title: Comparative chromosome painting between two marsupials: origins of an XX/XY1Y2 sex chromosome system.
abstract: Cross-species chromosome painting was used to investigate genome rearrangements between tammar wallaby Macropus eugenii (2n = 16) and the swamp wallaby Wallabia bicolor (2n = 10female symbol/11male symbol), which diverged about 6 million years ago. The swamp wallaby has an XX female:XY1Y2 male sex chromosome system thought to have resulted from a fusion between an autosome and the small original X, not involving the Y. Thus, the small Y1 should represent the original Y and the large Y2 the original autosome. DNA paints were prepared from flow-sorted and microdissected chromosomes from the tammar wallaby. Painting swamp wallaby spreads with each tammar chromosome-specific probe gave extremely strong and clear signals in single-, two-, and three-color FISH. These showed that two tammar wallaby autosomes are represented unchanged in the swamp wallaby, two are represented by different centric fusions, and one by a tandem fusion to make the very long arms of swamp wallaby Chromosome (Chr) 1. The large swamp wallaby X comprises the tammar X as its short arm, and a tandemly fused 7 and 2 as the long arm. The acrocentric swamp wallaby Y2 is a 2/7 fusion, homologous with the long arm of the X. The small swamp wallaby Y1 is confirmed as the original Y by its painting with the tammar Y. However, the presence of sequences shared between the microdissected tammar Xp and Y on the swamp wallaby Y2 implies that the formation of the compound sex chromosomes involved addition of autosome(s) to both the original X and Y. We propose that this involved fusion with an ancient pseudoautosomal region followed by fission proximal to this shared region.
authors.affiliation: School of Genetics and Human Variation, La Trobe University, Kingsbury Drive, Bundoora, Victoria 3083, Melbourne, Australia.
authors.forename: R
authors.initials: R
authors.lastname: Toder
authors.forename: R J
authors.initials: RJ
authors.lastname: O'Neill
authors.forename: J
authors.initials: J
authors.lastname: Wienberg
authors.forename: P C
authors.initials: PC
authors.lastname: O'Brien
authors.forename: L
authors.initials: L
authors.lastname: Voullaire
authors.forename: J A
authors.initials: JA
authors.lastname: Marshall-Graves
mesh_terms: D000818:Animals; D005260:Female; D017404:In Situ Hybridization, Fluorescence / Q000379:methods*; D007621:Karyotyping; D007614:Macropodidae / Q000235:genetics*; D008297:Male; D008394:Marsupialia / Q000235:genetics; D016133:Polymerase Chain Reaction; D013194:Staining and Labeling / Q000379:methods; D014960:X Chromosome; D014998:Y Chromosome