A whole-arm translocation (WAT8/9) separating Sumatran and Bornean agile gibbons, and its evolutionary features

Gibbons, like orangutans, are a group of threatened Asian apes, so that genetic monitoring of each species or subspecies is a pressing need for conservation programmes. We conducted a project to take, as far as possible, samples of known origin from wild-born animals from Sumatra and Borneo (Central Kalimantan) for genetic monitoring of agile gibbons. As a result, we found a whole arm translocation between chromosomes 8 and 9 (WAT8/9) specific to Sumatran agile gibbons. Furthermore, population surveys suggested that the form with the WAT8/9 seems to be incompatible with an ancestral form, suggesting that the former might have extinguished the latter from Sumatran populations by competition. In any case, this translocation is a useful chromosomal marker for identifying Sumatran agile gibbons. Population genetic analyses with DNA showed that the molecular genetic distance between Sumatran and Bornean agile gibbons is the smallest, although the chromosomal difference is the largest. Thus, it is postulated that WAT8/9 occurred and fixed in a small population of Sumatra after migration and geographical isolation at the last glacial period, and afterwards dispersed rapidly to other populations in Sumatra as a result of the bottleneck effect and a chromosomal isolating mechanism.