Ig light chain gene in the Siberian sturgeon (Acipenser baeri). Implications for the evolution of the immune system

Elasmobranch and teleost fish have their Ig light (L) chain loci organized in multiple clusters (V sub(L)-J sub(L)-C sub(L)). The V sub(L) segments of teleosts are in opposite transcriptional orientation to the C sub(L) genes, suggesting that in teleosts and elasmobranchs there may have been separate evolutionary events leading to this organization. To address this problem, the IgL locus from the Siberian sturgeon (Acipenser baeri) (representative of a branch between elasmobranchs and teleosts) was investigated. Sequence analysis of cDNA clones shows that sturgeon V sub(L) genes are most similar to those of teleosts, but that sturgeon C sub(L) genes are more similar to those of the sharks. Southern blot analyses of sturgeon erythrocyte DNA with V sub(L)- and C sub(L)-specific probes showed that there are more than 20 V sub(L) segments in both the tetraploid Siberian sturgeon and the diploid sterlet (Acipenser ruthenus), but only a few C sub(L) segments in the genome of the Siberian sturgeon and up to four C sub(L) segments in that of the sterlet. Screening of an unamplified genomic library gave more than 300 V sub(L)-positive and four C sub(L)-positive clones. None of these contained inserts positive for both probes. PCR analysis of a genomic C sub(L) clone using J sub(L)- and C sub(L)-specific primers suggested that upstream of the C sub(L) segment there are at least seven J sub(L) segments. It is concluded that sturgeons have a Kappa -like organization of their IgL locus and that the clustered organization of IgL loci in bony fish and sharks arose from two distinct evolutionary events.