The Resurrection of Grass

The resurrection grass Oropetium thomaeum, photographed by Diana Margaret Napper.
Even for a grass, the annual Oropetium thomaeum is not a very prepossessing plant. Only a couple of inches in height, it can be found as small tufts in arid or saline habitats in tropical Africa and Asia. In some parts of India, it has been recorded as a dominant grass species, probably because its small size makes it resistant to grazing while, for instance, its somewhat taller perennial relative O. roxburghianum is restricted to growing under shrubs that protect it from hungry ruminants (Gaff & Bole 1986).

Despite its modesty, O. thomaeum is not devoid of interest. It is an example of what is known as a 'resurrection plant', able to survive severe desiccation in a dormant state only to apparently come back to life after rain. Because O. thomaeum is a diploid species with a relatively small genome (one of the smallest of all grasses), it has been touted as potential useful in studying the genetic factors controlling drought tolerance (Bartels & Mattar 2002). According to this article, it has seen some use in attempts to rehabilitate degraded environments. Oropetium thomaeum has also been shown to differ from most plant species in that it is the shoot that emerges first from the germinating seed rather than the root (Dakshini & Tandon 1970).

Oropetium is a small genus of seven species of grasses in the subfamily Chloridoideae, with three species found in India and five in Africa (Phillips 1975). Despite the small number of species, members of Oropetium have been divided between no less than five genera in the past, reflecting a fair degree of disparity in their reproductive morphology. All, however, are relatively small narrow-leaved grasses forming dense tufts. Whether the African species possess the resurrective abilities of the Asian species remains, so far, unknown.

REFERENCES

Bartels, D., & M. Z. M. Mattar. 2002. Oropetium thomaeum: a resurrection grass with a diploid genome. Maydica 47 (3-4): 185-192.

Dakshini, K. M. M., & R. K. Tandon. 1970. An unusual type of germination of graminaceous seed. Annals of Botany 34 (2): 423-425.

Gaff, D. F., & P. V. Bole. 1986. Resurrection grasses in India. Oecologia 71: 159-160.

Phillips, S. M. 1975. A review of the genus Oropetium (Gramineae). Kew Bulletin 30 (3): 467-470.

The Rhodospirillales: It's Photosynthesis, But Not As You Know It

Colony of Rhodospirillum rubrum, from here.
For today's post, I'm going to take a look at the Rhodospirillales. This is a clade within the larger bacterial group known as the Alphaproteobacteria, other members of which include the disease-causing rickettsiae (thought to be close relatives of the ancestors of eukaryote mitochondria) and the nitrogen-fixing bacteria found in legume nodules. Rhodospirillales species are commonly found in fresh water, though some significant species have been found in other habitats. Many (but far from all) species of Rhodospirillales contain a chlorophyll-like pigment called bacteriochlorophyll a that functions like chlorophyll in catalysing photosynthesis. They also often contain other photosynthetic pigments, carotenoids, that are red in colour: hence the common use of prefixes such as 'rhodo-' and 'roseo-' in genus names in this group. Most photosynthetic Rhodospirillales differ from plants in being photoheterotrophs rather than photoautotrophs: that is, they cannot use carbon dioxide alone as the source of carbon in photosynthesis, and require other carbon sources (Rhodospirillum can grow photoautotrophically if conditions are optimal). Rather than using water as an electron donor, photosynthetic Rhodospirillales generally use other donors such as sulfide or hydrogen.

Individual of Magnetospirillum magneticum, showing the row of magnetosomes, photographed by Richard B. Frankel.
The members of the Rhodospirillales are divided between two families, the Rhodospirillaceae and Acetobacteraceae (Garrity et al. 2005). The majority of members of the Rhodospirillaceae are spiral in shape and grow in anoxic or microaerobic conditions. Acetobacteraceae, in contrast, are most often coccus- or rod-shaped and aerobic. Non-photosynthetic members of Rhodospirillaceae include the nitrogen-fixing genus Azospirillum, often found living in the soil or in association with plants (though, unlike the more familiar nitrogen-fixing genus Rhizobium, Azospirillum is not found specifically in association with root nodules). Also noteworthy is the genus Magnetospirillum, specimens of which possess a row of magnetite-containing bodies called magnetosomes down one side of the cell. These magnetic bodies are presumed to aid the bacterium in navigation relative to the earth's magnetic field. Magnetospirillum can be drawn from sediment through the use of a bar magnet; when observed under a microscope close to a magnet, they can be seen to swim parallel to the magnetic field lines. A cell suspension dropped onto a magnetic stirrer will be seen to flicker, as the cells change their direction of movement (and hence the direction of light scattering) to follow the spinning field.

