Platypus immunoglobulin M and the divergence of the two extant monotreme lineages

Abstract

A full-length cDNA clone encoding the platypus (Ornithorynchus anatinus) immunoglobulin M (IgM) heavy chain was isolated from a spleen cDNA library using a short-beaked echidna (Tachyglossus aculeatus) IgM constant region (Cµ) probe. The isolation of platypus IgM shows that O. anatinus, like all other examined jawed vertebrates, express a classical IgM molecule. Amino acid sequence comparisons of the constant regions of IgM reveals a high level sequence conservation between O. anatinus and T. aculeatus sequences (87%), and only approximately 48% identity between O. anatinus and therian Cµ sequences. The variable region of this clone belongs to clan 3, supporting the view that this family is used preferentially, if not exclusively by O. anatinus, as opposed to the use of all three variable region clans by T. aculeatus. Phylogenetic analysis of Cµ sequences supports the traditional Theria hypothesis and suggests that the O. anatinus and T. aculeatus lineages separated from their last common ancestor approximately 21 million years ago.