The swine Ig heavy chain locus has a single J(H) and no identifiable IgD

Sequence analysis of 84 V region cDNAs expressed with IgM, IgG and IgA from both adult and newborn swine suggested that their J(H) segments had been derived from the same germline J(H) Only a single hybridizing J(H) Segment could be identified in genomic DNA, in a J(H)-C-mu cosmid and in a Sad fragment of the cosmid extending 5 kb 5' of the Enh(H). The single germline J(H) Segment mapped 6 kb 5' to C-mu. This J(H) had a sequence identical to 40 of 42 J(H) segments expressed in a newborn piglet and 25 of 42 expressed by adult swine. None of the 19 J(H) segments which varied from the germline sequence were identical to each other and half of the nucleotide changes were silent. No cosmid DNA hybridizing with heterologous probes for C-delta could be found within 20 kb 3' of C-mu and C-delta could not be cloned from genomic or cDNA libraries. A conserved IgD fragment could be amplified from human, mouse and rat genomic DNA but not from rabbit, swine or cattle. We hypothesize that heavy chain organization and constituency in homeothermic vertebrates is correlated with the site of secondary antibody repertoire development and the mechanism(s) used.