Duplication of J kappa genes within genus Rattus

We have isolated a region containing the immunoglobulin kappa chain joining segments from a liver DNA library of the Australian rat Rattus villosissimus, and determined its nucleotide sequence. While the laboratory rat (Rattus norvegicus) had previously been shown to contain three recently duplicated copies of J kappa 2, R. villosissimus has only two. Furthermore, all three copies of J kappa 2 in R. norvegicus share an 11 bp deletion in their 5' flanking regions which is not evident in either copy of J kappa 2 in R. villosissimus. This suggests that the initial duplication events occurred separately in the two lineages, and were followed by a second duplication in R. norvegicus, all three duplications having occurred within the last 6-12 million years (although more complicated schemes involving gene conversion events cannot be excluded). These results indicate that there is a high degree of plasticity in this region of the genome, and that selective forces must exist which have maintained the number of expressible J kappa segments in humans (5) and rodents (4-6) within their narrow range.