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.
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.