Abstract
Marsupials typically possess very large, distinctive chromosomes that make them excellent subjects for cytogenetic analysis, and the high level of conservation makes it relatively easy to track chromosome evolution. There are two speciose marsupial families with contrasting rates of karyotypic evolution that could provide insight into the mechanisms driving genome reshuffling and speciation. The family Dasyuridae displays exceptional karyotype conservation with all karyotyped species possessing a 2n = 14 karyotype similar to that predicted for the ancestral marsupial. In contrast, the family Macropodidae has experienced a higher rate of genomic rearrangement and one genus of macropods, the rock-wallabies (Petrogale), has experienced extensive reshuffling. For at least some recently diverged Petrogale species, there is still gene flow despite hybrid fertility issues, making this species group an exceptional model for studying speciation. This review highlights the unique chromosome features of marsupial chromosomes, particularly for these two contrasting families, and the value that a combined cytogenetics, genomics, and epigenomics approach will have for testing models of genome evolution and speciation.
Highlights
Two observations led to the idea that chromosome rearrangements may play a role in speciation; chromosome numbers can vary greatly, even between closely related species, and hybrids resulting from crosses of individuals with different chromosome arrangements often experience reduced fertility [1,2]
The opossum, devil, and wallaby genomes have cytogenetic mapping data to anchor sequence to chromosomes [14,21], which is essential if these genomes are to be useful for studying marsupial chromosome evolution
The integrative breakage model [4] would predict these regions to have an open chromatin conformation so assessing the epigenetic landscape and transcription output of Petrogale chromosomes will be a crucial step towards testing this model and determining the features that may be making ancestral macropod (AnMac) chromosomes 5, 6, and 10 susceptible to rearrangement. Marsupials, with their distinctively large chromosomes, provide some excellent opportunities to test models of genome evolution and speciation but to do this there needs to be a closer union between cytogenetics, genomics, and epigenomics
Summary
Two observations led to the idea that chromosome rearrangements may play a role in speciation; chromosome numbers can vary greatly, even between closely related species, and hybrids resulting from crosses of individuals with different chromosome arrangements often experience reduced fertility [1,2]. Dasyuridae, consisting of over 70 species, has remarkable chromosome conservation with all and 1980s [19] These studies revealed some interesting observations that make marsupials good models for investigating the mechanisms behind chromosome evolution. Comparisons of the arrangement of these segments between species using chromosome painting make it clear that some chromosomes or chromosomal segments have been prone to rearrangement during marsupial evolution [20]. When a broader comparison across marsupials is made, it becomes clear that some of these segments are commonly involved in rearrangements, even between species within the family Dasyuridae with its highly conserved karyotype
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