The mitotic chromosomes of Marsupials and their bearing on taxonomy and phylogeny.

Abstract

Chromosome numbers of marsupials vary between 2n = 11 B 10 @ and 2n =24. Most species have 14 or 22 chromosomes. There is no evidence that polyploidy has occurred in marsupial evolution. The Dasyuridae have 12 metacentric autosomes, a small metacentric X-chromosome and a very small Y-chromosome (20% of living species have been studied) and the chromosomes of Myrmecobius fasciatus are typically like those of other Dasyuridae. The Peramelidae (30% of species have been studied) have chromosomes like the Dasyuridae except that X- and Y-chromosomes are much larger. The occurrence of similar chromosome numbers in Dasyuridae and Peramelidae is not necessarily evidence of affinity. The chromosomes of the Phascolomidae are similar in number and morphology to those of the Peramelidae and the resemblances are, almost certainly, due to parallel evolution. The chromosomes of Phascolarctos are unlike those of any of the Phalangeridae and this genus might be just as easily grouped with the Phascolomidae. The Phalangeridae have considerable chromosomal heterogeneity but less than 20% of species have been studied. Two species of Cercaertus have 12 metacentric autosomes and small sex chromosomes like all members of the Dasyuridae. This suggests that the primitive phalangers may have retained the chromosome number and morphology of possible dasyurid ancestors but the resemblances may be due to parallel evolution of similar chromosome number and morphology in separate groups. The chromosomes have been studied in more than 50% of Macropodinae. Cytological evidence suggests that Thylogale (3 species studied), Petrogale (2 species studied), and probably Lagorchestes (1 species studied), all with 22 chromosomes, are a related group. Onychogalea unguifer, with 20 chromosomes, may be derived from this group. There is no justification for the placing of Thylogale billardierii in the genus Protemnodon. Lagostrophus fasciatus has 2n = 24 and its placement in a monotypic genus is justified. Macropus major and all species of Protentnodon, except P. bicolor, are a related group with 16 chromosomes. M. robustus is possibly included in this group. M. rufus has 20 chromosomes and should perhaps be placed in the separate genus Megaleia. P. bicolor, with 11 chromosomes in the male and 10 in the female, differs from all other species of Protemnodon and this genus, as at present constituted, may be diphyletic. The relationships of P. bicolor are unknown. Setonix brachyurus has 22, mostly metacentric, chromosomes and its affinities are at present unknown. Three species of Bettongia (Potoroinae) have 22 chromosomes which are mostly metacentric. Hypsiprymnodon moschatus has 22 chromosomes which are mostly acrocentric. Both genera are very different cytologically from Potorous tridactylus.