The relationship of the marsupial families and the mammalian subclasses based on recombination activating gene-1.
Baker, M. L., Wares, J. P., Harrison G. A., and Miller, R. D.
Abstract:
Controversies remain over the relationships among several of the marsupial families and between the three extant groups of mammals; eutherians (placentals), metatherians (marsupials) and prototherians (monotremes). Two opposing hypotheses place the marsupials as either sister to the placental mammals (Therian hypothesis) or sister to the monotremes (Palimpsest or Marsupionta hypothesis). A nuclear gene that has proved useful for analyzing phylogenies of vertebrates is the recombination activation gene-1 (RAG1). RAG1 is a highly conserved gene in vertebrates and likely entered the genome by horizontal transfer early in the evolution of jawed vertebrates. Phylogenetic analysis was carried out on RAG1 from seven placental, 28 marsupials and all three living monotreme species. Analysis of RAG1 supports many of the traditional relationships among the marsupials and suggests a relationship between the Peramelidae marsupials and Notoryctidae, two species whose position in the phylogenetic tree has been enigmatic. A sister relationship between the American marsupial, Caenolestes with respect to all living marsupials is also suggested by RAG1. The relationship between the three groups of mammals is consistent with a therian history of mammals, with the monotremes as the sister group to a clade containing marsupials and placentals.