In the current issue of Heredity, Neaves and colleagues describe the results of their analysis of 12 microsatellite loci and the mtDNA of two kangaroo species -- western and eastern grey kangaroos. The two species are sympatric across part of Australia, basically a swath through western New South Wales. Neaves and colleagues describe substantial evidence for introgression of both autosomal loci and mtDNA into both populations:
A total of 7.6% of grey kangaroos sampled from the region of sympatry displayed evidence of introgression. Although no F1 hybrids were identified, 14 M. giganteus backcrosses and 3 M. fuliginosus backcrosses were detected. In addition to introgression at nuclear microsatellite loci, a single individual also exhibited introgression of mtDNA. The two phenotypic groups apparent within the region of sympatry corresponded (in 95% of individuals) to the two clusters identified by genetic analyses. Furthermore, the two phenotypic/genetic groups within the region of sympatry corresponded to representative allopatric samples of M. giganteus and M. fuliginosus from elsewhere in the distribution. Five of the M. giganteus backcrosses identified by genetic analyses were classified as M. fuliginosus based on overall phenotype. Geographically, hybrids were located throughout the region of sympatry.
This introgression has happened between the kangaroos despite the presence of prezygotic barriers that interrupt mating even in captivity:
Physical differences in the structure of the cloacal eminence as well as the production of species-specific odours by females may allow for species recognition (Kirsch and Poole, 1972). These characteristic differences are potentially among the features that result in the unidirectional hybridization observed in captivity, with male M. giganteus frequently failing to recognize female M. fuliginosus in oestrus.
In addition, there was male sterility in captive F1 hybrids. The authors expected a unidirectional bias in introgression owing to these factors, but the evidence says that gene flow apparently has gone both directions in the wild.
Sort of interesting -- I would actually have expected there to be fewer postzygotic isolating mechanisms in marsupials because the placenta-uterus interaction isn't there complicating matters. But cases of interspecific hybridization have apparently been rarely noted -- maybe that's because Australia is small enough that phylogeographic differentiation doesn't go as far for large species. In any event, this case is another one where F1 hybrids are basically absent in the area of sympatry, yet substantial historical introgression has clearly happened. That's based on a restricted sample of 12 autosomal loci -- we would expect to see much more significant effects at a few genes if the introgressive variant had a high adaptive value.
A model for ancient humans? Well, here's a case where 12 microsatellite loci seem sufficient to document substantial historical gene flow -- whereas in the human case described last week, there are more than 600 microsatellite loci to test the hypothesis. So the human case should have more power, all things being equal.
But the humans probably don't have as simple a prior population structure. The kangaroos have two well-defined lineages with a large zone of sympatry. Ancient humans may not have been highly differentiated (given the low Neandertal-human mtDNA coalescence time, for example) and may not have had substantial zones of sympatry -- they may have been much more similar populations interacting along a narrow boundary or cline. So the phylogeography in humans will be much more subtle.
Neaves LE, Zenger KR, Cooper DW, Eldridge MDB. 2010. Molecular detection of hybridization between sympatric kangaroo species in south-eastern Australia. Heredity 104:502-512. doi:10.1038/hdy.2009.137