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Abstract

In order to deduce the ancestral genome arrangement in the karyotypically diverse marsupial family Macropodidae, and to assess chromosome change in this family, chromosome-specific paints from the tammar wallaby (2n = 16) were hybridized to metaphase spreads from the two species proposed to represent the 2n = 22 ancestral karyotype, as well as species with derived 2n = 20 and 2n = 14 karyotypes. Identical patterns were observed in the two 2n = 22 species, from which the rearrangements to form the three derived karyotypes may be easily deduced to be 1, 3 and 4 different fusions, respectively. The identical Thylogale and Dorcopsis genomes may both be used to represent the pleisiomorphic macropodid chromosome complement. Variation in the X chromosome was also investigated by hybridizing an X-Y shared tammar wallaby 12-kb repeat element to chromosomes from the other four macropodid species, finding that it hybridized only to the most closely related species, and therefore is of recent origin.

Genome sequence of an Australian kangaroo, Macropus eugenii, provides insight into the evolution of mammalian reproduction and development

“…Thus, the basic marsupial X shares homology with the long arm and pericentric region of the human X [35,36]. The tammar Y shares only five genes with the degraded eutherian Y [86] (Figure 5).…”

“…Comparative studies show that the marsupial X and Y are representative of the ancestral mammalian X and Y chromosomes [35]. However, in the kangaroos, a large heterochromatic nucleolus organizer region became fused to the X and Y. Chromosome painting confirms the extreme conservation of kangaroo chromosomes [36] and their close relationship with karyotypes of more distantly related marsupials [37-40] so that genome studies are likely to be highly transferable across marsupial species.…”

Chromosome Evolution in Marsupials

“…Diploid numbers vary among macropods from the most derived and lowest diploid number karyotype for a marsupial, consisting of 2 n = 10 F /11 for the swamp wallaby ( Wallabia bicolor ) to 2 n = 24 for the banded hare wallaby ( Lagostrophus fasciatus ) [ 74 ]. Techniques such as G-banding [ 72 , 76 , 77 , 78 , 79 , 80 , 81 , 82 ], chromosome painting [ 39 , 83 , 84 ], and FISH with a telomeric probe used to detect chromosome fusions [ 85 , 86 , 87 , 88 , 89 ] have all helped to decipher the genomic reshuffling events among macropods, including a large number of interchromosomal rearrangements. For example, using probes for each tammar wallaby chromosome and FISH with a telomeric probe demonstrated that swamp wallaby chromosome 1 was formed by the fusion of ancestral macropod (AnMac) chromosomes 6, 10, and 4 and an inversion; chromosomes 2 and 3 were the result of a Robertsonian fusion [ 39 , 86 ].…”

Marsupial Genome Sequences: Providing Insight into Evolution and Disease

“…The alternative hypothesis proposed that the higher diploid number of 2 n = 22 was ancestral, with lower diploid numbers being the result of fusion events [180, 184]. This hypothesis was originally proposed based on its prevalence amongst marsupials [180], but chromosome painting showed that 2 n = 22 karyotypes present in different species were not equivalent [50, 185]. Nevertheless, this did not eliminate a 2 n = 22 ancestral karyotype that was subject to several fusion events very early in marsupial evolution to result in the common 2 n = 14 karyotype.…”