Dietary Breadth Predicts Toxin Expression Complexity in the Venoms of North American Gartersnakes

Selection on heritable phenotypic variation has played a prominent role in shaping the remarkable adaptations found across the Tree of Life. Complex ecological traits, such as snake venoms, are thought to be the products of selection because they directly link to fitness and survival. Snake venom increases the efficiency of prey capture and processing and is thus likely under intense selection. While many studies of snake venom have investigated the relationship between venom and diet, they have primarily focused on medically relevant front-fanged snakes. However, recent work has suggested that many non-front fanged snakes also rely on venom for subduing prey, despite having reduced toxicity toward humans. Here, we set out to uncover variation in toxin-producing genes, along with the ecological and evolutionary pressures impacting snake venom characteristics in the North American gartersnakes (Squamata: Natricidae: Thamnophis), a model group of non-front-fanged snakes. We annotated and analyzed Duvernoy's venom gland transcriptomes from 16 species representing all the major lineages within Thamnophis. We then generated measures of complexity of both toxins and dietary breadth. We found strong correlations between the complexity of toxin gene expression and phylogenetic diversity of diet, but no relationship between the complexity of the genetic makeup of the transcriptomes (allelic or sequence variation) and diet complexities. We also found phylogenetic signal associated with venom complexity, suggesting some influence of ancestry on venom characteristics. We suggest that, in non-front-fanged snakes, expression of toxins rather than sequence complexity is under strong selection by dietary diversity. These findings contradict similar studies from front-fanged snakes where increased transcriptomic complexity varies positively with dietary diversity, exposing a potential novel relationship between a complex phenotype-toxin expression-and its selective pressures-diet.