Analysis of a marsupial MHC region containing two recently duplicated class I loci

A 37-kb cosmid containing two complete major histocompatibility complex (MHC) class I chain loci from the opossum Monodelphis domestica was isolated, fully sequenced, and characterized. This sequence represents the largest contiguous genomic sequence reported for the MHC region of a nonplacental mammal. Based on particular conserved amino acid residues, and limited expression analyses, the two MHC-I loci, designated ModoUB and ModoUC, appear to encode functional MHC-I molecules. The two coding regions are 98% identical at the nucleotide level; however, their promoter regions differ significantly. Two CpG islands present in the cosmid sequence correspond to the two coding regions. Twelve microsatellites and six retroelements were also present in the cosmid. The retroelements share highest sequence homology to the CORE–SINE family of retroelements. Due to high sequence identity, it is very likely that ModoUB and ModoUC loci are products of recent gene duplication that occurred less than 4 million years ago.