Genetics: Marsupial genome sequenced
The first high-quality draft of a marsupial genome sequence is revealed.The genome of the grey, short-tailed opossum (Monodelphis domestica) offers interesting insights into the genetics of the immune system and the X chromosome.
Kerstin Lindblad-Toh and colleagues identified a wide range of immune genes. Some seem to be marsupial-specific, whereas others are shared with placental mammals. This, alongside the discovery of a novel type of T-cell receptor, indicates that marsupials had already evolved a complex immune system when they diverged from the placental mammal lineage some 180 million years ago — a suggestion that is at odds with previous claims of a primitive immune system.
The results also suggest that random inactivation of the X chromosome — a phenomenon seen in placental mammals whereby one random copy of the X chromosome is switched off to avoid a double dose of 'X genes' — appeared alongside the evolution of a complex genetic locus called the X inactivation centre (XIC). The XIC is lacking in the opossum genome, a finding that may help to explain why, in the opossum, it's always the paternally derived X chromosome that is silenced.
The newly sequenced genome seems to contain 18,000–20,000 protein-coding genes, most of which have counterparts in placental mammals. Opossum-specific genes mostly originate from the expansion and rapid turnover of gene families involved in immunity, sensory perception and detoxification.
The sequencing of the opossum genome marks an important point in genetics research and not just because of the special place occupied by the marsupial in the evolutionary tree. The animals provide a good model of malignant melanoma, and are useful in studies of regeneration because newborns can heal complete transections of spinal cord.
CONTACT
Kerstin Lindblad-Toh (Broad Institute of MIT and Harvard, Cambridge, MA, USA)

Nicole Davis (Scientific Communications Specialist, Broad Institute of MIT and Harvard, Cambridge, MA, USA)
Tel: +1 617 258 0952; E-mail: ndavis@broad.mit.edu

Geoff Spencer (Public Affairs Specialist, National Human Genome Research Institute, NIH, Bethesda, MD, USA)
Tel: +1 301 451 8325; E-mail: spencerg@mail.nih.gov

Leo Goodstadt (MRC Functional Genetics Unit, University of Oxford, UK) Co-author
This author can be contacted through the MRC press office:
Tel: +44 20 7637 6011; E-mail: press.office@headoffice.mrc.ac.uk