title: Characterisation of echidna IgM provides insights into the time of divergence of extant mammals.
abstract: 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 (Cmu)was isolated from a spleen cDNA library using a Trichosurus vulpecula probe. It has approximately 46.5% amino acid identity to marsupial and eutherian Cmus, and approximately 30% amino acid identity with Cmu from birds and reptiles. Phylogenetic analysis of mammalian Cmu provides strong support for the Theria hypothesis, with a sister grouping of the eutherians and marsupials to the exclusion of the monotremes. Cmu 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.
authors.affiliation: Evolutionary Biology Unit, Australian Museum, 6 College Street, Sydney, NSW 2010, Australia.
authors.forename: Katherine
authors.initials: K
authors.lastname: Belov
authors.forename: Lars
authors.initials: L
authors.lastname: Hellman
authors.forename: Desmond W
authors.initials: DW
authors.lastname: Cooper
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D015723:Gene Library; D007127:Immunoglobulin Constant Regions / Q000235:genetics; D007143:Immunoglobulin Heavy Chains / Q000235:genetics; D007075:Immunoglobulin M / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D017386:Sequence Homology, Amino Acid; D004442:Tachyglossidae / Q000235:genetics* / Q000276:immunology
chemical_list: D007127:Immunoglobulin Constant Regions; D007143:Immunoglobulin Heavy Chains; D007075:Immunoglobulin M