The immunoglobulin heavy chain repertoire in fish was investigated by cloning a total of 88 rearranged VDJ junctions from the head kidney B cell mRNA of a salmonid, the rainbow trout (Oncorhynchus mykiss). Trout DH segments are short and cannot be classified into independent DH families. Several of the ten identified putative DH segments had stretches of nucleotide sequence identity with mouse (DQ52, DFL 16.2 and Dsp 2.1), human (DM1) and chicken (DH4) DH. There was a clear preference for one or two of the three putative DH reading frames and a stop codon is often present in the less used reading frame. Four of the six JH segments are preferentially used, and analysis of the VH-DH and DH-JH junctions suggest the presence of N-nucleotides. The absolute size and size heterogeneity of the rainbow trout CDRH3 are smaller than those of the Xenopus, mouse and human CDRH3. About 75% of the 84 in-frame trout CDRH3 have 8, 9 or 10 residues and none of them have more than 11 residues. This homogeneization of the CDRH3 loop size may partly explain the restricted antibody diversity in lower vertebrates.

An Igh-V library was constructed from the head kidney cytoplasmic RNA of an 8.5-month-old non-immunized rainbow trout, Oncorhynchus mykiss, using the 5' RACE polymerase chain reaction. Six new Igh-V segments were identified, bringing to nine the number of Igh-V families actually defined in that species. A phylogenetic analysis shows that these nine Igh-V families can be classified into three major groups. The first includes the Igh-V1, Igh-V3, Igh-V4, and Igh-V7 families, and is homologous to the human and mouse Group III Igh-V families. The second includes the Igh-V5, Igh-V8, and Igh-V9 families and is more closely related to the Group I and Group II human and mouse Igh-V families. The third group includes the Igh-V2 and Igh-V6 families, which are not closely related to any other vertebrate Igh-V gene. Six Igh-J segments were characterized. They can recombine with Igh-V segments belonging to different families and there is a high level of junctional diversity between the Igh-V and Igh-J segments.