Diversity analysis of the immunoglobulin M heavy chain gene in Nile tilapia, Oreochromis niloticus (Linnaeus)

A full-length cDNA encoding the immunoglobulin (IgM) heavy chain gene of Nile tilapia was successfully cloned using the 5’ and 3’ RACE techniques. The complete cDNA of the Nile tilapia IgM heavy chain gene is 1,921 bp in length and has an open reading frame (ORF) of 1,740 bp, which corresponds to 580 amino acid residues. The deduced amino acid sequence of the Nile tilapia IgM heavy chain includes a typical secretory IgM heavy chain designated "On-sIgM" and a variable region that is connected to 4 constant regions to form the LH-VH-Cµ1-Cµ2-Cµ3-Cµ4 pattern. Comparisons of the nucleotide and amino acid sequences of On-sIgM with IgM heavy chains of other organisms showed the highest similarity scores of 62.6 and 55.4%, respectively, to the orange-spotted grouper (Epinephelus coioides). Structural analysis of 126 cDNAs encoding variable domains of the IgM heavy chain revealed that at least 9 VH families, 6 DH segments and 4 JH families were utilized using several mechanisms to generate the repertoire of antigen-binding domains. Variation analysis of the variable domains indicated that the amino acid sequences of the framework regions (FRs) were less variable than those of the complementarity determining regions (CDRs), among which the most variable was CDR3. Tissue expression profile analysis using quantitative real-time RT-PCR of healthy Nile tilapia showed that the IgM heavy chain gene was ubiquitously expressed in all 13 tested tissues, but the highest expression level was observed in the head kidney, followed by the spleen, intestine and peripheral blood leukocytes (PBLs). Furthermore, Southern blot analysis of the constant region of the IgM heavy chain gene of 3 different fishes indicated that Nile tilapia genomes may contain 2 copies of the IgM gene.   Key words: Nile tilapia, IgM heavy chain, variable region, diversity, secreted form, southern blot.