COMPARATIVE CHROMOSOME PAINTING BETWEEN 2 MARSUPIALS - ORIGINS OF AN XX XY1Y2 SEX-CHROMOSOME SYSTEM/

Cross-species chromosome painting was used to investigate genome rearr angements between tammar wallaby Macropus eugenii (2n = 16) and the sw amp wallaby Wallabia bicolor(2n = 10 female/11 male), which diverged a bout 6 million years ago. The swamp wallaby has an XX female:XY1Y2 mal e 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 Y-1 should represent the original Y and the large Y-2 the origin al autosome. DNA paints were prepared from flow-sorted and microdissec ted chromosomes from the tammar wallaby. Painting swamp wallaby spread s with each tammar chromosome-specific probe gave extremely strong and clear signals in single-, two-, and three-color FISH. These showed th at two tammar wallaby autosomes are represented unchanged in the swamp wallaby, two an 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 shor t arm, and a tandemly fused 7 and 2 as the long arm. The acrocentric s wamp wallaby Y-2 is a 2/7 fusion, homologous with the long arm of the X. The small swamp wallaby Y-1 is confirmed as the original Y by its p ainting with the tammar Y. However, the presence of sequences shared b etween the microdissected tammar Xp and Y on the swamp wallaby Y-2 imp lies that the formation of the compound sex chromosomes involved addit ion of autosome(s) to both the original X and Y. We propose that this involved fusion with an ancient pseudoautosomal region followed by fis sion proximal to this shared region.