Role of the nuclear pore in piRNA biogenesis and speciation. Swapnil Parhad¹, Jie Wang², Zhiping Weng², William Theurkauf¹. 1) Program in Molecular Medicine,Univ Massachusetts Med Sch, Worcester, MA; 2) Program in Bioinformatics and Integrative Biology, Univ Massachusetts Med Sch, Worcester, MA.

   Crosses between recently diverged species, which often carry distinct transposon families, can lead to hybrid lethality or sterility. Drosophila nucleoporin 160kDa (Nup160) has been implicated in hybrid incompatibility between Drosophila melanogaster and Drosophila simulans. The gene appears to be evolving rapidly under positive selection, which is often linked to a host-pathogen interaction. Piwi-interacting RNAs (piRNAs) repress transposons in the germline, several piRNA pathway genes appear to be evolving under positive selection, piRNA mutations lead to sterility and developmental arrest during early embryogenesis, and the piRNA machinery appears to be organized around nuclear pores. These observations led us to speculate that Nup160 and the transposon silencing machinery may be co-evolving in response to transposon invasion. To test this hypothesis, we have used transgenes expressing either D. melanogaster Nup160 (DmNup160) or D. simulans Nup160 (DsNup160) to rescue null mutants in the D. melanogaster Nup160 locus. We find that ubiquitous expression of DmNup160 rescues the lethal and sterile phenotypes associated with Nup160 null mutants. By contrast, expression of DsNup160 restores viability, but not fertility, and the resulting females produce eggs that fail to hatch due to mitotic division failure and developmental arrest during early embryogenesis. DsNup160 can therefore support housekeeping roles for the nuclear pore during D. melanogaster development, but is defective in a germline specific function. Small RNA sequencing shows that expression of DsNup160 leads to a global suppression of piRNA ping-pong amplification and significantly reduces expression of major chromosome 4 cluster, which produces piRNAs to telomeric transposons. These observations suggest that co-evolution of piRNA genes and Nup160 generates species specific interactions between nuclear pores and transposon silencing machinery that may contribute to hybrid incompatibility.