Elasmobranch and teleost fish have their Ig light (L) chain loci organized in multiple clusters (VL-JL-CL). The VL segments of teleosts are in opposite transcriptional orientation to the CL 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

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... lysis of cDNA clones shows that sturgeon VL genes are most similar to those of teleosts, but that sturgeon CL genes are more similar to those of the sharks. Southern blot analyses of sturgeon erythrocyte DNA with VL- and CL-specific probes showed that there are more than 20 VL segments in both the tetraploid Siberian sturgeon and the diploid sterlet (Acipenser ruthenus), but only a few CL segments in the genome of the Siberian sturgeon and up to four CL segments in that of the sterlet. Screening of an unamplified genomic library gave more than 300 VL-positive and four CL-positive clones. None of these contained inserts positive for both probes. PCR analysis of a genomic CL clone using IC and CL-specific primers suggested that upstream of the CL segment there are at least seven JL 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.