EVOLUTION OF MHC CLASS-I LOCI IN MARSUPIALS - CHARACTERIZATION OF SEQUENCES FROM KOALA (PHASCOLARCTOS-CINEREUS)

We demonstrate that koala (Phascolarctos cinereus) MHC class I constit utes a variable multigene family. A total of nine partial exon 2 and 3 major histocompatibility complex (MHC) class I sequences are presente d, including six sequences from at least three loci from one koala; Va riation was detected by examination of sequences from a number of indi viduals and family groups. The koala is the second marsupial species c haracterized to date, and comparisons reveal approximately 80% similar ity with sequences from the red-necked wallaby (Macropus rufogriseus). The latter sequences represent at least two, and probably three, diff erent loci. Phylogenetic analysis demonstrates that all koala sequence s are more related to one another than they are to any of the wallaby loci. This indicates that the koala sequences are probably not ortholo gous to the wallaby genes, and thus represent a new class I gene famil y. In addition, marsupial gene families cluster away from human gene f amilies, supporting a different origin of MHC genes for marsupials and eutherians.