Dividing cell of Stella vacuolata, from Garrity et al. (2005). Scale bar equals 1 µm.
Non-photosynthetic members of the Acetobacteraceae include the acetic acid bacteria (hence the name of the family). These gain energy by converting ethanol to acetic acid or, in more general terms, alcohol to vinegar. Some genera, such as Acetobacter and Acidomonas, are able to further oxidise the acetate produced to water and carbon dioxide; others, such as Gluconobacter, do not do so. One particularly distinctive genus in the Acetobacteraceae is Stella, species of which are found in soil and animal faecal matter. Cells of Stella are flat and, as suggested by their name, star-shaped with six points. Division of Stella involves the growth of a cross-wall down the midline of the cell, with each daughter cell retaining three of the parent's points to which they add three new points.

REFERENCES

Garrity, G. M., D. J. Brenner, N. R. Krieg & J. T. Staley. 2005. Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 2. The Proteobacteria, part C. The Alpha-, Beta-, Delta-, and Epsilonproteobacteria. Springer.

The Wolf in Time

Black-backed jackal pup Canis mesomelas, photographed by Blake Matheson.
The dogs of the genus Canis include some of the most familiar of all mammals: the wolf Canis lupus, the coyote C. latrans, and of course the domestic dog Canis familiaris. I have already discussed in an earlier post how these three, together with the golden (Canis aureus) and the Simien (C. simensis) jackals, form a cluster of closely related species (that I'll refer to as the 'wolf group') that are not always clearly separated. Today, I'll take things a bit further and look at the fossil history of the genus Canis.

Coyote Canis latrans, from Ryan Photographic.
The earliest taxa assigned to the genus Canis are known from the late Miocene, about six million years ago (Tedford et al. 2009). Early Canis have been identified in both Europe (C. cipio) and North America (C. ferox), though there is some uncertainty about whether the European C. cipio should be treated as Canis or assigned to the related, slightly earlier fossil genus Eucyon. Whatever the case, it doesn't appear to have been long before Canis populations were well and truly established on both continents. The North American Canis ferox was, as far as I can tell, probably not dissimilar to a modern coyote in appearance, and early Canis species probably also resembled coyotes in being fairly generalist predators. In the evolutionary analysis by Tedford et al. (2009), C. ferox was suggested to have begat C. lepophagus at the beginning of the Pliocene, which in turn begat two lineages: one leading to the modern wolf group, the other leading to three North American Plio-Pleistocene species (C. thooides, C. feneus and C. cedazoensis) that were smaller than their ancestor and probably similar in appearance to modern jackals. It is somewhat unfortunate that Tedford et al.'s analysis did not include the African side-striped (C. adustus) and black-backed (C. mesomelas) jackals, which molecular and morphological analyses have generally agreed lie outside the wolf group. Biogeography alone suggests that the North American 'jackals' were probably convergent rather than directly related to the modern African species, but it would be nice to know.

Mounted skeleton of dire wolf Canis dirus, from lora_313. This species probably weighed between 50 to 80 kg, which is comparable in size to a very large dog such as a bullmastiff or great dane.
The modern wolf group diversified in the late Pliocene, including a number of fossil species as well as the modern. The rate of diversification and spread of wolf-group Canis was such that palaeontologists refer to their appearance in the fossil record as the 'wolf event', and use it as a marker of the development of the colder tundra climate of the Pleistocene ice ages. Higher diversity in Eurasia suggests that it was probably the centre of diversification, with North American species derived from repeated colonisation. Significant among these was the relatively large C. armbrusteri, a close relative of the grey wolf C. lupus. Canis armbrusteri is notable as the probable ancestor of the late Pleistocene dire wolf C. dirus, made famous by its appearances in the works of Robert E. Howard* and similar authors. As well as being a dominant predator in North America, the dire wolf spread into northwestern South America. A similar large Canis species, C. nehringi, is also known from the same time in Argentina, but the analysis of South American canids by Prevosti (2010) was unable to clearly determine whether C. nehringi was a southern relative of C. dirus or a convergent relative of the Xenocyon lineage.

*A man who spent far too much time thinking about oiled chests if ever there was one.

