Evolution of the major histocompatibility complex: Isolation of class II β cDNAs from two monotremes, the platypus and the short-beaked echidna

Extant mammals are composed of three lineages: the eutherians, the marsupials and the monotremes. The majority of the mammalian major histocompatibility complex (MHC) data is based on the eutherian mammals, which generally have three classical MHC class II β chain gene clusters — DRB, DQB and DPB, as well as the non-classical DMB and DOB. Marsupial DMB, DAB and DBB have been characterised. Confusion still surrounds the relationship of the marsupial DAB and DBB genes with the classical eutherian class II clusters. Here we present the first monotreme MHC class II β chain sequences. Four MHC class II β chain sequences were isolated from a spleen cDNA library from the short-beaked echidna , and one from a spleen cDNA library from platypus using a brushtail possum DAB probe. Given the non-orthologous relationship of the monotreme sequences with marsupial and eutherian β chain clusters, we recommend that the five new monotreme sequences be assigned the nomenclature 'DZB', signifying the description of a new mammalian β chain cluster. Our analysis suggests that all mammalian β chain sequences (except DMB) evolved from a common ancestor. Maximum likelihood analysis places the monotreme β chain sequences at the base of the mammalian clade, indicating their ancestral status. However, within the mammalian clade, monophyletic clades are not robust, and elucidation of the order of gene duplication that gave rise to the present-day gene clusters is not yet possible.