The conserved marsupial karyotype: Chromosome painting and evolution

Abstract

Marsupials are an ancient group of mammals that fill diverse niches in Australia, New Guinea, the East Indies and America. These species separated from eutherians around 150 million years ago and the American species diverged from the Australian species around 70 million years ago. Compared to other mammalian species, the karyotypes of marsupials are highly conserved; their diploid numbers range from 2n = 10-32 but with a predominance of 2n = 14 or 2n = 22. The first chromosome comparative studies were performed mainly by searching for similarities in G-banding patterns leading to hypotheses on the ancestral marsupial karyotype and the chromosome rearrangement mechanisms that resulted in the karyotypes seen in the extant species. The advent of chromosome painting allowed chromosome comparisons to be based on chromosome-wide sequence similarities, which is a more accurate method than the indirect method of banding analysis. This chapter is divided into six sections. The first section describes early marsupial karyotype studies performed by G-banding and introduces hypotheses on marsupial chromosome evolution. The second explains chromosome painting techniques including flow karyotyping and flow sorting, and presents results in the form of chromosome paint images and chromosome homology maps. The third section describes marsupial chromosome evolution in terms of phylogeny, ancestral karyotypes, chromosome conserved regions, and mechanisms of chromosome rearrangements. The fourth section explains the role of centromere dynamics in marsupial chromosome evolution. The fifth section focuses on recent work on the sequenced genome of the opossum. This section is followed by concluding remarks.