Genetic And Molecular Analysis Of Lethal Hybrid Rescue, A Major-Effect Gene In D. Melanogaster -D. Simulans Hybrid Incompatibility
Post-zygotic hybrid incompatibility (HI) refers to barriers such as sterility and inviability that affect the reproductive success of zygotes from interspecies matings. Although the genetic basis for HI has been studied for decades, very few major-effect genes have been identified. Crosses between Drosophila melanogaster and its sibling species D. simulans produce incompatible hybrids. In the cross of D. melanogaster females to D. simulans males, the F1 daughters are semi-viable but sterile, and the sons are inviable. The D. simulans gene Lethal hybrid rescue (Lhr) was identified as contributing to these incompatibilities, on the basis that loss-of-function mutations suppress hybrid male inviability. I have investigated several properties of Lhr in order to understand how it causes HI. I found that LHR physically interacts with another HI protein, Hybrid male rescue (HMR), as well as heterochromatin proteins HP1 and HP6. Furthermore, LHR directly binds to the chromo-shadow domain of HP1 through a potentially novel HP1-interacting motif. Consistent with binding to HP1, LHR localizes to heterochromatic regions of polytene chromosomes. I have discovered that the pericentric localization of LHR depends on both HP1 and HP5, and that this localization may not be critical for causing hybrid male inviability. It was previously reported that Lhr is evolving rapidly in a manner consistent with positive selection. To understand how the divergence of Lhr affects its functions and interactions, I investigated the properties of other Lhr orthologs. First, I found that the yeast two-hybrid interactions with HMR, HP1 and HP6 are conserved for six additional LHR orthologs, and despite the extensive sequence divergence among LHR orthologs, that the properties of the HP1-interacting domain are also conserved. Second, three LHR orthologs co-localize with HP1 at heterochromatic regions of polytene chromosomes when expressed in D. melanogaster. And finally, I also found that the ability to induce hybrid male lethality, that was previously attributed to only D. simulans Lhr, is conserved for D. melanogaster Lhr. These results demonstrate that despite the extensive sequence divergence among Lhr orthologs, many properties remain conserved. Overall, my work contributes to understanding the function of Lhr in both pure species and in hybrids.
dissertation or thesis