Abstract
BackgroundIn therian mammals heteromorphic sex chromosomes are subject to meiotic sex chromosome inactivation (MSCI) during meiotic prophase I while the autosomes maintain transcriptional activity. The evolution of this sex chromosome silencing is thought to result in retroposition of genes required in spermatogenesis from the sex chromosomes to autosomes. In birds sex chromosome specific silencing appears to be absent and global transcriptional reductions occur through pachytene and sex chromosome-derived autosomal retrogenes are lacking. Egg laying monotremes are the most basal mammalian lineage, feature a complex and highly differentiated XY sex chromosome system with homology to the avian sex chromosomes, and also lack autosomal retrogenes. In order to delineate the point of origin of sex chromosome specific silencing in mammals we investigated whether MSCI exists in platypus.ResultsOur results show that platypus sex chromosomes display only partial or transient colocalisation with a repressive histone variant linked to therian sex chromosome silencing and surprisingly lack a hallmark MSCI epigenetic signature present in other mammals. Remarkably, platypus instead feature an avian like period of general low level transcription through prophase I with the sex chromosomes and the future mammalian X maintaining association with a nucleolus-like structure.ConclusionsOur work demonstrates for the first time that in mammals meiotic silencing of sex chromosomes evolved after the divergence of monotremes presumably as a result of the differentiation of the therian XY sex chromosomes. We provide a novel evolutionary scenario on how the future therian X chromosome commenced the trajectory toward MSCI.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0215-4) contains supplementary material, which is available to authorized users.
Highlights
In therian mammals heteromorphic sex chromosomes are subject to meiotic sex chromosome inactivation (MSCI) during meiotic prophase I while the autosomes maintain transcriptional activity
All DNA fluorescence in situ hybridisation (FISH) probes either targeting pseudoautosomal region (PAR) or sex chromosomes co-localised with the DAPI intense mass indicating its primary composition is sex chromatin
All SYCP1 positive nuclei with staining patterns characteristic of advanced synapsis showed a consistent and obvious presence of a singular large circular DAPI void we describe as the nucleolar like body (NLB) (Fig. 2a)
Summary
In therian mammals heteromorphic sex chromosomes are subject to meiotic sex chromosome inactivation (MSCI) during meiotic prophase I while the autosomes maintain transcriptional activity The evolution of this sex chromosome silencing is thought to result in retroposition of genes required in spermatogenesis from the sex chromosomes to autosomes. The hallmark feature of meiotic pachytene cells in male mammals is the transcriptional silencing of genes residing on unpaired sex chromosome DNA, termed Meiotic Sex Chromosome Inactivation (MSCI) [3]. This response silences unpaired sequences by utilising DNA damage repair (DDR) pathway components and recruiting chromatin remodelling factors [4, 5]. Therian MSCI is postulated to prevent the exchange of genetic material between heterologous sequences or enable checkpoint avoidance [8, 9]
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