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Rabeling, C., J. M. Brown & M. Verhaagh (in press). Newly discovered sister lineage sheds light on early ant evolution. Proceedings of the National Academy of Sciences of the USA.
I have learnt through Alex Wild of a paper currently in press for PNAS describing a new species of ant. With more than 12,000 ant species already on the books, this may sound like something of a non-event, but trust me, it's not. This new ant, Martialis heureka, has been placed not only in a new genus, but in a whole new subfamily. In fact, Martialis appears to be the sister group to all other living ants. A quick request later, and the lead author, Christian Rabeling, was kind enough to send me a copy of the paper*.
*For those without ready journal access, don't be afraid to contact authors of articles you might be interested in and ask if they have spare copies. Most researchers will be all too happy to help you.
As yet, Martialis is known only from a single worker specimen collected in 2003 in leaf litter from the Brazilian Amazon. One of the authors had found two workers in a soil sample five years previously, but these specimens were subsequently lost. The species name heureka (Greek for "Give me a towel!") reflects this history of disappointment and elation. The genus name Martialis refers to the unusual appearance of the specimen which looked like it may as well have come from Mars. Martialis is a pale-coloured, eyeless ant with long, thin, pincer-like mandibles. Because of the absence of eyes and its being found in litter and soil samples, the authors infer that Martialis lives hypogaeically (under the ground) or in some other low-light habitat such as within logs. However, the new species lacks any noticeable adaptations for digging, so it may inhabit pre-existing cavities such as rotting roots or burrows made by other animals. The fine mandibles, unlike those of any other ant, may be used for drawing out soft-bodied burrowing prey such as insect larvae. Features such as the presence of a sting indicate that Martialis belongs to the basal grade of ants, and molecular analysis of the specimen indicated Martialis to be sister to all other ants.
Interestingly, Rabeling et al.'s analysis also corroborates an earlier analysis by Brady et al. (2006) in finding the basalmost clade in the ants other than Martialis to be the Leptanillinae, another small hypogaeic subfamily. If this topology is correct, it is possible that the ancestor of all ants was hypogaeic. However, the current analysis was unable to statistically reject a number of alternative rootings.
The earliest fossil record of ants from the Cretaceous consists of the extinct subfamily Sphecomyrminae and a single species of the basal subfamily "Ponerinae" (Dlussky, 1999 - recent analyses indicate that the Ponerinae as previously recognised should be divided into a number of subfamilies, and I don't know whether the Cretaceous species would belong to the Ponerinae in the stricter sense). Dlussky (1999) recognises a separate family Armaniidae from the Cretaceous closely related to Formicidae, but Wilson (1987) argued that the "armaniids" are most likely winged castes of Sphecomyrminae. While the basal lineages of living ants might be hypogaeic, cryptic forms, the Sphecomyrminae, previously identified by its morphology as sister group to living ants, were wasp-like, probably epigaeic forms. Alternative positions for Sphecomyrminae within the crown clade seem unlikely in light of the absence of other ant fossils from the Cretaceous. I personally suspect that despite their basal position, there is a strong possibility that the hypogaeic lifestyle was acquired independently in Leptanillinae and Martialis rather than being ancestral for living ants as a whole.
REFERENCES
Brady, S. G., T. R. Schultz, B. L. Fisher & P. S. Ward. 2006. Evaluating alternative hypotheses for the early evolution and diversification of ants. Proceedings of the National Academy of Sciences of the USA 103 (48): 18172-18177.
Dlussky, G. M. 1999. The first find of the Formicoidea (Hymenoptera) from the lower Cretaceous of the Northern Hemisphere. Paleontologicheskii Zhurnal 1999 (3): 62-66 (transl. Paleontological Journal 33 (3): 274-277).
Wilson, E. O. 1987. The earliest known ants: an analysis of the Cretaceous species and an inference concerning their social organization. Paleobiology 13 (1): 44-53.
How, uh, hypogaeic is the hypogaeic life-style? In other words, if we're thinking about how likely or unlikely a particular phenotypic change through evolution might be, I'd like to know how distinct from other possible phenotypic states that particular character state is.
ReplyDeleteMost ants that I've met (not that I've met many at all) seem to live underground, in burrows they excavate themselves. How many distinct life-histories are included in that description?
In ants, Hypogaeic and Epigaeic normally refer to foraging habit. While most ants live in the soil, the Epigaeic ones forage in the open and the Hypogaeic ones rarely do. Some species do blur the line- these tend to be intermediate in morphology- but ants that are both pale and eyeless are hardly ever seen in the open.
ReplyDeletebut ants that are both pale and eyeless are hardly ever seen in the open.
ReplyDeleteDoes this mean the phenotype of interest here is the presence/absence of eyes and body pigments, rather than life-history per se? There's an obvious corelation between eyes-and-pigments and foraging habitat, which allows us to infer with reasonable confidence the life-history of a species from a single collected worker.
Eyes seem like something that can be lost more easily than (re-)gained. Same with body pigmentation, to a somewhat lesser degree. Foraging habitat would seem to be a trait constrained by morphological traits, rather than something more-or-less free to evolve independently.
Does any of this rambling discussion bring us closer to a confident inference of the ancestral foraging habitat of ants 70 million years ago? I kind of feel like my brain is spinning its wheels here. Help?
Does this mean the phenotype of interest here is the presence/absence of eyes and body pigments, rather than life-history per se?
ReplyDeleteWell, inasmuch as there is a correlation between life-history and phenotype, you're right.
Eyes seem like something that can be lost more easily than (re-)gained. Same with body pigmentation, to a somewhat lesser degree.
I agree with you. As I indicated in the post, I suspect that Martialis represents a derived lineage in its own right rather than a particularly plesiomorphic one.
Foraging habitat would seem to be a trait constrained by morphological traits, rather than something more-or-less free to evolve independently.
I would suspect that morphology would follow foraging behaviour rather than the other way around, but that's a whole different discussion topic.