Evaluating the Relationship between Spermatogenic Silencing of the X Chromosome and Evolution of the Y Chromosome in Chimpanzee and Human

Chimpanzees and humans are genetically very similar, with the striking exception of their Y chromosomes, which have diverged tremendously. The male-specific region (MSY), representing the greater part of the Y chromosome, is inherited from father to son in a clonal fashion, with natural selection acting on the MSY as a unit. Positive selection might involve the performance of the MSY in spermatogenesis. Chimpanzees have a highly polygamous mating behavior, so that sperm competition is thought to provide a strong selective force acting on the Y chromosome in the chimpanzee lineage. In consequence of evolution of the heterologous sex chromosomes in mammals, meiotic sex chromosome inactivation (MSCI) results in a transcriptionally silenced XY body in male meiotic prophase, and subsequently also in postmeiotic repression of the sex chromosomes in haploid spermatids. This has evolved to a situation where MSCI has become a prerequisite for spermatogenesis. Here, by analysis of microarray testicular expression data representing a small number of male chimpanzees and men, we obtained information indicating that meiotic and postmeiotic X chromosome silencing might be more effective in chimpanzee than in human spermatogenesis. From this, we suggest that the remarkable reorganization of the chimpanzee Y chromosome, compared to the human Y chromosome, might have an impact on its meiotic interactions with the X chromosome and thereby on X chromosome silencing in spermatogenesis. Further studies will be required to address comparative functional aspects of MSCI in chimpanzee, human, and other placental mammals

Flickin' the ubiquitin switch: The role of H2B ubiquitylation in development

The reversible ubiquitylation of histone H2B has long been implicated in transcriptional activation and gene silencing. However, many questions regarding its regulation and effects on chromatin structure remain unanswered. In addition, while several studies have uncovered an involvement of this modification in the control of certain developmental processes, a more general understanding of its requirement is lacking. Herein, we present a broad overview of the pathways known to be regulated by H2B ubiquitylation, while drawing parallels between findings in disparate organisms, in order to facilitate continued delineation of its spatiotemporal role in development. Finally, we integrate the findings of recent studies into how H2B ubiquitylation affects chromatin, and cast an eye over emerging areas for future research