Transcriptional Down Regulation of Spermatogenesis Genes in Drosophila Interspecies Hybrids

Abstract

In Drosophila, crossing two closely related species can generate viable but sterile male offspring,typically an outcome of postzygotic reproductive isolation. Hybrids between species of the Drosophila simulans clade show disruption mainly after the meiosis stage of the spermatogenesis pathway, which eventually affects the production of mature sperm. Whole genome investigations (using microarray) of the clade identified that misregulation in sterile hybrids was caused by post-meiotic breakdown. However, either the use of non species-specific genomic platforms or the choice of tissue sampling (whole bodies rather than testes) has made the results of previous investigations prone to ambiguity.Down regulation of spermatogenesis genes specific to interspecies hybrid testes was further supported by lack of differences in gene expression in hybrid ovaries (interspecific), hybrid whole body samples (interspecific) and hybrid testes (intrapsecific) when compared to their respective parental species. However a preliminary protein assay did not suggest a difference in expression between D. mauritiana and interspecies sterile hybrid for bam and sa. These results suggest that misregulation in hybrid sterile males is solely transcriptional and exclusive to testes in interspecies hybrids. The results presented in this thesis do not support down regulation driven by hybrid male sterility. It is possible that transcriptional down regulation of spermatogenesis genes in interspecific hybrids could be the result of rapid divergence experienced by the male genome among the closely related species of Drosophila (i.e. the male sex drive hypothesis). Alternatively, allometric changes due to subtle testes-specific developmental abnormalities in sterile interspecific hybrids, as suggested by morphological analysis, might also contribute to differences in gene expression.