THE MARSUPIAL MHC - THE TAMMAR WALLABY, MACROPUS-EUGENII, CONTAINS ANEXPRESSED DNA-LIKE GENE ON CHROMOSOME

In the placental mammal major histocompatibility complex (MHC) three m ain families of class II genes, DR, De, and DP, have been recognized. Each family contains genes that code for one or more A- and B-chains. Recent evidence has indicated that a fourth family can be described, t he DN/DO family. These four families arose sometime early in mammalian evolution. Our purpose was to deduce the MHC of an early mammalian an cestor of marsupials and eutherians. Using primers designed to conserv ed regions in exon 2 and exon 3 of the DQA gene we amplified an 830-bp band from the total genomic DNA of the marsupial, Macropus eugenii (t ammar wallaby). However, sequence analysis of cloned genomic products showed that the primers had amplified three genes, two of which appear ed to be alleles at one locus, while the other gene belonged to a clos ely related locus. Phylogenetic analysis showed that both these loci w ere most closely related to the human (HLA-DNA) and mouse (H-20a) DNA genes, with a bootstrap support of 78%. Expression of only one locus c ould be detected by RT-PCR from spleen RNA. In situ hybridization to t ammar wallaby chromosomes mapped these genes to one region on the long arm of chromosome 1, indicating the position of the MHC in marsupials . Related A-chain genes were detected in monotremes, and human by sout hern blotting, and very faint bands were observed in the chicken. Hybr idization with a tammar DNA-like gene on several marsupial species sho wed evidence of at least three DNA-like loci in the tammar wallaby, at least one in the koala, but none in the kowari. This indicates that t he organization of the class II MHC may be more dynamic in marsupial t han in placental mammals, but, in contrast to a previous study on the MHC of a marsupial, we cannot conclude that the class II gene families of placental and marsupial mammals evolved from different ancestral g enes.