In their evolutionary arms race between rattlesnakes and their prey, rodents, birds, and other reptiles often develop resistance to snakes’ venom. However, a new study led by the University of Colorado Boulder (CU Boulder) and the University of Texas at Arlington (UT Arlington) has found that rattlesnakes maintain a broad and diverse genetic toolkit that encodes snake venom, allowing them to rapidly adapt as their local prey and conditions change.
“The existence of these resistance mechanisms in prey led us to wonder: If there’s selection pressure imposed back on the snakes, then it might make sense evolutionarily to have a more expanded venom arsenal,” said study lead author Drew Schield, a postdoctoral researcher in Ecology and Evolutionary Biology at CU Boulder. “We found these rattlesnakes had a more diverse venom repertoire, more genetic tools in the toolkit, than their venom composition alone might suggest.”
For decades, scientists thought that the co-evolution between rattlesnakes and their prey would be driven by a process called “directional selection,” in which snake venom would become highly specialized, evolving to effectively kill specific prey. However, by studying samples from 68 rattlesnakes belonging to two different species in the western United States, Dr. Schield and his colleagues have found that venom evolution is in fact driven by “balancing selection,” an evolutionary process where multiple versions of a gene are maintained instead of eliminated.
“Our findings help explain decades of seemingly contradictory theory and evidence for what drives the extreme variation observed in snake venoms. It turns out that the arms-race between snakes and prey ends up favoring the constant re-shuffling of venom variants that are favored, leading to the retention of lots of venom variants over time, some of which are ancient,” said study co-author Todd Castoe, a professor of Biology at UT Arlington.
Thus, while directional selection may have driven the origins of venom, in more recent times, there may have been an equilibrium shift towards balancing selection favoring a more diverse venom repertoire. “These evolutionary mechanisms ramp up the complexity that you’re contending with when you develop antivenoms, as venom composition within the same species but in different geographic regions might be totally different,” Dr. Schield concluded.
The study is published in the journal Nature Ecology & Evolution.