title: Lack of sex chromosome specific meiotic silencing in platypus reveals origin of MSCI in therian mammals.
abstract: BACKGROUND
  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. 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.
  
  
  RESULTS
  Our 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.
  
  
  CONCLUSIONS
  Our 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.
authors.affiliation: The Robinson Research Institute, School of Biological Sciences, University of Adelaide, Adelaide, 5005, South Australia, Australia. tasman.daish@adelaide.edu.au.
authors.forename: Tasman J
authors.initials: TJ
authors.lastname: Daish
authors.affiliation: The Robinson Research Institute, School of Biological Sciences, University of Adelaide, Adelaide, 5005, South Australia, Australia. aaron.casey@adelaide.edu.au.
authors.forename: Aaron E
authors.initials: AE
authors.lastname: Casey
authors.affiliation: The Robinson Research Institute, School of Biological Sciences, University of Adelaide, Adelaide, 5005, South Australia, Australia. frank.grutzner@adelaide.edu.au.
authors.forename: Frank
authors.initials: F
authors.lastname: Grutzner
mesh_terms: D000818:Animals; D044127:Epigenesis, Genetic; D019143:Evolution, Molecular; D020869:Gene Expression Profiling; D020868:Gene Silencing; D007150:Immunohistochemistry; D017404:In Situ Hybridization, Fluorescence; D008297:Male; D008540:Meiosis; D049469:Meiotic Prophase I; D010987:Platypus / Q000235:genetics*; D012730:Sex Chromosomes / Q000235:genetics*