Molecular genetics underlying reproductive isolation of species

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The research in this proposal will use fruit flies of the Drosophila genus to investigate the molecular mechanisms that ensure that interspecific hybrids cannot reproduce. More specifically, we will investigate the hypothesis that Vasa, a rapidly-evolving protein that is expressed in germ cells during oogenesis, and that is required maternally for specification of primordial germ cells in progeny embryos, is an important player in limiting the fertility of interspecific hybrids. Eukaryotic genomes including those of Drosophila contain many copies of transposable elements (TEs), whose mobility has to be suppressed in order to maintain chromosome integrity. Suppression of TE transposition involves small RNAs called Piwi-interacting RNAs (piRNAs) that target TE-derived mRNAs, and subsequently destabilize them and repress their translation. In interspecific Drosophila hybrid females, TEs are derepressed, raising the possibility that this system has a fundamental role in establishing the reproductive isolation of species. If this is so, then it would be expected that the responsible protein or proteins would evolve very rapidly, so that a protein from one species would be sufficiently different to not function in a different species. Vasa, which has recently been implicated in piRNA processing, has regions that diverge extensively, even between closely related Drosophila species such as D. melanogaster and D. simulans. To examine the relationships between Vasa, piRNA mediated processes, and reproductive isolation, we will first identify which domains of Vas are required for piRNA-mediated processes by analyzing expression of TE-encoded genes, and piRNA expression, in females that only express one of a set of site-specific mutated forms of Vasa. Next, we will examine the extent to which Vasa orthologues from other species can function in D. melanogaster, by introducing these genes transgenically into a vasa mutant background. Finally, based on the results we obtain we will replace various domains of Vasa from non-rescuing species with D. melanogaster counterpart domains to attempt to restore function. This will enable us to map particular domains of Vasa that must be derived from the host species (in this case, melanogaster) for functionality. In the longer term extending beyond the scope of the present proposal we will investigate other piRNA-effector genes in a similar way. We will combine mutational analysis, molecular analysis, and protein structure information produced by others to understand particular molecular associations underlying reproductive isolation. For example, we will test whether a particular critical segment of Vasa interacts with a particular partner protein in a species-specific way. The expected impact of this work will include publications in high-quality international scientific journals, and the trainees funded by the grant (as well as the PI) will present their results at national and international conferences. The trainees will also complete PhD thesis projects that will have involved gaining facility in state-of-the-art molecular genetic and imaging techniques, which will enable them to pursue research careers in either the public or private sectors.