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Sunday, May 31, 2009

Long-term effects of hybridization in primates

One of my research interests is hybridization in primates, and the possible role it played in hominin evolution. It’s a sticky subject, so it’s always fun to find good papers on real-life examples of hybridization between different primate ‘species.’ In this vein, Burrell et al. (2009) report that the kipunji—a highly endangered papionin monkey from a small area in Tanzania—has an mtDNA haplotype from its yellow baboon (Papio cynocephalus) neighbors. Morphologically, the (living) monkey looks more like mangabeys (Lophocebus), though it has some baboon-like affinities, too.The authors posit that the most likely reason for this is inter-generic hybridization in the past, between Papio cynocephalus (yellow baboons) and Lophocebus sp. (mangabey monkeys).


The authors suggest a scenario in which in a marginal environment, Lophocebus (or Cercocebus?) males mated with some P. cynocephalus females. The hybrids, then, back-crossed into the respective parent species—thus baboon mtDNA was brought into a mangabey population. From here, the habitat favored nuclear genes of mangabeys, hence the overall mangabey appearance. Even though mtDNA is often (by necessity) assumed to be selectively neutral for phylogenetic studies such as these, it is not inconceivable that the baboon mtDNA persisted in the population because of selection, too.


The authors note that the test of the hybrid-origin hypothesis will come from nuclear DNA. If the kipunji truly represents the meshing of two genera’s genomes, then it should have a large amount of mangabey nuclear DNA. However, if the nuclear genome is all Papio that would mean that the kipunji’s ancestors were baboons whose morphology (and niche?) converged on that of mangabeys. Even this outcome would be a bit incredible, given the apparent pervasiveness of homoplasy within the papionins. In fact, the few nuclear genes known for the specimen either cluster in Papio, or are phylogenetically ambiguous. But for the moment, mtDNA and morphology support hybrid-origins. This is especially remarkable, since hybridization between genera, above the species level, leading to a stable taxon has not been documented before.


It is unclear whether the kipunji represents an instance of hybrid (or “secondary”) speciation, in which hybrids thrive in an environment while individuals of the parental species don’t, or just an intense case of gene transfer between species (I suppose if you’re getting a whole mitochondrial genome, it’s not really introgression). Nevertheless, the paper provides an amazing example of the potential evolutionary significance of hybridization in primates. Nice.


References

Burrell AS, Jolly CJ, Tosi AJ, and Disotell TR. 2009. Mitochondrial evidence for the hybrid origin of the kipunji, Rungwecebus kipunji (Primates: Papionini). Molecular Phylogenetics and Evolution 51(2):340-348.

Wednesday, May 27, 2009

Field Update 1: Things Fall Apart

So, I've been here for a week now, pretty much left to my own devices. What have I learned from my first real trip to the field (that's right, the field in a building)?

Well, the project I was so psyched about before I came will not work out. The sample is just too small, that is, not enough specimens preserve all the traits I need. I contacted Dr. Miriam Zelditch at UM who is a pretty amazing scientist working on integration, among other things. She confirmed my suspicion that even with resampling, such a small empirical sample size will not work out. I still think it's a neat idea, so I'm contemplating heading over to the Mammals collection to test it out on larger samples of extant primates. So Life Lesson #1 (which I've learned elsewhere): often, things don't work out they way you'd intended.

Everyone at the museum has been lovely so far. Very friendly and very helpful. And the fossils! I've only examined things Swartkrans and Kromdraai (I found out that some of the Kromdraai teeth I looked at today might be early Homo instead of Australopithecus robustus). But the real fossil are amazing! I never realized the extent to which the fossils were reconstructed--I don't think I've ever seen so much glue before. In fact, one specimen had a note from the late Dr. Charles Lockwood pointing out that it appears to have been reconstructed (glued together) slightly incorrectly. So it's not all taphonomy after all...

But then the taphonomy is incredible, too. Taphonomy literally means "burial laws" in Greek, and it's the study of how fossils come to look the way they do--that is, what happened after the organism died, how long did it take to be buried, what geological processes affected it, etc? The South African cave systems have interesting taphonomy: Many, or most, Swartkrans specimens may have been victims of carnivores like leopards, that subsequently fell into the caves. Once in the caves, they were buried by other debris and bones that fell in the caves. Many of the specimens have been distorted--squished flat or contorted in other weird ways. COB 101, for example, is part of a cranium that has been flattened on itself, such that the forehead is right next to the hard palate (I'm told they call the specimen "Pancake" around here).

So even where there are fairly complete specimens, many have been 'morphed' from having spent over 1 million years under the moving earth, further preventing quantitative analyses. Milford likes to note that with fossils, the data don't speak for themselves. Earlier in the week he asked me if the fossils had talked to me yet. I replied that they seem to be whispering, "try harder." Really, though, they were yelling at me, "You suck! You'll never figure us out, you hack." Milford replied that the fossils, in their condition, are telling me (and other paleontologists) to be innovative. Which I believe is true. So, here's Life Lesson #2 (I haven't tasted the fruits of the lesson, but I sincerely believe it): Most research questions are answerable; however, some require more creativity than others.

Monday, May 18, 2009

Summer Research

I've been pretty busy since classes let out, so it's been a while since I've posted anything. Two years of grad school down, n more to go. Hopefully no more than four more....

In a few hours, I leave lovely Ann Arbor, MI for my summer research, to the Transvaal Museum in Pretoria, South Africa. So far as I can tell, this museum has good collections of cultural artifacts, recent mammals, and tons of fossils. Many hominin remains are curated here as well, namely Australopithecus from the region--A. africanus from Sterkfontein and Makapansgat, and A. robustus from Swartkrans, Kromdraai, and I think also maybe Cooper's Cave(?).

I'll be focusing on the A. robustus collection. As with many fossil groups, the sample is largely teeth, and most other cranial remains are highly fragmentary--there are only a few relatively complete crania (including a remarkably well preserved skull, DNH 7 from Drimolen, which I don't think is at the Transvaal and that I doubt I'll get to examine. Oh well). The main project will examine the relative independence of many of the cranial, facial, and dental features in A. robustus, since these have been important in debates about whether the A. robustus and A. boisei (the latter from E. Africa) are more closely related to one another than to other hominins. Basically I'm going to test a few developmental/functional models that have been proposed, by applying a resampling procedure (that I'm still sort of in the process of developing). Hopefully it will be an interesting (and successful...) way of examining morphological integration in a fossil sample.

There are a few other projects, but this is the one I'm most interested in. I'll do my best to keep Lawnchair readers (whoever they might be) updated. Here's to what I hope will be a productive summer!