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 organ ized in multiple clusters (V-L-J(L)-C-L). The V-L segments of teleosts are in opposite transcriptional orientation to the C-L genes, suggest ing that in teleosts and elasmobranchs there may have been separate ev olutionary events leading to this organization. To address this proble m, the IgL locus from the Siberian sturgeon (Acipenser baeri (represen tative of a branch between elasmobranchs and teleosts) was investigate d. Sequence analysis of cDNA clones shows that sturgeon V-L genes are most similar to those of teleosts, but that sturgeon C-L genes are mor e similar to those of the sharks. Southern blot analyses of sturgeon e rythrocyte DNA with V-L- and C-L-specific probes showed that there are more than 20 V-L segments in both the tetraploid Siberian sturgeon an d the diploid sterlet (Acipenser ruthenus), but only a few C-L segment s in the genome of the Siberian sturgeon and up to four C-L segments i n that of the sterlet. Screening of an unamplified genomic library gav e more than 300 V-L-positive and four C-L-positive clones. None of the se contained inserts positive for both probes. PCR analysis of a genom ic C-L clone using J(L)- and C-L-specific primers suggested that upstr eam of the C-L segment there are at least seven), segments, It is conc luded that sturgeons have a kappa-like organization of their IgL locus and that the clustered organization of IgL loci in bony fish and shar ks arose from two distinct evolutionary events.