Unsolved mysteries about the evolution of sex chromosome regulation

Abstract Sex chromosomes have evolved repeatedly across eukaryotes and are frequently associated with chromosome-wide regulatory mechanisms that modulate gene expression in a sex-, tissue-, and stage-specific manner. In somatic tissues, dosage compensation is often invoked to counteract gene dose imbalances arising from Y or W chromosome degeneration, whereas in the germline, sex chromosomes are commonly downregulated or silenced during gametogenesis. These processes have traditionally been studied in isolation and in just a handful of model organisms. However, recent comparative work across diverse taxa reveals substantial variation in both their extent and underlying mechanisms, and has challenged canonical views derived from model systems. Here, we synthesize current knowledge of sex chromosome regulation across animals and highlight key unresolved questions. First, it remains unclear which expression imbalances dosage compensation actually corrects, and why its outcomes vary from gene-specific to chromosome-wide and from partial to complete compensation. Second, the evolutionary drivers and functional consequences of germline downregulation, including meiotic sex chromosome inactivation and alternative mechanisms, are still poorly understood despite their widespread occurrence. Third, the origins of chromosome-scale regulatory systems remain elusive, with emerging evidence suggesting repeated co-option or rewiring of preexisting regulatory pathways. We further consider whether somatic and germline regulatory systems are mechanistically and evolutionarily linked and how existing regulatory architectures constrain or facilitate sex chromosome turnover and gene content evolution. Finally, we address the largely unexplored fate of regulatory systems after they become redundant following sex chromosome transitions. Resolving these questions will require the combination of comparative genomics, functional assays, and cytological analyses across a broad diversity of nonmodel organisms. Such efforts are essential for establishing general principles governing the evolution of chromosome-wide regulatory systems and their role in shaping genome evolution.