in The Phytophactor
Linnaeus' Legacy #7
The newest edition of Linnaeus' Legacy is now available at The Ethical Palaeontologist. This month's keywords: hiking boots, gay sex, irritability, type specimens, chimaeras, dwarf woodpecker, dragon taxonomy, not insects, monkey brain, Raeticodactylus, Species Plantarum, plantains, Darwin's Garden, rapid evolution, squid necropsy, decline of the British cat, postage stamps, gardeners' binomials.
Southern Crustacean Relicts
This week's highlight taxon is the Phreatoicidea, a suborder of isopods restricted to freshwater habitats in ex-Gondwanan continents. This is not a particularly large group - only about a hundred species have been described, though it is estimated that at least that number again remain undescribed. A reasonably high proportion of the species are known from subterranean habitats*, including the first species to be described, Phreatoicus typicus. Knott (1986) listed eleven subterranean species, which at the time was about a quarter of the total known diversity (over half the known species have been described since then). The diversity of the suborder is also heavily centred in Australasia - 94 species have been described from there, in contrast to four species from South Africa and only two from India (Wilson, 2008), but again this is probably heavily biased by the fact that almost all taxonomic work on this group has been conducted in Australia. For instance, Knott (1986) refers to possible undescribed species from India - these species seemingly still have not appeared in print twelve years later. The supposed Gondwanan distribution of phreatoicids also makes their apparent absence from South America very interesting, but how confident can we be that they are truly absent from that continent?
*I should make it clear that "subterranean" does not necessarily mean "cave-dwelling". Caves actually only make up a small proportion of the subterranean habitat, and only one cave-dwelling phreatoicid species is known (Knott, 1986). The majority of subterreanean species are sediment-dwelling forms whose habitats can extend right down into groundwater aquifers. Phreatoicus typicus, for instance, was originally described from a well near Christchurch in New Zealand, into which it would have emerged from the surrounding bedrock.
Most people imagine isopods as dorsoventrally flattened animals, like their most familiar representatives the woodlice and slaters. Phreatoicids, however, represent an exception to this rule, being fairly high-vaulted, narrow animals. Stygobiotic forms tend to be more elongated. Phylogenetically, phreatoicids are one of the most basal groups of isopods, and have one of the earliest fossil records. The Palaeozoic phreatoicids (or, technically, stem-phreatoicids) Palaeophreatoicidae are known from marine sediments, but since the Triassic all known representatives have been freshwater. Phreatoicids are detritivores feeding primarily on decaying vegetation or on the micro-organisms associated with the former, but may occassionally be carnivorous (Wilson, 2008). Phreatoicids are a significant part of the pholeteros - the specific faunal assemblage of organisms associated with the burrows of larger animals such as freshwater crayfish.
Taxonomically, the Phreatoicidea have been a difficult group. For many years the classificatory sytem used was that established by G. E. Nicholls in the early 1940s, which divided phreatoicids between two families, each divided into a number of subfamilies. Unfortunately, the features used to separate these taxa have been shown to be largely artificial, and a high degree of variation can occur between closely-related species or even within examples of the one species. Wilson & Keable (2002)revised the classification somewhat through phylogenetic analysis, recognising three families and abandoning Nicholls' subfamilies. The suborder as a whole seems to be characterised by fairly slow morphological evolution. Gouws et al. (2004) showed that at least one supposed species, the South African Mesamphisopus capensis, is divisible on genetic and morphometric grounds into a number of potential cryptic species.
Like many freshwater and subterranean organisms, many phreatoicids have very restricted distributions and are placed at significant risk of human activities. Genetic studies show that each separate aquifer may have its own isolated population (Wilson, 2008). Indeed, a number of species are believed to have already gone extinct, due to factors such as increasing groundwater salinity as a result of deforestation (Knott, 1986) or alteration and exhaustion of water supplies and aquifers (Wilson, 2008). Unfortunately, the lack of taxonomic resolution within the group, as well as the difficulty of surveying the habitats of subterranean species in particular, make it very difficult to assess the risk to individual species.
REFERENCES
Gouws, G., B. A. Stewart & S. R. Daniels. 2004. Cryptic species within the freshwater isopod Mesamphisopus capensis (Phreatoicidea: Amphisopodidae) in the Western Cape, South Africa: allozyme and 12S rRNA sequence data and morphometric evidence. Biological Journal of the Linnean Society 81: 235-253.
