The genetic code of marsupials has been documented by an international team led by Dr Kathy Belov, a postdoctoral fellow from the University of Sydney’s Faculty of Veterinary Science.
This major paper, which is published in the most recent issue of PLoS Biology, details the evolution of an important cluster of immune genes, known as the MHC, using the genetic sequence of Monodelphis domestica (the grey, short tailed opossum) a marsupial found in South America.
Entitled "Reconstructing an Ancestral Mammalian Immune Supercomplex from a Marsupial MHC", the paper is the result of international collaboration between the University of Sydney, Australian National University (ANU), the Walter and Eliza Hall Institute of Medical Research, the University of New Mexico, Texas A&M, and the Southwest Foundation for Biomedical Research and the University of Pittsburgh.
The gene sequence points towards the existence of an ancestral ‘immune supercomplex’ which contained various immune genes in a single region of the genome.
"Mapping the opossum MHC has allowed us to deduce what the MHC of ancestral mammals looked like," Dr Belov says. "We think it contained several different types of immune genes in a single complex."
"These genes are no longer found in a single complex in any living animal but are scattered over various chromosomes. We have named this complex ‘The Immune Supercomplex’."
"The clues we unearthed by looking at different genomes are also helping us to understand how our own intricate immune system evolved from the relatively simple immune system seen in lower vertebrates such as birds and fish."
"Interest in marsupial and monotreme genomes comes from their important positions in vertebrate evolution," says Dr Belov.
"Comparing genes of placental mammals, such as the human and the mouse, is not very efficient because their genes can be so similar it is hard to pinpoint regions that remain unchanged because they serve a particular purpose."
"In contrast, comparison of distantly related genes, such as the chicken and human, can be difficult, because the sequences are so different."
"Marsupial and monotreme genomes fill this gap. They are easily aligned with placental mammal genomes, yet are different enough to pinpoint regions that have important functions and therefore have been conserved for long periods of time."
"The monotremes (represented today only by the platypus and the echidna) split off from other mammals 210 million years ago. The remaining marsupials split from the main (placental) group about 180 years ago."
"Understanding the immune system of marsupials and monotremes will help us to conserve our native species," she said.