Meiotic Sex Chromosome Inactivation: Conservation across the Drosophila genus

The inherent differences between sex chromosomes in males and females create conflicts in gene expression, driving the evolution of regulatory mechanisms such as Meiotic Sex Chromosome Inactivation (MSCI), a process that transcriptionally silences the sex chromosomes during male meiosis. In this study, we explore the evolutionary dynamics of MSCI within the Drosophila genus by analyzing transcriptomes across different stages of spermatogenesis in D. melanogaster and its progressively more distant relatives, D. simulans, D. willistoni, and D. mojavensis. Stage-enriched bulk RNA sequencing, showing a strong correlation in spermatogenic gene expression patterns among these species, revealed that MSCI dates back to the early evolution of the Drosophila genus, impacting the regulation of both coding and long non-coding RNAs. Notably, for newly evolved genes, X-linked genes show higher expression levels than autosomal genes during mitosis and meiosis, indicating that MSCI predominantly regulates older genes. In contrast, newly evolved autosomal genes exhibit a gradual increase in expression throughout spermatogenesis, reaching their peak in the post-meiotic phase. During this phase, the expression of X-linked new genes decreases, eventually aligning with that of autosomal genes. This expression pattern suggests that haploid selection plays a crucial role in the regulation of new genes, with monoallelic expression of the X chromosome providing an advantage across all stages of germline development, while autosomal gene expression gains a selective edge primarily in the post-meiotic phase. Together, these findings provide new insights into the evolution of sex chromosomes and highlight the critical role of MSCI in shaping gene expression profiles in Drosophila.