The opossum MHC genomic region revisited

The gray short-tailed opossum Monodelphis domestica is one of the few marsupial species for which a high quality whole genome sequence is available and the major histocompatibility complex (MHC) region has been annotated. Previous analyses revealed only a single locus within the opossum MHC region, designated Modo-UA1, with the features expected for encoding a functionally classical class I alpha-chain. Nine other class I genes found within the MHC are highly divergent and have features usually associated with non-classical roles. The original annotation, however, was based on an early version of the opossum genome assembly. More recent analyses of allelic variation in individual opossums revealed too many Modo-UA1 sequences per individual to be accounted for by a single MHC class I locus found in the genome assembly. A reanalysis of a later generation assembly, MonDom5, revealed the presence of two additional loci, now designated Modo-UA3 and UA4, in a region that was expanded and more complete than in the earlier assembly. Modo-UA1, UA3, and UA4 are all transcribed, although Modo-UA4 transcripts are rarer. Modo-UA4 is also relatively non-polymorphic. Evidence presented support the accuracy of the later assembly and the existence of three related class I genes in the opossum, making opossums more typical of mammals and most tetrapods by having multiple apparent classical MHC class I loci.