A NOVEL CHIMERIC IG HEAVY-CHAIN FROM A TELEOST FISH SHARES SIMILARITIES TO IGD

IgD is considered to be a recently evolved Ig, being previously found only in primates and rodents, Here we describe, from a teleost fish (t he channel catfish, Ictalurus punctatus), a novel complex chimeric Ig heavy chain, homologous, in part, to the heavy chain (delta) of IgD. I n addition to alternative secretory or membrane-associated C termini, this chimeric molecule contains a rearranged variable domain, the firs t constant domain of mu, and seven constant domains encoded by a delta gene homolog. Identification of the catfish gene as delta is based on the following properties: sequence relatedness to mammalian delta; a location within the IgH locus that is immediately downstream of the mu gene; separate terminal exons for the secretory and membrane forms; c oexpression with the complete mu chain in some but not all B cells. Th ese results (i) suggest that IgD is an ancient immunoglobulin that was present in vertebrates ancestral to both the mammals and the ray-finn ed fishes, and (ii) raise the possibility that this Ig isotype may hav e served an as yet unidentified important function early in the evolut ion of the immune system.