Chromosome Painting in Marsupials.

To generate a model of mammalian karyotype evolution and to extend our knowledge of the evolutionary relationships of mammals, it is helpful to deduce ancestral chromosomal arrangements. Of particular interest would be a global comparison among the 3 major extant groups of mammals, since this would reveal the most ancient arrangements of the mammalian genome and allow us to follow chromosomal rearrangements in each lineage. Marsupials diverged from eutherian ("placental") mammals about 130 million yr ago (Figure 1), and monotremes about 170 million yr ago (Graves and Watson 1991). The genomic arrangement in a common ancestor could be deduced by comparing the ancestral genomes of the 3 groups deduced from interspecies and interordinal comparisons. However, it has been difficult to construct an ancestral mammalian chromosomal arrangement because of the wide variety of karyotypes observed in mammals. Neither comparative gene mapping nor chromosome banding studies are adequate for this task. Comparative chromosome banding has provided a powerful tool to establish interspecies chromosomal homologies within some orders (Yunis and Prakash 1982) and even between some orders (Nash and O'Brien 1982), but genomic shuffling within and between other eutherian orders is beyond the resolution of banding methods. Chromosome bands lack the detail necessary to establish homology except over large regions and are reliable only between closely related species. Comparative mapping of many individual genes in several species provides a good test of genetic homology, which has revealed the presence of large chromosomal segments conserved in the vertebrate genome for up to 400 million yr (Wakefield and Graves 1996). However, it is time consuming, and whole chromosomal homology must be inferred from limited single-gene comparisons. A new strategy for analyzing chromosomal evolution and for comparing genomes in mammals is comparative chromo-