Assessment of the Fertility of Hybrids in Drosphila Athabasca

Coyne and Orr (1989 and 1997) have shown that sympatric species pairs show an enhanced level of prezygotic, or sexual, isolation. This phenomenon could potentially be explained by either the theory of reinforcement, which requires hybrid costs, or parapatric/sympatric speciation, which does not require hybrid costs. In order to determine which one is the right explanation, this study looks at the fertility of hybrids in Drosophila athabasca, one of the most rapid cases of speciation in the genus Drosophila. The species are composed of three behaviorally distinct races: West Northern (WN), Eastern A (EA) and Eastern (B). This study focuses on WN and EA only. First, fertility was examined at the F1 levels. By setting up crosses of pure races as well as hybrid (type1: WN female x EA male and type 2: EA female x WN male), it was determined that the fertility F1 hybrids did not display postzygotic disadvantages compared to the pure races. However, F1 hybrid type 1 may experience some level of behavioral incompatibility, as suggested by longer developmental time compared to pure race crosses. Second, fertility of the F2 generation was studied. The results showed some significant fertility reduction in hybrid type 1 crosses. In addition, both types of hybrid crosses experienced longer developmental time compared to the pure races. There was evidence that this reduction could be due to both prezygotic and postzygotic disadvantage. When F2 hybrids were backcrossed with pure race flies, all females mated while half of the males did not, indicating problems with males could lead to the fertility reduction in F2 hybrid type 1. On overall, results of this study do not provide enough evidence for reinforcement, as this theory demands very strong hybrid costs at the F1 level. On the other hand, the level of behavioral incompatibility suggests that this system might be a case of parapatric/sympatric speciation. Future studies should examine the fertility of hybrids in competition with pure race flies (competition assay) and the system’s ecology