Many options, few solutions: over 60 million years snakes converged on a few optimal venom formulations

Abstract

Gene expression changes contribute to complex trait variations in both individuals and populations. However, how gene expression influences changes of complex traits over macroevolutionary timescales remains poorly understood. Being comprised of proteinaceous cocktails, snake venoms are unique in that the expression of each toxin can be quantified and mapped to a distinct genomic locus and traced for millions of years. Using a phylogenetic generalized linear mixed model, we analysed expression data of toxin genes from 52 snake species spanning the three venomous snake families, and estimated phylogenetic covariance, which acts as a measure of evolutionary constraint. We find that evolution of toxin combinations is not constrained. However, while all combinations are in principle possible, the actual dimensionality of phylomorphic space is low, with envenomation strategies focused around only four major toxins: metalloproteases, three-finger toxins, serine proteases, and phospholipases A2. While most extant snakes prioritize either a single or a combination of major toxins, they are repeatedly recruited and lost. We find that over macroevolutionary timescales the venom phenotypes were not shaped by phylogenetic constraints, which include important microevolutionary constraints such as epistasis and pleiotropy, but more likely by ecological filtering that permits a few optimal solutions. As a result, phenotypic optima were repeatedly attained by distantly related species. These results indicate that venoms evolve by selection on biochemistry of prey envenomation, which permit diversity though parallelism and impose strong limits, since only a few of the theoretically possible strategies seem to work well and are observed in extant snakes.