Molecular determinants and evolutionary dynamics of wobble splicing

Alternative splicing at tandem splice sites (wobble splicing) is widespread in many species, but the mechanisms specifying the tandem sites remain poorly understood. Here, we used synaptotagmin I as a model to analyze the phylogeny of wobble splicing spanning more than 300 My of insect evolution. Phylogenetic analysis indicated that the occurrence of species-specific wobble splicing was related to synonymous variation at tandem splice sites. Further mutagenesis experiments demonstrated that wobble splicing could be lost by artificially induced synonymous point mutations due to destruction of splice acceptor sites. In contrast, wobble splicing could not be correctly restored through mimicking an ancestral tandem acceptor by artificial synonymous mutation in in vivo splicing assays, which suggests that artificial tandem splice sites might be incompatible with normal wobble splicing. Moreover, combining comparative genomics with hybrid minigene analysis revealed that alternative splicing has evolved from the 3' tandem donor to the 5' tandem acceptor in Culex pipiens, as a result of an evolutionary shift of cis element sequences from 3' to 5' splice sites. These data collectively suggest that the selection of tandem splice sites might not simply be an accident of history but rather in large part the result of coevolution between splice site and cis element sequences as a basis for wobble splicing. An evolutionary model of wobble splicing is proposed.