Leopard horse: Fossils, phenotypes and genotypes

I wish I were talking about some beastly horse-big-cat hybrid, or at least a carnivorous horse. Instead… a ton of anthropology-related papers came out today in PNAS, and possibly the coolest one is a study that compares the DNA of Pleistocene fossil and modern horses with different coat colors/patterns, and then ties this in with Paleolithic cave art. A crazy confluence of four-field anthropology right there.

Modern horses and their depictions in Late Pleistocene French caves (Pruvost et al. 2011)
Melanie Pruvost and colleagues (in press) noted that the depiction of spotted horses at the site of Pech-Merle (they give 24 kya) could mean one of two things: (1) either the early human painters were depicting horses they actually saw on the landscape at the time, or (2) they were just being fanciful and frivolous, creative and carefree with their cavern canvas. Now, some modern horse breeds have a similar spotted, “leopard” phenotype, and a genetic basis for this is understood. So Pruvost and pals examined DNA from fossil horse bones from European sites dating to 20 – 2 kya to see if these mottled mares roamed the lands of the cave-painters. Sure enough, several samples show evidence for the mutation causing leopard spots.

This is pretty cool for evolutionary biology and paleontology. A major question in biology is how an individual’s genes (genotype) relate to overall appearance/behavior (phenotype). To a certain extent, physical variation between organisms arises from genetic variation, so when we see things evolve through the fossil record, this ought to correspond with some genetic changes as well. But linking genes to appearances isn’t so easy (especially for extinct animals). Pruvost and colleagues’ study is a step in this direction, though. Plus, the recent sequencing of the fossil Neandertal (Green et al. 2010) and Denisovan (Reich et al. 2010) genomes makes it possible to try to figure out if/how humans’ unique physical traits reflect our genes. In fact, even before these genomes were fully sequenced, Carles Lalueza-Fox and team (2007) identified a mutation on Neandertals’ MC1R gene, strongly suggesting the Neandertals sampled had light skin and red hair.

But the genetic basis for skeletal phenotypes is harder to discern. For example, Green et al. (2010) identified the unique human version of the RUNX2 gene as having come under strong natural selection since the disappearance of Neandertals. The authors noted that because mutations of RUNX2 in humans are associated with a cleidocranial dysplasia affecting the form of the skull and shoulders, and because humans and Neandertals differ in some aspects of their skulls and shoulders, then RUNX2 variation between humans and Neandertals is likely related to visible differences in their skeletons. But that’s about as much as could be said at the moment – RUNX2 is involved in bony development of the entire skeleton, interacting with other various genes in various places during ontogeny. So while it’s tempting, it’s still a little early to link RUNX2, or pretty much any other development-related gene, with physical differences between humans and our fossil relatives. But one day!

A Neandertal’s ruddy locks have never preserved in the fossil record, but its bones are very well known. In an ironic twist, we may have a better understanding of the genetic basis of variation in a soft-tissue (for which there are no fossils), than we do for the skeleton (for which we have lots of fossils).

And maybe one day I’ll get that leopard horse I was hoping for.

Green, R., Krause, J., Briggs, A., Maricic, T., Stenzel, U., Kircher, M., Patterson, N., Li, H., Zhai, W., Fritz, M., Hansen, N., Durand, E., Malaspinas, A., Jensen, J., Marques-Bonet, T., Alkan, C., Prufer, K., Meyer, M., Burbano, H., Good, J., Schultz, R., Aximu-Petri, A., Butthof, A., Hober, B., Hoffner, B., Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E., Russ, C., Novod, N., Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Z., Gusic, I., Doronichev, V., Golovanova, L., Lalueza-Fox, C., de la Rasilla, M., Fortea, J., Rosas, A., Schmitz, R., Johnson, P., Eichler, E., Falush, D., Birney, E., Mullikin, J., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D., & Paabo, S. (2010). A Draft Sequence of the Neandertal Genome Science, 328 (5979), 710-722 DOI: 10.1126/science.1188021

Pruvost, M., Bellone, R., Benecke, N., Sandoval-Castellanos, E., Cieslak, M., Kuznetsova, T., Morales-Muniz, A., O’Connor, T., Reissmann, M., Hofreiter, M., & Ludwig, A. (2011). Genotypes of predomestic horses match phenotypes painted in Paleolithic works of cave art Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1108982108

Reich, D., Green, R., Kircher, M., Krause, J., Patterson, N., Durand, E., Viola, B., Briggs, A., Stenzel, U., Johnson, P., Maricic, T., Good, J., Marques-Bonet, T., Alkan, C., Fu, Q., Mallick, S., Li, H., Meyer, M., Eichler, E., Stoneking, M., Richards, M., Talamo, S., Shunkov, M., Derevianko, A., Hublin, J., Kelso, J., Slatkin, M., & Pääbo, S. (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia Nature, 468 (7327), 1053-1060 DOI: 10.1038/nature09710

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