Drosophila ananassae inhabits most of the tropical and subtropical regions of the world. In contrast to D. melanogaster and D. simulans, populations of D. ananassae exhibit a distinct genetic population substructure through most of their geographic range. Studies of D. ananassae populations from Trinity Beach (Australia), Apia (Samoa), Nadi (Fiji) and Java (Indonesia) and its sister species, D. pallidosa from Malololelei (Samoa) and Nadi (Fiji) showed significant levels of prezygotic mating discrimination. However, it is unclear whether postzygotic isolation exists, and if fitness of hybrids from matings between populations and between D. ananassae and D. pallidosa is lower than fitness of offspring from matings within populations. Such postzygotic reproductive isolation among populations of D. ananassae would indicate that the populations may be in the early stages of speciation. In this study, I determined the extent to which postzygotic reproductive barriers exist among populations of D. ananassae and D. pallidosa from Australia, Samoa, Fiji, and Indonesia. I measured hybrid sterility and hybrid inviability as components of hybrid fitness of offspring from crosses between populations of D. ananassae and D. pallidosa. I found there is measurable postzygotic isolation between D. ananassae and D. pallidosa as species, but that there little measurable postzygotic isolation among populations of D. ananassae.
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