A long form of the skate IgX gene exhibits a striking resemblance to the new shark IgW and IgNARC genes

Differential screening has been used to identify cDNAs encoding a long form of IgX in Raja eglanteria (clearnose skate). Comparisons of the IgX long f orm with the previously described short-form IgX cDNAs and the genomic IgX locus indicate that the V and two 5' C regions of the short and long forms of IgX are >90% identical at the nucleotide level. Differences between the V sequences of the long- and short-form IgX genes are concentrated in compl ementarity determining regions, suggesting that these forms are derived thr ough alternative splicing of the same genomic loci or transcription of high ly related loci. The extreme conservation of nucleotide sequence, including third position codons, among different cDNAs as well as the near identity of nucleotide sequence in the intervening sequences of germline IgX, IgX sh ort-form sterile transcripts and IgX long-form sterile transcripts indicate that the multiple IgX loci are recently diverged from one another and/or a re under intense gene correction. Phylogenetic analyses of the known cartil aginous fish immunoglobulin loci demonstrate that the long form of IgX is o rthologous to IgW/IgNARC (NARC) and is most consistent with: 1) the diverge nce of the IgX/IgW/NARC and IgM-like loci from a common ancestral locus pri or to the divergence of the cartilaginous/bony fish lineages and 2) the div ergence of the NAR locus from the IgX/IgW/NARC gene(s) after the cartilagin ous/bony fish split but prior to the shark/skate split, approximately 220 m illion years ago.