I suspect that many people, asked where you’d find marsupials, would answer ‘Australia’ & leave it at that. But while we may be most familiar with Skippy & his ilk, those pouched mammals across the Tasman have close cousins half a world away – in South America.
In fact, there are 3 orders (a grouping that sits above species, genus, & family in the taxonomic hierarchy) of South American marsupials and 4 from Australiasia. While marsupials as a whole last shared a common ancestor with our own lineage, the placental mammals, about 130 million years ago, the relationships between the various marsupial groups aren’t all that clear, although it’s generally accepted that the group originated in South America. Of the American taxa the most familiar is probably the large opossum species that’s now found in North America, having moved north following the formation of the Panamanian isthmus.. The most unusual is the ‘little mountain monkey’, Dromicops gliroides, found only in southwestern South America. In a taxonomic reshuffle, Dromiciops was placed in a grouping with the Australian marsupials (on the basis of details of the ankle joint, among other things), but this has generated quite a bit of discussion as it could suggest a ‘back migration’ from Australia back to the New World. At the very least, the marsupial family tree is complex & hasn’t proved particularly easy to resolve.
In a new paper in PLoS Biology Maria Nilsson & her colleagues describe how they used retroposon insertions to clarify the marsupials’ complex history. These DNA sequences had already been used to sort out relationships and divergences among the placental mammals, & so Nilsson & her team were hopeful that this technique could be applied to the marsupials as well. Retroposons are bits of DNA that have been reverse-transcribed from RNA molecules & randomly inserted into chromosomes. This randomness is important – if two or more taxa share a particular retroposon, in the same chromosomal location, scientists can be fairly sure that this genuinely indicates relatedness & is not the result of convergent evolution (Nilsson et al, 2010). And if that retroposon is missing in a third species, then it’s reasonable to assume that species #3 diverged from the others before they acquired this particular DNA sequence. It’s also possible to get an idea of just when various species diverged from each other, because "young retroposed elements can insert into older elements, but older, inactive elements are not capable of inserting into younger ones" (Nilsson et al. 2010).
First, though, the team needed to identfy retroposon sequences to use in their analysis. This was made possible by the fact that the genomes of the South American opossum (Monodelphis domestica) and the tammar wallaby (Macropus eugenii) were recently sequenced. Nilsson & her colleagues screened these 2 genomic sequences and identified 3 groups of retroposon insertions that could then be used to search for phylogentically-useful genetic markers in the various opossums and the Australasian marsupials. They also looked for markers that might indicate that marsupials are ‘monophyletic’ ie that there is a single evolutionary origin for the group.
On the monophyly question, the research team found 10 markers that were incorporated in the marsupial genome – but which are absent from other mammals – after the time when placentals and marsupials diverged (which happened around 130 mya) and before modern marsupials evolved (70-80 mya). So there’s a single trunk for the marsupial family tree.
That left another 43 "phylogenetically informative retroposon markers" (Nilsson et al. 2010) that could be used to sort out the ‘internal’ history of the marsupials themselves. Analysis of the distribution of these markers (which on reading the paper sounds a hugely complex task!) indicated that a) the opossums, or Didelphimorphia, are on the oldest branch of the marsupial family tree; b) that the ‘Australidelphia’ – that’s the Australian groups plus the cute ‘little mountain monkey’ of southwestern South America – is a monophyletic group; and c) that Dromiciops is the oldest member of the Australidelphia, evolving prior to the migration across to what was to become Australia. Overall, the team concluded that there was "only a single effective migration from South America to Australia, which is remarkable given that South America, Antarctica, and Australia were connected in the South Gondwanan continent for a considerable time" (Nilsson et al. 2010).
Figure 2. Phylogenetic tree of marsupials derived from retroposon data, from Nilsson et al, 2010.
The names of the seven marsupial orders are shown in red, and the icons are representative of each of the orders: Didelphimorphia, Virginia opossum; Paucituberculata, shrew opossum; Microbiotheria, monito del monte; Notoryctemorphia, marsupial mole; Dasyuromorphia, Tasmanian devil; Peramelemorphia, bilby; Diprotodontia, kangaroo. Phylogenetically informative retroposon insertions are shown as circles. Gray lines denote South American species distribution, and black lines Australasian marsupials. The cohort Australidelphia is indicated as well as the new name proposed for the four “true” Australasian orders (Euaustralidelphia).
Nilsson MA, Churakov G, Sommer M, Tran NV, Zemann A, Brosius J, & Schmitz J (2010). Tracking marsupial evolution using archaic genomic retroposon insertions. PLoS biology, 8 (7) PMID: 20668664