Knott, B. 1986. Isopoda: Phreatoicidea. In Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) (L. Botosaneanu, ed.) pp. 486-492. E. J. Brill / Dr. W. Backhuys: Leiden.
Knott, B., & S. A. Halse. 1999. Pilbarophreatoicus platyarthricus n.gen., n.sp. (Isopoda: Phreatoicidea: Amphisopodidae) from the Pilbara Region of Western Australia. Records of the Australian Museum 51: 33-42.
Wilson, G. D. F. 2008. Global diversity of Isopod crustaceans (Crustacea; Isopoda) in freshwater. Hydrobiologia 595: 231–240.
Wilson, G. D. F., & S. J. Keable. 2002. New Phreatoicidea (Crustacea: Isopoda) from Grampians National Park, with revisions of Synamphisopus and Phreatoicopsis. Memoirs of the Museum of Victoria 59 (2): 457-529.
Carnivalia
Three new carnivals over the last week:
For the greener side of life, see Berry Go Round at Foothills Fancies.
If you like things to be a little rockier, see The Boneyard at Familiarity Breeds Contempt.
And if you like that squishy feeling between your toes, there's always Circus of the Spineless at Deep Sea News.
How Irritable is This Bird?
It's definitely intriguing how the meanings of words can change over the years. A Blog Around the Clock recently presented a quote that claimed that Charles II described the newly-built St Paul's Cathedral as "awful, pompous and artificial". This was entirely complimentary - "awful" indicates something that inspires awe, "pompous" roughly means "majestic", while "artificial" means that it indicated a great deal of artifice on the part of its builder - and "artifice" would have meant "technical or artistic skill", having not yet gained its current connotation of duplicity and deception. "Automaton" is another word that has changed significantly in meaning. It literally means "self-moving", and so when it was first coined would have indicated the independent, undirected movement and action of the subject - almost the complete opposite of its current usage.
I bring up those examples to segue into another change in meaning that I came across yesterday. I happen to have in front of me a copy of the Proceedings of the Zoological Society of London for 1866, in which (amongst other things) W. Lilljeborg presented a revised classification for birds. In explaining the principles on which he based his classification, Professor Lilljeborg notes:
Irritability seems to us to be the most distinguishing character for birds, and this should consequently be taken into consideration more than others with regard to their classification.
I'm sorry? You're classifying birds according to how irritable they are? Fortunately, Lilljeborg further explains, "We do not by irritability mean the muscular strength alone, but vivacity and activity generally". Lilljeborg was basing his classification on a principle that evolution had proceeded from less derived, more sluggish forms to more derived and accomplished forms. All horribly Scala Naturae-based, of course, and more than a little inaccurate in some regards:
The swimmers seem to us the lowest, from their showing a tendency to the lowest form of vertebrated animals - the fish-form. In the [penguins], where the wings resemble fins, and where they, as in all other diving birds, serve as such, we have this form most strongly designated. The heavy, clumsy structure, with small wings and short legs, also makes them generally less active than other birds, and shows a lower development of the type of bird.
A description obviously heavily biased by observations of penguins almost solely on land, not in the water where they are far more adept. I also had to snicker somewhat at Lilljeborg's complaint about the previous tradition of placing the birds of prey (rather than the passerines he favoured) at the "summit" of avian evolution:
A system that places the dirty vultures highest, does not seem to us to indicate a correct idea of the nature of birds.
Lest I give you too negative impression of Lilljeborg's skills as a systematist, his final classification stands up fairly well by the standards of the time. The classification itself was generally based on good anatomy - the philosophical considerations mostly affected what order he listed things in. Still, I couldn't read that section on "irritability" without getting the mental image of Professor Lilljeborg, his fingers heavily bandaged, making his way down a row of birds sitting in cages, poking each one in turn with a chopstick and seeing how long it took to snap back. One suspects he probably left the eagle until last.
REFERENCES
Lilljeborg, W. 1866. Outlines of a systematic review of the class of birds. Proceedings of the Zoological Society of London 1866: 5-20.
The Nature of Nanoarchaeum
The Archaea have received a great deal of attention in recent years as the supposed Third Domain of life. They are a group of prokaryotes that differ from all others (the Eubacteria) in a number of significant ways - they have a RNA-protein translation system more similar to that of eukaryotes than Eubacteria, they lack the murein that makes up the cell wall of most Eubacteria, and they have a unique tetraether cell membrane that differs from that of the other domains. The study of Archaea has also been influenced by the association of most cultured representatives with extreme environments as thermophiles or halophiles, though environmental PCR studies suggest that Archaea may be much more abundant in the general environment than the cultured diversity suggests (Forterre et al., 2002).
