Genes of the constant regions of functional immunoglobulin heavy chain of Japanese flounder, Paralichthys olivaceus

IgM and IgD genes of the Japanese flounder were cloned and characterized from a genomic bacterial artificial chromosome (BAC) library. The mu gene contained four constant region exons (Cmu1-Cmu4), and two transmembrane exons (muTM1 and muTM2), which conforms to the organizational pattern of all other vertebrate mu-chain genes examined so far. In the same BAC clone, the delta gene, which is homologous to the IgD gene in mammals and teleost fish, was found immediately (0.9 kb) downstream of the IgM gene. This gene encoded seven exons (Cdelta1-Cdelta7) and two deltaTM exons (deltaTM1 and deltaTM2) and had no duplication of Cdelta1-Cdelta2, as found in Atlantic cod, or Cdelta2-Cdelta3-Cdelta4, as found in Atlantic salmon and channel catfish. Phylogenetic and sequence analyses strongly suggest that teleost delta is more closely related to non-IgM isotypes than IgM isotypes. The heavy chain (IgH) locus of Japanese flounder, which encodes mum, mus and deltam, was found to be fully functional.