Dholes Cuon alpinus, from Rajnish Pradhan.
Xenocyon is itself relevant to the history of Canis: first appearing in the late Pliocene, Xenocyon lycaonoides is probably the ancestor of the modern African hunting dog Lycaon pictus and the Asian dhole Cuon alpinus, forming a hypercarnivorous lineage specialised for collaborative hunting of large prey. Phylogenetic analyses of modern taxa have varied as to whether Lycaon and Cuon are the sister group of modern Canis, or whether they are in fact more closely related to the wolf group than are C. adustus or C. mesomelas, rendering Canis paraphyletic. Removal of the latter two species from Canis into separate genera as Schaeffia adusta and Lupulella mesomelas to preserve monophyly has been suggested, but almost universally ignored (as well as failing to resolve the status of the non-wolf-group fossil Canis species). Tedford et al. (2009) even nested the Xenocyon lineage within the wolf group itself, as sister to the Canis lupus-C. dirus group, but one might suspect the influence of convergences to large size and hypercarnivory. Prevosti (2010) placed Lycaon and Cuon in a more standard position just outside the wolf group, but did not consider as many fossil Canis species as Tedford et al.

Remains of Cynotherium sardous (plus some smaller mammal), from here.
The Xenocyon lineage was undoubtedly Eurasian in origin, but the primarily Eurasian X. lycaonoides did spread into northern North America, and a second species X. texanus was found in the Pleistocene of (surprisingly) Texas. The modern dhole Cuon alpinus was also present in North America in the latest Pleistocene, with remains of at least four individuals found in a cave in northeastern Mexico, as well as being found in Europe (Tedford et al. 2009). Also a member of the Xenocyon lineage was the Pleistocene Cynotherium sardous, found on the Mediterranean islands of Sardinia and Corsica (which were a single island when the Mediterranean sea level was lower). Though descended from hypercarnivorous ancestors, Cynotherium became adapted in its island habitat to hunting smaller prey (such as the Sardinian lagomorph Prolagus sardus). Though it retained the simplified dentition of a hypercarnivore, it became smaller and the skull became less reinforced, as befits an animal no longer wrestling down large ungulates (Lyras et al. 2006).

REFERENCES

Lyras, G. A., A. A. E. Van Der Geer, M. D. Dermitzakis & J. De Vos. 2006. Cynotherium sardous, an insular canid (Mammalia: Carnivora) from the Pleistocene of Sardinia (Italy), and its origin. Journal of Vertebrate Paleontology 26 (3): 735-745.

Prevosti, F. J. 2010. Phylogeny of the large extinct South American canids (Mammalia, Carnivora, Canidae) using a "total evidence" approach. Cladistics 26: 456-481.

Tedford, R. H., X. Wang & B. E. Taylor. 2009. Phylogenetic systematics of the North American fossil Caninae (Carnivora: Canidae). Bulletin of the American Museum of Natural History 325: 1-218.

Exogone sexoculata, a Worm of the Interstitial

Drawings of head and representative chaetae of Exogone sexoculata, from San Martín (2005).
The sort-of-randomly chosen subject of today's post is the marine annelid worm Exogone (Parexogone) sexoculata, a member of the interstitial fauna around the coast of Australia. A previous record of this species from Italy has since been re-identified as the related species E. gambiae (Lanera et al. 1994). Exogone sexoculata is found among sand, mud, algae and dead coral, and in depths of up to 24 m (San Martín 2005). Despite the species name, it actually has four eyes, plus two eyespots that lack the lenses of the true eyes. As this is a fairly standard arrangement for Exogone species, E. sexoculata can't really claim to have one of the most distinguishing of species names.

Unidentified species of Exogone (in its epitokous form, perhaps?) from the Biodiversity Institute of Ontario.
Exogone sexoculata is a member of the family Syllidae, whose sometimes dramatic reproductive habits were discussed in an earlier post. Like other syllids, most Exogone species go through the process known as epitoky, where reproductively mature individuals metamorphose into a more mobile form, with enlarged eyes and parapodia. In the subfamily including Exogone, Exogoninae, the entire animal transforms into an epitoke (rather than epitokes budding off as may happen in other syllids). After releasing its gametes in the water column, the epitoke returns to the substrate and transforms back into the interstitial form. Females of Exogoninae brood their fertilised eggs attached to their body, and juvenile worms hatch out without going through a planktonic larval stage. In Exogone, the brooded eggs are attached on the underside of the female's body, whereas in other genera brooding may be dorsal. Exogone is also distinguished from other exogonine genera by the absence of a covering of papillae, the presence of only a single pair of tentacular cirri, and palps that are fused along all or almost all of their length. Exogone sexoculata is distinguished from other species in the genus by the absence of dorsal cirri on the second chaeta-bearing segment, its long median antenna, and by features of the chaetae (San Martín 2005).

