title: In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes.
abstract: Two centuries after the duck-billed platypus was discovered, monotreme chromosome systems remain deeply puzzling. Karyotypes of males, or of both sexes, were claimed to contain several unpaired chromosomes (including the X chromosome) that form a multi-chromosomal chain at meiosis. Such meiotic chains exist in plants and insects but are rare in vertebrates. How the platypus chromosome system works to determine sex and produce balanced gametes has been controversial for decades. Here we demonstrate that platypus have five male-specific chromosomes (Y chromosomes) and five chromosomes present in one copy in males and two copies in females (X chromosomes). These ten chromosomes form a multivalent chain at male meiosis, adopting an alternating pattern to segregate into XXXXX-bearing and YYYYY-bearing sperm. Which, if any, of these sex chromosomes bears one or more sex-determining genes remains unknown. The largest X chromosome, with homology to the human X chromosome, lies at one end of the chain, and a chromosome with homology to the bird Z chromosome lies near the other end. This suggests an evolutionary link between mammal and bird sex chromosome systems, which were previously thought to have evolved independently.
authors.affiliation: Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, Australian Capital Territory 2601, Australia. frank.gruetzner@anu.edu.au
authors.forename: Frank
authors.initials: F
authors.lastname: Grützner
authors.forename: Willem
authors.initials: W
authors.lastname: Rens
authors.forename: Enkhjargal
authors.initials: E
authors.lastname: Tsend-Ayush
authors.forename: Nisrine
authors.initials: N
authors.lastname: El-Mogharbel
authors.forename: Patricia C M
authors.initials: PC
authors.lastname: O'Brien
authors.forename: Russell C
authors.initials: RC
authors.lastname: Jones
authors.forename: Malcolm A
authors.initials: MA
authors.lastname: Ferguson-Smith
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Marshall Graves
mesh_terms: D000818:Animals; D001717:Birds / Q000235:genetics*; D020223:Chromosome Painting; D019143:Evolution, Molecular; D005260:Female; D008297:Male; D008322:Mammals / Q000235:genetics*; D008540:Meiosis / Q000235:genetics*; D008677:Metaphase; D008969:Molecular Sequence Data; D010987:Platypus / Q000235:genetics*; D012730:Sex Chromosomes / Q000235:genetics*; D019849:Sex Determination Processes; D013094:Spermatozoa / Q000166:cytology / Q000378:metabolism; D016615:Telomere; D014960:X Chromosome / Q000235:genetics; D014998:Y Chromosome / Q000235:genetics