Immunoglobulin variable-region diversity in the zebrafish

Abstract.

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_H3, 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_H1 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.