaluation of the vaid taxa, both recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1-184.

Ueshima, R., & T. Asami. 2003. Single-gene speciation by left-right reversal. Nature 425 (6959): 679.

More than One Way to Skin a Cat (or Fertilise a Female)

Different male morphs of the beetle Onthophagus mouhoti. Photo from Bayblab.

Most of you will probably be aware of the existence in many species of sexual dimorphism, when males and females of the same species are significantly (and consistently) different in appearance. Indeed, considering that we ourselves as humans belong to one of the species showing sexual dimorphism, then if you're not aware of its existence you probably need your parents to explain some things to you. However, in some species sexual variation doesn't just stop there. Allow me to present you with the concept of male dimorphism.

Most researchers would agree that sexual selection is a major factor in the differences between sexes. Sexual selection is often referred to as something separate from and often opposed to natural selection, but as with so many popular distinctions the difference is somewhat artificial. Basically, sexual selection makes the fairly obvious point that if a certain form of male is more favoured in reproduction, then it will tend to become predominant in the population. This may lead to divergence between the sexes because males and females may be under different selective pressures. For instance, male deer grow large antlers that they use in fighting each other for access to females. The females themselves do not conduct such fights, so for them all that headgear would be little more than an annoyance, and they don't have them. Often features arising from sexual selection appear downright disadvantageous for the individuals possessing them - bright colours or loud calls in a bird may attract more females, but they also attract more predators. It has been suggested that this may not be entirely a coincidence - because sexually selected features usually act as a proxy for indicating the reproductive fitness of an individual, then development of such inconvenient characters indicates the male has fitness to spare.

Nevertheless, it is exactly this conflict between selective pressures that is thought to underly male dimorphism. In the majority of known cases, the two forms of male that are present in the population are one in which the sexually selected features are well-developed (large size, ornamentation, etc.) while the other form is less accentuated and more similar to the female.

The best-known example of male dimorphism occurs in Atlantic salmon where the population contains both large males and smaller "jacks". Salmon live most of their lives in the sea and travel back into fresh water to breed. When they reach their breeding grounds, the large males will occupy a territory and start gathering a harem of females that they defend from rival males. Jacks, on the other hand, do not defend a territory. Instead, they wander around the breeding grounds trying to find females that are not being well-defended by a large male. When the guarding male is distracted, the opportunistic jack will rush in and attempt to mate with his females while his back is turned. Males intermediate in size between large males and jacks are absent, and would probably be strongly selected against, being too small to effectively defend a harem and too large to mate in secret. One might also wonder why the two forms persist in the population rather than one entirely displacing the other - I suspect that this may be because the selective advantage of each form actually increases proportionally to the percentage of the other form in the population. Sneak-mating jacks are more favoured in the presence of a large density of harem-guarding males, as the guards will keep each other's attention engaged. Harem-guarders have an advantage over jacks in having females readily on hand rather than having to seek out unguarded females, and would be advantaged if there were less other large males with which to compete.

Minor males may have other advantages over major forms. In some species of mite, major males develop thickened and sharply terminated third legs that are used in fights between males (which are almost invariably fatal for the loser). Minor males lack these spines, and so are unable to compete directly with majors. However, minor males live for longer than majors, probably because they have not invested as much energy into developing the secondary sexual features (Radwan & Bogacz, 2000). Similarly, antlerless deer known as 'hummels' occur at low frequencies within the population and have a reputation for greater overall health than their antlered counterparts. Auld (1887) relayed the comments of a Lord Lovat on the subject:

Sometimes stags have no horns. These are called humle stags. If naturally so, and otherwise perfect, they will thrash any other stags of their own, or even considerably greater weight. We have known several of them undisputed masters of their own herds.

REFERENCES

Auld, R. C. 1887. Hornless ruminants. The American Naturalist 21 (8): 730-746.

Radwan, J., & I. Bogacz. 2000. Comparison of life-history traits of the two male morphs of the bulb mite, Rhizoglyphus robini. Experimental and Applied Acarology 24: 115-121.