Hybrid sterility and evolution in Hawaiian Drosophila: differential gene and allele-specific expression analysis of backcross males. Drosophila

The Hawaiian Drosophila are an iconic example of sequential colonization, adaptive radiation and speciation on islands. Genetic and phenotypic analysis of closely related species pairs that exhibit incomplete reproductive isolation can provide insights into the mechanisms of speciation. D. silvestris from Hawai’i Island and D. planitibia from Maui are two closely-related allopatric Hawaiian picture-winged Drosophila that produce sterile F1 males but fertile F1 females, a pattern consistent with Haldane’s rule. Backcrossing F1 hybrid females between these two species to parental species gives rise to recombinant males with three distinct fertility patterns despite a similar genomic background: fertile (motile sperm), sterile (no sperm), and effectively sterile (immotile sperm). We found that these three reproductive morphologies of backcross hybrid males produce divergent gene expression profiles in testes, as measured with RNA sequencing. There was a total of 71 genes significantly differentially expressed between backcross males with no sperm compared to those backcross males with motile sperm and immotile sperm, but no significant differential gene expression between backcross males with motile sperm and backcross males with immotile sperm. All of these genes were underexpressed in males with no sperm, including a number of genes with previously known activities in adult testis. An allele-specific expression analysis showed overwhelmingly more cis-divergent than trans-divergent genes, with no significant difference in the ratio of cis- and trans-divergent genes amongst the male fertility groups. Overall, the results indicate that the regulation of gene expression involved in sperm production likely diverged relatively rapidly between these two closely-related species.