REFERENCES

Lanera, P., P. Sordino & G. San Martín. 1994. Exogone (Parexogone) gambiae, a new species of Exogoninae (Polychaeta, Syllidae) from the Mediterranean Sea. Bolletino di Zoologia 61 (3): 235-240.

San Martín, G. 2005. Exogoninae (Polychaeta: Syllidae) from Australia with the description of a new genus and twenty-two new species. Records of the Australian Museum 57: 39-152.

A Devonian Pterygote?

I was going to write a post today on Strudiella devonica, the new fossil insect described from the Late Devonian of Belgium in today's Nature (Garrouste et al. 2012). Unfortunately, there's a limit to what I can really say. The stratigraphic significance of the specimen is undeniable: it sits well within a gap of about sixty million years that previously divided the earlier known insect fossils from the lower Devonian from the earliest known unequivocal winged insects in the mid-Carboniferous. Unfortunately, and I say this in the nicest possible way, the specimen itself is roadkill:

Photograph and interpretative drawing of Strudiella devonica, from Garrouste et al. (2012). Scale bar equals 1 mm.
Ah well, we simply have to work with what we're given, don't we? I think it's fairly reliable that this is, indeed, an insect: there seems a clear separation into a well-defined head, thorax, and legless abdomen, with no more than three legs visible on a single side of the thorax. The forward-protruding mandibles and antennae with broader basal segments are also insect-like rather than entognath-like (so it's not a stem dipluran or something like that). However, Garrouste et al. suggest that this specimen not only represents an insect, but also a crown-group pterygote. This I feel is a little more problematic.

Assignment of Strudiella to pterygotes relies on two characters: the relatively elongate legs, and the appearance of the mandibles. I suspect that it would not be that difficult for elongate legs to evolve convergently, and the supposed Carboniferous dipluran Testajapyx appears to have relatively long hind legs at least (Kukalova-Peck 1987). The mandible structure is a bit more difficult to hand-wave, though: Garrouste et al. interpret Strudiella as having an orthopteroid mandible, which is believed to be a synapomorphy of the Metapterygota, the particular clade within the pterygotes uniting the Neoptera and Odonata.

Hexapod phylogeny with representative mandible types, from Engel & Grimaldi (2004).
The earliest hexapods possessed a mandible with a single articulation (the condyle) to the head; such a mandible is still present in springtails, diplurans and bristletails. The clade uniting silverfish and pterygotes developed a second articulation (the acetabulum) on the inside of the mandible. In silverfish and mayflies, the acetabulum is anterior to the condyle, and the acetabular articulation is relatively loose. In the metapterygotes, the acetabulum has moved back to become more level with the condyle, and the mandible's articulation with the head is a lot more solid. The fossil remains of Strudiella do not appear to show the mandible articulation itself, but the general shape and orientation of the triangular mandible is more similar to the metapterygote arrangement than to the more basal morphology. Besides, such a morphology is has more clearly been demonstrated in the Lower Devonian Rhyniognatha, known only from a pair of preserved mandibles that are even older than Strudiella (Engel & Grimaldi 2004).

The ultimate question, then, is: is this one character enough to cement these taxa as crown pterygotes, with the implication that winged insects must have evolved considerably earlier than their fossil record currently indicates? Strudiella itself shows no sign of wings; Garrouste et al. suggest that it may be a nymph of a winged adult. I would counter that it also doesn't appear to possess any incipient wing buds, but of course it is debatable whether the preservation is good enough to be confident on this point.

If winged insects have been around since the Early Devonian, why do we find no direct evidence of them until the mid-Carboniferous? Wings are among the most commonly preserved insect remains—to the extent that if, as the adage goes, mammalian palaeontology is all about 'the tooth, the whole tooth, and nothing but the tooth', insect palaeontology often threatens to be 'all in vein'. For my part, I'm not inherently opposed to the idea of Devonian winged insects, but I don't think I'd really be willing to accept them until we're shown the actual wings.

REFERENCES

Engel, M. S., & D. A. Grimaldi. 2004. New light shed on the oldest insect. Nature 427: 627-630.

Garrouste, R., G. Clément, P. Nel, M. S. Engel, P. Grandcolas, C. D’Haese, L. Lagebro, J. Denayer, P. Gueriau, P. Lafaite, S. Olive, C. Prestianni & A. Nel. 2012. A complete insect from the Late Devonian period. Nature 488: 82-85.

Kukalova-Peck, J. 1987. New Carboniferous Diplura, Monura, and Thysanura, the hexapod ground plan, and the role of thoracic side lobes in the origin of wings (Insecta). Canadian Journal of Zoology 65: 2327-2345.