Abstract
BackgroundSex chromosomes of extant eutherian species are too ancient to reveal the process that initiated sex-chromosome differentiation. By contrast, the neo-sex chromosomes generated by sex-autosome fusions of recent origin in Tokudaia muenninki are expected to be evolutionarily ‘young’, and therefore provide a good model in which to elucidate the early phases of eutherian sex chromosome evolution. Here we describe the genomic evolution of T. muenninki in neo-sex chromosome differentiation.ResultsFISH mapping of a T. muenninki male, using 50 BAC clones as probes, revealed no chromosomal rearrangements between the neo-sex chromosomes. Substitution-direction analysis disclosed that sequence evolution toward GC-richness, which positively correlates with recombination activity, occurred in the peritelomeric regions, but not middle regions of the neo-sex chromosomes. In contrast, the sequence evolution toward AT-richness was observed in those pericentromeric regions. Furthermore, we showed genetic differentiation between the pericentromeric regions as well as an accelerated rate of evolution in the neo-Y region through the detection of male-specific substitutions by gene sequencing in multiple males and females, and each neo-sex–derived BAC sequencing.ConclusionsOur results suggest that recombination has been suppressed in the pericentromeric region of neo-sex chromosomes without chromosome rearrangement, whereas high levels of recombination activity is limited in the peritelomeric region of almost undifferentiated neo-sex chromosomes. We conclude that PAR might have been formed on the peritelomeric region of sex chromosomes as an independent event from spread of recombination suppression during the early stages of sex chromosome differentiation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0514-y) contains supplementary material, which is available to authorized users.
Highlights
Sex chromosomes of extant eutherian species are too ancient to reveal the process that initiated sex-chromosome differentiation
We show that recombination have been suppressed in the pericentromeric region while recombination have been activated in the peritelomeric region of neo-sex chromosomes, indicating pseudoautosomal region (PAR) might have evolved during the early stage of sex chromosome differentiation
bacterial artificial chromosome (BAC) clones used for fluorescence in situ hybridization (FISH) were screened based on genome information from mouse and rat, using as a reference the neo-sex–linked genes we identified in a previous study [16]
Summary
Sex chromosomes of extant eutherian species are too ancient to reveal the process that initiated sex-chromosome differentiation. The neo-sex chromosomes generated by sex-autosome fusions of recent origin in Tokudaia muenninki are expected to be evolutionarily ‘young’, and provide a good model in which to elucidate the early phases of eutherian sex chromosome evolution. It is needed to understand the both processes of recombination suppression and PAR formation during early sex chromosome differentiation for revealing the eutherianspecific feature in Y degeneration. It remains unknown when and how PAR is formed in the sex chromosomes. The neo-sex chromosomes generated by sex-autosome fusions of recent origin are expected to be evolutionarily young Such neo-sex chromosomes provide a good model in which to elucidate the early phases of eutherian-specific sex chromosome evolution
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