Transcriptome Differences between Alternative Sex Determining Genotypes in the House Fly, Musca domestica. Musca domestica

Sex determination evolves rapidly, often because of turnover of the genes at the top of the pathway. The house fly, Musca domestica, has a multifactorial sex determination system, allowing us to identify the selective forces responsible for the evolutionary turnover of sex determination in action. There is a male determining factor, M, on the Y chromosome (Y^M), which is probably the ancestral state. An M factor on the third chromosome (III^M) has reached high frequencies in multiple populations across the world, but the evolutionary forces responsible for the invasion of III^M are not resolved. To test if the III^M chromosome invaded because of sex-specific selection pressures, we used mRNA sequencing to determine if isogenic males that differ only in the presence of the Y^M or III^M chromosome have different gene expression profiles. We find that more genes are differentially expressed between Y^M and III^M males in testis than head, and that genes with male-biased expression are most likely to be differentially expressed between Y^M and III^M males. This suggests that male phenotypes, especially those related to male fertility, are more likely to be affected by the male-determining chromosome, supporting the hypothesis that sex-specific selection acts on alleles linked to the male-determining locus driving evolutionary turnover in the sex determination pathway. We additionally find that III^M males have a "masculinization" gene expression profile, suggesting that the III^M chromosome has accumulated an excess of male-beneficial alleles because of its male-limited transmission. Overall design: 3 replicates of testes and male heads from a Y^M and a III^M strain, and one sample of head and ovary from each of the strains

性别决定机制演化迅速，这通常由通路上游基因的更替所驱动。家蝇（Musca domestica）拥有多因子性别决定系统，为我们研究正在发生的性别决定进化更替的选择压力提供了绝佳模型。其Y染色体上存在雄性决定因子M（Y^M），该类型大概率为祖先状态。第三条染色体上的M因子（III^M）已在全球多个种群中达到较高频率，但驱动III^M入侵的进化力量尚未明确。为验证III^M染色体的入侵是否源于性特异性选择压力，我们采用mRNA测序（mRNA sequencing）技术，检测仅在Y^M或III^M染色体存在上存在差异的同基因背景雄性个体的基因表达谱。研究发现，与头部组织相比，睾丸组织中Y^M与III^M雄性个体间的差异表达基因更多；且雄性偏好表达基因在两者间最易出现表达差异。这表明雄性表型（尤其是与雄性生育力相关的表型）更易受雄性决定染色体的影响，支持了“性特异性选择作用于与雄性决定位点连锁的等位基因，进而推动性别决定通路的进化更替”这一假说。我们还发现，III^M雄性个体呈现“雄性化”的基因表达谱，提示III^M染色体因仅在雄性中传递，已积累了大量雄性有益等位基因。实验设计概况：分别获取Y^M与III^M品系的睾丸组织与雄性头部组织，各设置3个生物学重复；同时采集每个品系的头部与卵巢组织各1份样本。