Genomic organization of the immunoglobulin light chain gene loci in Xenopus tropicalis: Evolutionary implications

Abstract

Based on presently available genome data, we characterized the genomic organization of all three light chain gene ( ρ, σ and type III) loci in Xenopus tropicalis. The ρ gene locus in X. tropicalis, structurally similar to the κ gene loci in mammals, was shown to contain a single C ρ gene and nine J ρ segments. The σ locus also contains a single C gene, although two distinct C σ genes have previously been found in Xenopus laevis (most likely due to chromosome polyploidy). Four J σ gene segments were identified upstream of the C σ . The type III light chain gene locus, spanning approximately 170 kb DNA, structurally resembles the topology of mammalian λ gene loci, containing three C genes (C III1–3). C III2 and C III3 are both preceded by single, unique, J genes, whereas C III1 contains three J gene segments. Furthermore, two additional J gene segments, termed J IIIx1 and J IIIx2, were found in the intron separating V III2 and pV III1 (a pseudogene). Based on BLAST searches against the X. tropicalis EST database, all the C genes identified in this study were shown to be functional. On the basis of similarity of protein sequences, genomic organization and chromosomal location of the light chain genes among frogs and mammals, our data strongly support the previous suggestions that the ρ genes belong to the κ gene lineage, whereas the type III genes share a common origin with the λ genes.