Marsupial radiations revealed

Living metatherians exist in two strongholds: the Neotropics (Mexico, Central America, and South America) and the Australasian region (Australia, Tasmania, New Guinea, and nearby islands). One hypothesis to explain this odd distribution proposes that metatherians arose in North America, invaded South America in the Cretaceous, move on to Antarctica and Australia in the late Cretaceous or early Tertiary.

In addition to the debate over marsupial origins, mammalogists have long debated the phylogenetic relationships among the various orders of Australasian and Neotropical marsupials. For example, the tiny “monito del monte” (Dromiciops gliroides) survives in South America, but experts place it in its own order Microbiotheria, allied with the Australasian marsupials. The close relationship between the South American order Microbiotheria and the four remaining Australasian orders suggests a complex pattern of early dispersal among the continents.

Shedding some light on these events is an analysis of the recently sequenced genomes of the South American opossum (Monodelphis domestica) and the Australian tammar wallaby (Macropus eugenii). Using retroposon insertion patterns, Nilsson and colleagues (2010) resolved the early branching patterns among the marsupial orders. Retroposons are fragments of DNA that have been reverse transcribed by RNA and inserted randomly into the genome. They are informative because the insertion sites are random and duplicate insertions are very, very unlikely. Thus, organisms that share retroposed elements at identical locations in their genomes are likely to share this pattern via common descent (e.g. are closely related).


Figure 1. Phylogenetic tree of marsupials based on retroposon insertion patterns (From Nilsson et al., 2010)

Now that the genomes are available for two marsupials, retroposon insertion patterns provide a unique opportunity to resolve marsupial relationships. Nilsson and coworkers confirmed the monophyly of marsupials by finding 10 markers in the metatherian genome that accumulated after their split from placental mammals (approximately 130 million years ago). Their analysis also confirms the sister group relationship between Microbiotheria and the remaining Australasian marsupial orders. Furthermore, the South American marsupials are ancestral to the Australasian marsupials, suggesting a single migration from South America to Australia. The authors propose the name Euaustralidelphia (aka, “true Australidelphia”) for the four exclusively Australasina groups: orders Notoryctemorphia,
Dasyuromorphia, Peramelemorphia, and Diprotodontia.

References

Nilsson M.A., Churakov G., Sommer M., Tran N.V., Zemann A., Brosius, J., and J. Schmitz. 2010. Tracking marsupial evolution using archaic genomic retroposon insertions. PLoS Biology, 8(7): e1000436. doi:10.1371/journal.pbio.1000436.