ISOLATION OF MHC CLASS-II DMA AND DMB CDNA SEQUENCES IN A MARSUPIAL -THE GRAY SHORT-TAILED OPOSSUM (MONODELPHIS-DOMESTICA)

We report the cDNA sequences for the DMA and DMB family of Mhc genes o f the gray short-tailed opossum, Until now DM sequences were available only in eutherian mammals. The marsupial sequences indicate that both members of the family are old and probably diverged from other classi cal class II families about the time of the radiation of jawed vertebr ates some 450 million years ago. We examine the evolutionary rates of equivalent sets of classical and nonclassical genes to check for rate heterogeneity. We find the alpha-1 domain of the DR genes to be untypi cally conservative in its evolutionary mode. The DM genes appear to ev olve at rates typical of other class II genes, indicating that their p lacement at the root of class II gene evolutionary trees may be justif ied.