Molecular genetics of ecological diversification: duplication and rapid evolution of toxin genes of the venomous gastropod Conus

Abstract

Predatory snails in the marine gastropod genus Conus stun prey by injecting a complex mixture of peptide neurotoxins. These conotoxins are associated with trophic diversification and block a diverse array of ion channels and neuronal receptors in prey species, but the evolutionary genesis of this functional diversity is unknown. Here we show that conotoxins with little amino acid similarity are in fact products of recently diverged loci that are rapidly evolving by strong positive selection in the vermivorous cone, Conus abbreviatus, and that the rate of conotoxin evolution is higher than that of most other known proteins. Gene duplication and diversifying selection result in the formation of functionally variable conotoxins that are linked to ecological diversification and evolutionary success of this genus.

Figure 1

Neighbor-joining tree reconstructed from Kimura two-parameter distances computed from comparisons of entire conotoxin sequences (including 3′ untranslated regions). Bootstrap values >50% are indicated on branches. Roman numerals labeling branches within groups identify the representative sequences used in Dn and Ds analyses and Tables 1 and 2. Total numbers of sequences in each group from each individual: A1, n1 = 6, n2 = 9; A2, n1 = 19, n2 = 14; A3, n1 = 1, n2 = 0; A4, n1 = 5, n2 = 5; A5, n1 = 1, n2 = 0; A6, n1 = 5, n2 = 5; A7, n1 = 9, n2 = 2; A8, n1 = 1, n2 = 0; A9, n1 = 4, n2 = 14; L1, n1 = 26, n2 = 43; L2, n1 = 11, n2 = 0.

Figure 2

Sliding window analysis of average Dn and Ds estimates for all toxin sequence comparisons. Codons 1–42 primarily include the prepro region; codons 43–63 only contain toxin sequences terminating before the stop codon; because the presumed cleavage site varies in position, window position 36–49 includes about a 1:1 ratio of prepro/toxin codons (see Table 1; gaps in the alignment are excluded from this codon-numbering scheme).