Evidence for Initiation of Post-Zygotic Reproductive Isolation between Drosophila ananassae and D. pallidosa as Indicated by Reduction in the Fertility of Hybrid Males

There are several barriers to preclude the gene flow between diverging populations. On the basis of their temporal nature, these can be broadly categorized into two forms: pre- and post-zygotic. Post-zygotic reproductive isolation can manifest in the form of reductions in hybrid fertility. Keeping this fact in view, in the present study, we studied sterility in hybrids of D. ananassae and D. pallidosa. Surprisingly a distinguishable pattern of infertility was found in the hybrids. This pattern, referred to as Haldane’s rule, is often observed in hybrids of recently diverged populations or species. Reduction in the fertility of hybrids provides the clue of incipient kind of post-zygotic reproductive isolation in these two sibling species. This is the first report of hybrid sterility in this species pair. However, hybrid sterility is not very prominent especially when compared to that of other species pairs with the similar divergence time. Thus, on the basis of our results, we conclude that either sexual isolation between these sibling species is sufficient and does not require the aid of post-zygotic isolation to preclude gene flow or rate of divergence between D. ananassae and D. pallidosa is very slow in comparison to other species pair or even races of some species.

Engineering recombination between diverged yeast species reveals genetic incompatibilities

The major cause of the sterility of F1 hybrids formed between Saccharomyces cerevisiae and Saccharomyces paradoxus is anti-recombination. The failure of homologous chromosomes from the different species to recombine causes them to mis-segregate, resulting in aneuploid gametes, most of which are inviable. These effects of anti-recombination have previously impeded the search for other forms of incompatibility, such as negative genetic interactions (Bateson-Dobzhoansky-Muller incompatibilities). By suppressing the meiotic expression of MSH2 and SGS1, we could increase recombination and improve hybrid fertility seventy-fold. This allowed us to recover meiotic tetrads in which all four gametes were viable, ensuring that segregation had occurred properly to produce perfectly haploid, not aneuploid, recombinant hybrid gametes. We sequenced the genomes of 84 such tetrads, and discovered that some combinations of alleles from different species were significantly under-represented, indicating that there are incompatible genes contributing to reproductive isolation.