Supplementary Material for: <i>Sox100B</i>, a <i>Drosophila</i> Group E Sox-domain Gene, Is Required for Somatic Testis Differentiation

Sex determination mechanisms are thought to evolve rapidly and show little conservation among different animal species. For example, the critical gene on the Y chromosome, <i>SRY</i>, that determines sex in most mammals, is not found in other animals. However, a related Sox domain transcription factor, <i>SOX9</i>, is also required for testis development in mammals and exhibits male-specific gonad expression in other vertebrate species. Previously, we found that the <i>Drosophila</i> orthologue of <i>SOX9</i>, <i>Sox100B</i>, is expressed male-specifically during gonad development. We now investigate the function of <i>Sox100B</i> and find, strikingly, that <i>Sox100B</i> is essential for testis development in <i>Drosophila</i>. In <i>Sox100B</i> mutants, the adult testis is severely reduced and fails to interact with other parts of the reproductive tract, which are themselves unaffected. While a testis initially forms in <i>Sox100B</i> mutants, it fails to undergo proper morphogenesis during pupal stages, likely due to defects in the pigment cells. In contrast, no substantive defects are observed in ovary development in <i>Sox100B</i> mutant females. Thus, as is observed in mammals, a <i>Sox9</i> homolog is essential for sex-specific gonad development in <i>Drosophila</i>, suggesting that the molecular mechanisms regulating sexually dimorphic gonad development may be more conserved than previously suspected.