Research to provide unparalleled insights into mammalian evolution
31 January 2006
The genetic code of marsupials has been documented for the first time 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.”
Dr Belov’s lab (the Australasian Wildlife Genomics Group) is working on the evolution of the mammalian immune system and is currently focusing on the immune system of the opossum, the Tammar wallaby and the platypus, because the genomes of these species are currently being sequenced.
The opossum genome has already been sequenced by the Broad Institute in the US. The Tammar wallaby project is Australia’s first large-scale genome project. Sequencing is currently being carried out at the Australian Genome Research Facility (AGRF) in Australia and at the Baylor Centre in the US. Dr Belov and Veterinary Science PhD student Hannah Siddle are working closely with the ANU-based ARC Centre for Kangaroo Genomics analysing the Tammar wallaby genome sequence as it becomes available. The platypus genome is being sequenced at Washington University in the US and the results will be published later this year.
“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."
Dr Belov has just received an ARC Discovery grant to fully characterise the MHC of the Tammar Wallaby. This work will be done in collaboration with the Sanger Centre in the UK and Macquarie University.
“Understanding the immune system of marsupials and monotremes will help us to conserve our native species” she said.
Contact: Jake O'Shaughnessy
Phone: +61 2 9351 4312
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