Characterisation of echidna IgM provides insights into the time of divergence of extant mammals

The immunobiology of monotremes is poorly understood. In this paper, we describe the characterisation of the heavy chain of IgM from Tachyglossus aculeatus, the short-beaked echidna. The echidna heavy chain constant region of IgM (Cμ was isolated from a spleen cDNA library using a Trichosurus vulpecula probe. It has approximately 46.5% amino acid identity to marsupial and eutherian Cμs, and approximately 30% amino acid identity with Cμs from birds and reptiles. Phylogenetic analysis of mammalian Cμ provides strong support for the Theria hypothesis, with a sister grouping of the eutherians and marsupials to the exclusion of the monotremes. Cμ sequences suggest that monotremes and therians separated approximately 170 million years ago (mya), marsupials and eutherians separated approximately 130mya, and Australian and American marsupials separated approximately 65mya.