<b>An update on the Genetics, Cellular and Evolutionary implications of X chromosome demasculinization</b>

Sex chromosomes and autosomes evolve in different trajectories depending on gene expression during gametogenesis. The X chromosome gene content across animal taxa shows opposite tendencies in different species implicating the underlying genetic basis of adaptive changes. The constraints imposed on the X chromosome by meiotic sex chromosome inactivation (MSCI) and related spermatogenesis-specific regulatory processes underscore the dynamic interplay between developmental regulation and evolutionary adaptation. The ying–yang functions of MSCI results in distinctive genomic architecture in which the X chromosome is enriched for early spermatogenic genes while the late spermatogenic functions are relocated or duplicated to autosomes (Wei et al., 2024). In the present review using comparative evidence across taxa as a template we explore principles of sex-linked repression and germline expression constrains. Major observations include differential allele expression level of X-linked genes in males compensating for the degeneration of the Y-linked homologs. Faster evolve of ampliconic gene families on Y chromosome. Therefore, even in predominantly diploid organisms, haploid selection may shape sex chromosome evolution and reproductive strategies. This form of selection acting on genes expressed in gametes, where the absence of allele masking enables more effective purging of deleterious mutations and more efficient fixation of beneficial ones. The far ranging implications of these mechanisms include sperm biology, fish conservation genetics and susceptibility to Human Diseases. In summary, chromosome-wide regulatory mechanism act as an evolutionary filter, redistributing essential reproductive functions and reinforcing male-specific transcriptional programs.