piRNA silencing contributes to interspecies hybrid sterility and reproductive isolation in Drosophila

The piRNA pathway is an adaptive mechanism for maintaining the stability of the genome by repression selfish genomic elements such as transposons. As a host defense mechanism the piRNA pathway is rapidly evolving with different sets of piRNA silencing different targets in closely-related species. In male germline of D.melanogaster, repression of Stellate genes by piRNA generated from Supressor of Stellate, Su(Ste), locus is required for male fertility, but both Su(Ste) piRNA and their targets are absent in other Drosophila species. Here we showed that D.melanogaster genome contain non-coding genomic repeats that have sequence similarity to vasa, protein-coding host gene essential for germline development. In male germline Vasa Related (Varel) repeats produce abundant piRNA that are anti-sense to vasa, however, vasa mRNA escape silencing due to imperfect complementarity between piRNA and vasa sequences. Unexpectedly, we found that Varel piRNA of D.melanogaster target vasa of closely related D. mauritana species in interspecies hybrids. The same hybrids show derepression of harmful Stellate locus due to the absence of Su(Ste) piRNA. Thus, the piRNA pathway contributes to the reproductive isolation between Drosophila species, causing hybrid male sterility via misregulation of two different host factors. Overall design: Examination of piRNA production in small RNA libraries and transcription level of Varel repeats and vasa transcripts in RNA-seq libraries from D.melanogaster, D.mauritiana and interspecies hybrids