Allopatric populations of Drosophila ananassae: a genetically unique species

Doleschall described Drosophila ananassae for the first time in 1858 from Ambon Island, Indonesia. This species belongs to the ananassae complex of the ananassae subgroup of the melanogaster species group. It is a cosmopolitan and domestic species and mainly circumtropical in distribution. It is of common occurrence in Indian subcontinent. In 1930s, Kikkawa and Moriwaki in Japan started genetical studies on D. ananassae and showed that it is a genetically unique species characterized by spontaneous male recombination and hypermutability. Another unique genetic feature of this species is absence of genetic coadaptation in its geographic populations which is considered as an exception to the genetic coadaptation concept of Dobzhansky. In a recent paper published in Ethology, Ecology & Evolution, Paul et al. (2024) have presented evidence that two allopatric populations of D. ananassae derived from Port Blair and Mysuru exhibit different degrees of trade-offs for evolving response against bacterial infection. Based on their results, the authors have suggested that two geographically isolated populations of a given species respond differently under a given selection pressure and gradually may diverge from each other in nature to develop a species barrier. Thus, the authors believe in allopatric mode of speciation which comes under geographic speciation suggested by Mayr. The importance of these studies is two allopatric populations of D. ananassae exhibit different degrees of trade offs for evolving resistance against bacterial infection. These populations may diverge in nature to develop species barrier ie. may be reproductively isolated. These findings in this recent paper are briefly discussed in the light of previous findings pertaining to trade-off studies, genetic differentiation in geographic populations, allopatric mode of speciation, a pair of sibling species: D. ananassae and D. pallidosa, and speciation in south Pacific populations of D. ananassae.