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Abstract

Reciprocal chromosome painting and G-banding were used to compare the karyotypes of three Australian marsupials (Sminthopsis crassicaudata, Macropus eugenii, Trichosurus vulpecula) and one South American marsupial (Monodelphis domestica). The results revealed only a limited number of rearrangements between these species and that the four karyotypes can be described as different combinations of fifteen conserved segments. Five chromosomes are totally conserved between M. domestica (pairs 1, 2, 5, 8 and the X) and the presumed 2n = 14 Australian ancestral karyotype, while M. domestica pairs 3 and 6 and 4 and 7 would have been involved in fusion/fission rearrangements. Chromosome comparisons are presented in a chromosome homology map. Although the species studied diverged 70 million years ago, the karyotype of Monodelphis domestica is highly conserved in relation to those of Australian marsupials.

Molecular cytogenetic and genomic insights into chromosomal evolution

“…Recently Westerman et al (2010), using a combination of cytogenetics and sequence-based phylogenetics, have argued that the karyotype of the opossum (n¼9) is highly conserved in relation to those of Australian marsupials confirming previous hypotheses (Rens et al, 2001). Monodelphis domestica groups within the basal Didelphimorphia (Nilsson et al, 2010;Westerman et al, 2010) and is thought to have undergone two fissions from the hypothesized marsupial ancestral karyotype of n¼7 (Rens et al, 2001). If the marsupial ancestral estimate is correct (our small sample size precludes an estimate for Marsupialia given that only one fully sequenced genome is available), a dramatic decrease in chromosome number appears to have occurred in the marsupial lineage (presumably by serial fusion events) since its divergence from the mammalian common ancestor (with n¼23-see mammalian ancestral configuration discussed below) B138 mya (Hallström and Janke, 2010).…”

“…Among marsupials, the South American opossum (Monodelphis domestica) is the only marsupial for which pair-wise alignments with the human genome are possible. Recently Westerman et al (2010), using a combination of cytogenetics and sequence-based phylogenetics, have argued that the karyotype of the opossum (n¼9) is highly conserved in relation to those of Australian marsupials confirming previous hypotheses (Rens et al, 2001). Monodelphis domestica groups within the basal Didelphimorphia (Nilsson et al, 2010;Westerman et al, 2010) and is thought to have undergone two fissions from the hypothesized marsupial ancestral karyotype of n¼7 (Rens et al, 2001).…”

Kangaroo gene mapping and sequencing: insights into mammalian genome evolution

“…This made it possible to compare homologous regions in different species. In collaboration with Willem Rens and Malcolm Ferguson-Smith we compared chromosomes between closely related Australian marsupials, and then between Australian and American marsupials (Rens et al 2001(Rens et al , 2003. Reciprocal painting between species from all the major marsupial families showed that all marsupial karyotypes comprise different arrangements of the same 19 conserved segments.…”

Sex chromosomes and sex determination in weird mammals

“…It is possible to recognize a "basic" 2n = 14 karyotype, with near identity of G-band patterns, in each of the major marsupial groups (Rofe and Hayman, 1985), from which other marsupial karyotypes are easily derived largely by simple fusions and fissions. Chromosome painting has dramatically demonstrated this equivalence (De Leo et al, 1999;Rens et al, 2001). Chromosome painting does not work between marsupial and eutherian autosomes, but comparative gene mapping demonstrates considerable homology (Samollow and Graves, 1998).…”