Evolutionary diversification of multigene families: Allelic selection of toxins in predatory cone snails

In order to investigate the evolution of conotoxin multigene families among two closely related vermivorous Conus species, we sequenced 104 four-loop conotoxin mRNAs from two individuals of Conus ebraeus and compared these wi th sequences already obtained from Conus abbreviatus. In contrast to the di versity of conotoxin sequences obtained from C. abbreviatus, only two commo n sequence variants were recovered from C. ebraeus. Segregation patterns of the variants in these two individuals and restriction digests of four-loop conotoxin amplification products from nine additional individuals suggest that the common variants are alleles from a single locus. These two putativ e alleles differ at nine positions that occur nonrandomly in the toxin-codi ng region of the sequences. Moreover, all substitutions are at nonsynonymou s sites and are responsible for seven amino acid differences among the pred icted amino acid sequences of the alleles. These results imply that conotox in diversity is driven by strong diversifying selection and some form of fr equency-dependent or overdominant selection at conotoxin loci, and they sug gest that diverse conotoxin multigene families can originate from duplicati ons at polymorphic loci. Furthermore, none of the sequences recovered from C. ebraeus appeared to be orthologs of loci from C. abbreviatus, and attemp ts to amplify orthologous sequences with locus-specific primers were unsucc essful among these species. These patterns suggest that venoms of closely r elated Conus species may differ due to the differential expression of conot oxin loci.