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An Analysis of Marsupial Interordinal Relationships Based on 12S rRNA, tRNA Valine, 16S rRNA, and Cytochromeb Sequences


Angela Burk,1 Michael Westerman,2 Diana J. Kao,1 John R. Kavanagh,1 and Mark S. Springer1,3

1 Department of Biology, University of California, Riverside, California 92521.
2 Department of Genetics and Human Variation, La Trobe University, Bundoora, Victoria 3083, Australia.
3 To whom correspondence should be addressed, e-mail:

 

ABSTRACT
The basal split among living marsupials is traditionally placed between the cohorts Ameridelphia and Australidelphia. Ameridelphia includes all American forms excepting the South American Dramicuipx gliroides (Order Microbiotheria). Australidelphia includes all Australasian taxa plus Dromiciops gliroides. DNA data support Eometatheria {Dromiciaps + Diprotodontia + Dasyuromorphia + Notoryctemorphia) but do not resolve the position of bandicoots, whether with other australidelphians or with ameridelphians. Also, the most robust molecular trees (DNA hybridization, multigene studies) exhibit minimal branch subdivision and raise the possibility of artifactual associations owing to long branch attraction. We analyzed data sets that consisted of complete sequences tor four mitochondrial genes (cytochrome b, 12S rRNA, tRNA valine, 16S rRNA). One data set included 14 marsupial taxa. A second data set included 14 marsupials as well as outgroup sequences (one monotreme; 20 placentals). Phylogenetic analyses included parsimony, minimum evolution, maximum likelihood, and quartet puzzling. When phylogenetic analyses were restricted to just the marsupial sequences, there was 75 to 96% boostrap support for the separation of Ameridelphia versus Australidelphia. This suggests that either one or both of these groups are monophyletic. Also, there was 71 to 98% bootstrap support for the separation of Eometatheria versus Ameridelphia + Peramelina. Nonmonophyly of several a priori clades was accepted by at least some statistical tests including the following: Diprotodontia + Peramelina, Notoryctemorphia + Peramelina, Diprotodonlia + Notoryctemorphia, and the monophyly of Australasian marsupials. With the inclusion of outgroup sequences, there was reduced bootstrap support for associations among marsupial orders and statistical tests failed to reject all interordinal associations that were tested.

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