Marsupial chromosome DNA content and genome size assessed from flow karyotypes: invariable low autosomal GC content

Abstract

Extensive chromosome homologies revealed by cross-species chromosome painting between marsupials have suggested a high level of genome conservation during evolution. Surprisingly, it has been reported that marsupial genome sizes vary by more than 1.2 Gb between species. We have shown previously that individual chromosome sizes and GC content can be measured in flow karyotypes, and have applied this method to compare four marsupial species. Chromosome sizes and GC content were calculated for the grey short-tailed opossum (2n = 18), tammar wallaby (2n = 16), Tasmanian devil (2n = 14) and fat-tailed dunnart (2n = 14), resulting in genome sizes of 3.41, 3.31, 3.17 and 3.25 Gb, respectively. The findings under the same conditions allow a comparison between the four species, indicating that the genomes of these four species are 1-8% larger than human. We show that marsupial genomes are characterized by a low GC content invariable between autosomes and distinct from the higher GC content of the marsupial × chromosome.

Keywords: chromosome profile; mammalian evolution; marsupial genome.

Figure 1.

Chromosome profiles based on flow karyotype measurements showing the relationship between chromosome size and GC content in the grey short-tailed opossum (2n = 18) (a), tammar wallaby (2n = 16) (b), Tasmanian devil (2n = 14) (c) and fat-tailed dunnart (2n = 14) (d) compared with human (2n = 46) (e) at the same scale. X chromosomes are indicated by X in each profile. Marsupial autosomes are variable in size, but have an invariable GC content. By contrast, marsupial X chromosomes have the highest GC content in each genome.

Figure 2.

Comparison of total genome size and GC content between four marsupials and human. Marsupial genome sizes are larger than those of human in excess of 10%. Low GC content in marsupials can be distinguished from the comparatively high GC content in human.