The genomic origins and evolutionary path to a key innovation in the world's most venomous snakes

Evolutionary innovation is a key driver of the colonization of new environments and the adaptive radiations of major groups. Novel traits typically evolve through the modification of pre-existing characters but the genetic paths underlying their origin have been challenging to trace, and the general requirements for and relative order of different kinds of gene mutations have been difficult to assess. Here, we trace the genomic origins of four procoagulant venom toxins (factor X, factor V, group I phospholipase A2, and Kunitz-type toxins) that collectively underlie a novel, especially potent blood-clotting venom type in the recently evolved Australian brown snake and taipan clade. We discover evidence for a previously unknown fifth toxin, coagulation factor VII, and show that the toxins evolved through two distinct genetic paths. The factor X and factor V toxins evolved through the sequential de novo co-option of ancestral clotting factor proteins that entailed their heterotopic expression in the venom gland, the fixation of segmental duplications containing each locus, and subsequent gain-of-function mutations that rendered factor X and factor V constitutively active. In contrast, the phospholipase A2 and Kunitz-type toxins evolved by modifying the functions of neurotoxins that were part of the venom arsenal. Our findings support models in which innovative mutations in single-copy genes precede gene duplication in the evolution of novel proteins and offer a rare view into the genesis of a complex trait that has played a central role in a major adaptive radiation. Significance StatementThis study investigates how an entirely new blood-clotting venom type evolved during the recent radiation of Australias iconic venomous snakes. We traced the key genetic events that occurred on the evolutionary path to one of the worlds most potent venoms. We found that the novel venom activity evolved through the sequential co-option of multiple proteins of the snakes own blood-clotting system, followed by the modification of two venom neurotoxins into proteins with procoagulant activities. We suggest that these unique de novo gene co-options are seminal events that can unlock new ecological strategies, which in turn, may enable major adaptive radiations.