Compared to the dog's breakfast that is our current understanding of eubacterial phylogeny, the phylogeny of Archaea seems much clearer. Most studies indicate a basal division of cultured Archaea between two major clades, the Crenarchaeota and Euryarchaeota. Crenarchaeota is the smallest of the two groups and the cultured representatives are all hyperthermophiles, though again PCR samples indicate uncultured mesophilic examples. The Euryarchaeota are much more diverse, including halophiles and methanogens as well as thermophiles. Environmental PCR sequences suggesting archaeal taxa lying outside these two major clades, and one group of them has received the provisional name of Korarchaeota, but until these taxa are properly characterised their existence remains uncertain.
So far, the only well-characterised archaeum that has been suggested to lie outside the two main clades is 'Nanoarchaeum equitans'* (Huber et al., 2002). 'Nanoarchaeum' is a minute (400 nm) organism that lives in hyperthermophilic environments as an obligate associate of another archaeum, Ignicoccus. Attempts to culture 'Nanoarchaeum' independently have failed - even growth on a medium of puréed Ignicoccus or separated from an Ignicoccus culture by a semipermeable membrane proved impossible. 'Nanoarchaeum' can only grow in direct cell-to-cell contact with living Ignicoccus. 'Nanoarchaeum' also has the one of the smallest genomes of any organism known, with only about 490,000 base pairs. For comparison, the human genome includes about 3000,000,000 base pairs, that of a lungfish 130,000,000,000 base pairs, and that of the largest known virus, Mimivirus, 1200,000.
*I haven't italicised 'Nanoarchaeum' because it's not yet a technically valid taxon. To be valid, a prokaryote name requires publication or validation in the International Journal of Systematic and Evolutionary Microbiology, and this doesn't seem to have happened for 'Nanoarchaeum'. Because of the stringent culture requirements of the Prokaryote Code of Nomenclature, 'Nanoarchaeum' may not be eligible for validation, though it may be recognisable as a Candidatus, the Prokaryote Code's provisional class for a taxon that cannot be cultured independently but can be well-characterised environmentally.
The reduced genome of 'Nanoarchaeum' is reflective of its inability to synthesise many of the metabolites it requires to survive, which it draws instead from its Ignicoccus host. This is not unusual among parasitic organisms, though 'Nanoarchaeum' is unusual among super-reduced parasites in its extracellular rather than intracellular position relative to the host, probably as a consequence of the general absence of phagocytosis among prokaryotes. 'Nanoarchaeum' is also comparable to other super-reduced parasites in another way. When it was first described, analysis of ribosomal genes suggested a position for 'Nanoarchaeum' divergent from all other Archaea, which lead to its promotion as a new archaeal phylum. However, studies of comparable eukaryotic parasites have indicated that they have usually undergone exceedingly rapid evolution that has exaggerated their differences from their more mediocre relatives. Ribosomal genes in particular seem to be very prone to such long-branch distortion. The prime example in recent years has been the Microsporidia, intracellular parasites that were once thought on the basis of ribosomal genes to be one of the earliest-diverging branches of eukaryotes, but have since been reinterpreted as highly-evolved fungi.
With this in mind, I was not overly surprised to see the results of Brochier et al. (2005). Brochier et al. took a slightly different tack to understanding the phylogeny of 'Nanoarchaeum' - as well as using a wider range of genes, Brochier et al. also calculated the open reading frames (ORFs) of the entire 'Nanoarchaeum' genome (an ORF is a section of the genome that could potentially code for a protein) and compared them to those of other Archaea. The ORF analysis found that 'Nanoarchaeum' shared the majority of its ORFs with Euryarchaeota, and was particularly similar to members of the order Thermococcales. This result was corroborated by a number of the gene analyses. The basal position of 'Nanoarchaeum' in the ribosomal trees would therefore appear to be due to long-branch attraction, possibly exacerbated by lateral gene transfer with its Crenarchaeota host.
REFERENCES
Brochier, C., S. Gribaldo, Y. Zivanovic, F. Confalonieri & P. Forterre. 2005. Nanoarchaea: representatives of a novel archaeal phylum or a fast-evolving euryarchaeal lineage related to Thermococcales? Genome Biology 6: R42.
