Deadly innovations: Molecular phylogenetics and evolution of phospholipase A₂ toxins in viperid snake venoms

Snake venoms have surged as model systems in evolutionary biology thanks to the dynamic diversification and accelerated evolution of many toxin families. Among these, phospholipase A₂ (PLA₂) constitute a prime example as they are ubiquitous across the venomous snake radiation and have evolved a wide variety of pathophysiological activities. This is especially true in vipers (family Viperidae), one of the most successful and medically significant venomous snake lineages worldwide. In this study, we gathered publicly available sequences of viper venom PLA₂s to recreate the molecular phylogeny and toxicological evolution of this toxin family to date. Furthermore, we determined the selection regimes regulating the evolution of these toxins with a comparative approach that combines multiple methodologies of phylogenetic reconstruction and analysis of selection signatures. Our phylogeny confirms the basal position of Asp49 PLA₂s (proteins with Asp at position 49), while derived clades, such as the non-enzymatic Lys49 myotoxins and the poorly characterised Ser49 type, are nested within. Neurotoxicity arose on multiple independent occasions (all within the Asp49 clade), with monomeric and dimeric forms only distantly related to each other. Positive Darwinian selection was widespread across the viper PLA₂ tree, in line with previous research. However, purifying selection was also preponderant (perhaps due to structural constraints imposed by the pathophysiological targets of these toxins) and relatively neutral substitutions were observed in certain clades. Overall, this study provides novel insights into the evolutionary history of viper venom PLA₂s through a comprehensive phylogenetic framework and highlights the need for complementary genomic and functional research into these toxins.