The marsupial mitochondrial genome and the evolution of placental mammals
Janke, A, Feldmaier-Fuchs, G, Thomas, WK, von Haeseler, A & Pӓӓbo, S 1994, Genetics, vol. 137, no. 1, pp. 243-256.
The entire mitochondrial genome of the American opossum, Didelphis vinginiana, was sequenced here and its distinguishing features described. A phylogenetic tree was created from the genetic sequence which demonstrated that rodents formed an earlier mammalian branch than did primates. The rate of evolution of the mitochondrial genome is clocklike, and this knowledge combined with genetic data indicated that the mouse and rat diverged as long as 35 million years ago.
The opossum’s mitochondrial genome codes for 22 tRNAs, two rRNAs and 13 proteins, which is consistent with other mammalian mitochondrial genomes. Nucleotides within the control region have varying levels of conservation compared to placental mammals, including two conserved sequence blocks which match two of the three blocks identified in placental mammals. Regions containing repeated motifs varied in number of repeats between opossum individuals. The tRNA for lysine has a reduced dihydrouridine arm in the opossum genome, similarly to placentals, suggesting this reduction occurred in a common ancestor. Serine’s tRNA has a different number of nucleotides to the chicken and frog genomes previously sequenced, but the same modification as placentals, again suggesting the change occurred in a common ancestor. The tRNA genes for alanine, asparagine and cysteine are rearranged around the replication origin of the L strand due to deletions, duplications and a new origin of replication. This variation has been seen in other marsupials previously, although the hairpin loop in the origin of L strand replication is longer than in placentals. The tRNA for aspartic acid contains the anticodon GCC, rather than the normal GTC, due to posttranscriptional modification, which has also been observed in Australian marsupials. The ribosomal RNA genes and protein coding genes were found to be conserved with other vertebrates, although different initiation and termination codons were present compared to humans and frogs. Variations in first and second positions of tRNA codons of various mammalian genomes were used to create a phylogenetic tree, with the final tree based on a maximum likelihood surface. This tree places primates and ungulates as sister taxa, with rodents as an earlier divergence, both contradicting and supporting various other studies. The rodent lineage had previously been suggested to have twice the rate of molecular evolution than other placentals; however, this study supports a molecular clock model with its protein gene homology and proposed phylogeny, suggesting the rates may actually be the same.
A greater understanding of the evolution of opossums increases knowledge of marsupial evolution, and hence how marsupials relate to other groups such as placental mammals.
Summarised by Laura Wait
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