Phospholipase A2 diversity and polymorphism in European viper venoms: Paradoxical molecular evolution in Viperinae

We report the diversity and polymorphism of phospholipase A2 (PLA2) transcripts from snakes belonging to nine European viper subspecies. This diversity results in the expression of a combination of six PLA2 species—ammodytin I1, ammodytin I2, ammodytin L, ammodytoxin, vaspin A and vaspin B—with 19 known isoforms of the first five of these species. Most of the European viper venoms studied contained either a myotoxin or a neurotoxin, and all contained ammodytin I1 and ammodytin I2. There is no evidence that a given pattern of PLA2 species constitutes a taxonomic criterion, and isoform analysis would be required for such discrimination. Analysis of the phylogenetic relationships between PLA2 species from European vipers and those of other members of the Viperinae revealed a strong correlation between the geographical source of the viper and the clustering seen for the different isoforms, for each PLA2 species.The Ka/Ks values calculated for the mature protein-coding region of paralogous genes showed that ratios for pairs including vaspin B or one ammodytoxin isoform were greater than 1.09, whereas those for most of the remaining pairs were less than 1. Different patterns of mutation were observed in comparisons of the different PLA2 isoforms. The mechanisms directing a mutation toward a precise exon remain unresolved.