Shaw, D.; Ross, W. D.; Lambert, A. V.; White, M. A.

Sex chromosomes often evolve unique patterns of gene expression in spermatogenesis. In many species, sex-linked genes are downregulated during meiosis in response to asynapsis of the heterogametic sex chromosome pair (meiotic sex chromosome inactivation; MSCI). Our understanding of this process has been limited to a handful of species, including mammals, Drosophila, and C. elegans. Based on findings from these taxa, MSCI has been viewed as likely a conserved process. However, in other groups like teleost fish, our understanding of this process is limited. Teleost fish are a noteworthy group to investigate because sex chromosomes can rapidly evolve between closely related species. Transcriptional profiling of spermatogenesis at the single-cell level is a useful approach to investigate whether MSCI occurs in other species with independently derived sex chromosomes. Here, we investigate whether MSCI occurs in the threespine stickleback fish (Gasterosteus aculeatus), which have an X and Y chromosome that evolved less than 26 million years ago. Using single-cell RNA-seq, we found that the X and Y chromosomes do not have a signature of MSCI, maintaining gene expression across meiosis. Using immunofluorescence, we also show the threespine stickleback do not form a condensed sex body around the X and Y, a conserved feature of MSCI in many species. We did not see patterns of gene content evolution documented in other species with MSCI. Y-linked ampliconic gene families were expressed across multiple stages of spermatogenesis, rather than being restricted to post-meiotic stages, like in mammals. Our work shows MSCI does not occur in the threespine stickleback fish and has not shaped the evolution of the Y chromosome. In addition, the absence of MSCI in the threespine stickleback suggests this process may not be a conserved feature of teleost fish and argues for additional investigation in other species.