Ontogenetic co-option of myotoxin expression variation in island Eastern Diamondback Rattlesnake (Crotalus...

Rapid adaptive evolution often arises from standing genetic variation within populations because of the rarity of de novo beneficial mutations and the time needed for their fixation. Eastern Diamondback Rattlesnakes (Crotalus adamanteus), a model species in the study of venom evolution, frequently inhabit islands, providing a useful model for studying the types of genetic variation leading to rapid venom evolution. In C. adamanteus, the primary axes of venom expression variation across the range are ontogeny (i.e., adult-juvenile) and regional differences in myotoxin (crotamine) expression; for the latter, myotoxin expression generally increases with latitude independent of age class due to differences in gene copy number. Prior work, however, found that on Caladesi Island (Florida), a juvenile and adult C. adamanteus exhibited expression patterns inconsistent with those observed across the species' range; here, the juvenile expressed high levels of myotoxin which was largely absent in the adult. To determine whether this variation reflected the co-option of existing axes of variation (i.e, myotoxin being integrated into the ontogenetic network) or standing polymorphism in myotoxin expression in the island population, we analyzed venom protein expression in 19 C. adamanteus of varying body sizes across three young islands (<5000 years old), including Caladesi Island, off the west coast of Florida. We found clear ontogenetic differences in venom expression across all three islands, including a decline in myotoxin expression with age. The incorporation of myotoxin expression into ontogeny has not been observed elsewhere within C. adamanteus. The novel integration of the two largest axes of variation via co-option of myotoxin into the ontogenetic venom expression network likely facilitated rapid adaptation in these island populations. Overall, rapid venom evolution on these islands appeared to be biased not only towards standing genetic variation but large, existing axes of variation previously subjected to selection.