Immunoglobulin variable-region diversity in the zebrafish

As part of an investigation of the immune system in the developing zebrafish, Danio rerio, we cloned and characterized a genomic V-H segment and a number of cDNAs encoding IgM heavy chains. The genomic V-H has the characteristic features of V-H in other vertebrates, including a leader segment interrupted by a short intron, and regulatory and recombination signal sequences. The V-H sequences of 75 cDNA clones fell into four distinct groups or families. All of these families cluster most closely with other teleost V-H; one of the families, V(H)3, is also closely related to other vertebrate V-H. Analysis by Southern blotting suggests that there are a number of germline genes corresponding to each family. Most variability within the V(H)1 family is confined to codons 30-32, at the end of the first framework region and in the beginning of the first complementarity-determining region. The nucleotide sequence of the genomic V-H in this region contains motifs associated with a relatively high frequency of somatic mutation. There is much variability in segments of the clones corresponding to the third complementarity-determining region. The cDNA sequences encoding the fourth framework region are consistent with the contribution of four distinct J(H) segments.