Michael Kubisch is a geneticist and reproductive physiologist working at the Tulane National Primate Research Center in the New Orleans area. He’s sent us his take on a recent paper on the genetics of the water buffalo. We really welcome this kind of contribution from our readers. Keep ’em coming! Here’s what Michael has to say:
Results of an interesting study by Chinese researchers have just been published, describing an extensive analysis of the genetics of Chinese swamp water buffalo (Bubalus bubalis). The Chinese swamp-type buffalo differs from the Indian river-type buffalo by the fact that it has 48 chromosomes compared to the 50 found in the latter. There is a third subspecies, the wild water buffalo, which may still exist in Southeast Asia, although its population size and genetic status are unknown and the animal is listed on the IUCN red list as being threatened. Based on analysis of mitochondrial DNA (which is solely inherited from one’s mother and consequently ideal for tracing maternal inheritance patterns), it appears that river and swamp buffalo split about 28,000 years ago with a further split in the swamp buffalo into two maternal lines taking place about 18,000 years ago. The genetic diversity varies between the two swamp buffalo matrilines in China and the authors suggest that the difference between the two lines might in part be due to the fact that occasional genetic introgression from wild swamp buffalo might have taken place into one of the lines. Interestingly, domestication of water buffalo seems to have occurred independently in India and China, most likely as a result of rice cultivation. Substantial numbers of water buffalo outside exist Asia, among other countries in Italy, where, as any cheese afficionado will know, their milk is used for the production of mozzarella.
What a wonderful blog on biodiversity. I will definitely come back to read more, thanks!
Just to be clear, the accepted story today is that at the species level there seems to be just one kind of water buffalo, Bubalus bubalis. What the research Michael points to shows is that there seem to be two distinct types among domestic water buffalo, a Chinese type and an Indian type. The two separated about 28,000 years ago. Furthermore, among the Chinese swamp water buffalo there are two distinct lines, which apparently separated about 18,000 years ago. So the populations were already well separated by the time the animals were domesticated, maybe 5000 years ago. In other words, it was not domestication as such that created the distinct populations; rather, the existence of the distinct types of water buffalo so much earlier than domestication suggests that early rice farmers domesticated the animals at least three times independently.
It is perfectly reasonable to assume independent domestication events, but these splits are all very recent, on an evolutionary timescale. I wonder if there is archaeological or other independent evidence for three independent domestications. An alternative hypothesis would be a single domestication and a molecular clock that ticks faster than currently thought. Would there be an effect of domestication on the rate of change of mt-DNA, perhaps because of a higher number of generations per unit time, particularly in a period of rapid population growth after domestication?
I’m pretty sure they measured the clock tick the standard way, by comparison to an outgroup. That would get rid of the generation time objection. But I can’t access the paper right now.
Later … yup. They estimated rate of divergence by a comparison with Bos taurus, which would deal with different generation lengths in the different lineages. There are fossils too, but I’m not entirely sure what they mean.