title: Sex chromosome silencing in the marsupial male germ line.
abstract: In marsupials, dosage compensation involves silencing of the father's X-chromosome. Because no XIST orthologue has been found, how imprinted X-inactivation occurs is unknown. In eutherians, the X is subject to meiotic sex chromosome inactivation (MSCI) in the paternal germ line and persists thereafter as postmeiotic sex chromatin (PMSC). One hypothesis proposes that the paternal X is inherited by the eutherian zygote as a preinactive X and raises the possibility of a similar process in the marsupial germ line. Here we demonstrate that MSCI and PMSC occur in the opossum. Surprisingly, silencing occurs before X-Y association. After MSCI, the X and Y fuse through a dense plate without obvious synapsis. Significantly, sex chromosome silencing continues after meiosis, with the opossum PMSC sharing features of eutherian PMSC. These results reveal a common gametogenic program in two diverse clades of mammals and support the idea that male germ-line silencing may have provided an ancestral form of mammalian dosage compensation.
authors.affiliation: Howard Hughes Medical Institute and Department of Molecular Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
authors.forename: Satoshi H
authors.initials: SH
authors.lastname: Namekawa
authors.forename: John L
authors.initials: JL
authors.lastname: VandeBerg
authors.forename: John R
authors.initials: JR
authors.lastname: McCarrey
authors.forename: Jeannie T
authors.initials: JT
authors.lastname: Lee
mesh_terms: D000818:Animals; D017404:In Situ Hybridization, Fluorescence; D008297:Male; D008540:Meiosis / Q000502:physiology*; D008856:Microscopy, Fluorescence; D009893:Opossums / Q000235:genetics / Q000502:physiology*; D012671:Seminiferous Tubules / Q000166:cytology; D012728:Sex Chromatin / Q000235:genetics / Q000502:physiology*; D049951:X Chromosome Inactivation / Q000235:genetics / Q000502:physiology*