Forterre, P., C. Brochier & H. Philippe. 2002. Evolution of the Archaea. Theoretical Population Biology 61 (4): 409-422.
Huber, H., M. J. Hohn, R. Rachel, T. Fuchs, V. C. Wimmer & K. O. Stetter. 2002. A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont. Nature 417: 63-67.
Linnaeus'Legacy #6 is coming up!
It's Linnaeus' Legacy time again! May 5th's edition will be happening at The Ethical Palaeontologist, so get your submissions in to julia (at) ethicalpalaeontologist.com, leave them in the comments here, or use the submission form at Blog Carnival.
We also need hosts for future editions!
We also need hosts for future editions!
Soft Waxy Scales
The Hemiptera (true bugs) are one of the definite contenders for the insect order containing the most oddballs (Coleoptera and Hymenoptera are probably their competitors). Hemiptera are well marked as a group by their specialised sucking mouthparts, but within the Hemiptera a wide range of body plans have arisen. The scale insects (Coccinea) are perhaps one of the oddest groups of all, and it is one of the scale families, the Ortheziidae, that is our current Taxon of the Week.
Scale insects get their name from the adult females, which have completely abandoned the joys of mobility and live their lives on a single spot, sucking the sap from a host plant. To protect themselves they secrete a covering of sticky wax or a hardened scale. Because of their sedentary lifestyle, indulgences such as legs or eyes are unnecessary, and have become reduced or lost. Only close inspection of the adult, or of the males or nymphs, would identify these creatures as even being insects. Those scales that are significant to humans are mostly plant pests, though some species are used to produce lacquer or the red dye known as cochineal (yep, gramophone records were once made from crushed insects).
Scales of both sexes first hatch out of their eggs as highly mobile nymphs called 'crawlers', with fully developed legs and antennae (Williams, 1991). This is the dispersal phase of their life cycle - not only can they crawl around, but they are also small enough to be easily blown by the wind. Once they find a suitable host plant and moult to the next instar, scale nymphs become pretty much immobile, and lose all the paraphernalia of their youth. While females pretty much remain in this state for the rest of their life, males do things quite differently. They feed for the second and third instars, then enter a non-feeding pupal stage before emerging as the winged adult (the adult males of a few species lack wings). Adult male scales also don't feed and lack mouthparts - they will only live for a short time while they find a mate. Male scales are also one of the few groups of winged insects, in addition to Diptera (flies) and Strepsiptera, to have lost one of the pairs of wings (the first time I ever saw one, I was not yet aware of this and it confused me immensely). Because of their brief lifespan, male scales are relatively rare overall, though I get the impression that they can appear in large numbers in the right season. However, they are also of microscopic size, so are not likely to be noticed.
Scale insects are divided between a number of families. They are often divided into two superfamilies, the Orthezioidea (archaeococcids) and Coccoidea (neococcids) (Koteja, 2000), though other authors combine them all into the Coccoidea. However, the archaeococcids are united only by primitive characters and are assumed to be paraphyletic and ancestral to neococcids. The Ortheziidae (ensign scales) is one of the most basal of the families of Coccinea, and one of the earliest families known from the fossil record, in the Lower Cretaceous (Koteja, 2000) - however, the Coccinea fossil record is extremely poor and should be treated with caution (most female scales are distinguished by microscopic characters not usually preserved in fossils, and the great difference between males and females makes them impossible to identify with each other unless specimens are preserved in the process of mating). Characters giving away the basal position of ortheziids include the presence of abdominal spiracles in the female (lost in neococcids), and compound eyes in the male (in neococcids the compound eye has disintegrated into a row of separate simple eyes). Nymphs and adult females secrete symmetrical plates of wax on their backs, while the female also secretes a wax ovisac at the end of the abdomen in which she incubates her eggs. This is the 'ensign' referred to in the common name.
The Ortheziidae are not a particularly large family by insect standards - about 200 species are known. As with other scales, a number of species have been spread around the world along with infected host plants, and some can cause trouble as pest species.
REFERENCES
Koteja, J. 2000. Advances in the study of fossil coccids (Hemiptera: Coccinea). Polskie Pismo Entomologiczne 69: 187-218.
Williams, D. J. 1991. Superfamily Coccoidea. In The Insects of Australia, 2nd ed. vol. I pp. 457-464. Melbourne University Press.
Subscribe to:
Posts (Atom)