Venom Characterization of Three Rear-Fanged Snake Species from the Rio Grande Valley of Texas

The lower Rio Grande Valley of southern Texas is a biologically diverse, semitropical region where a number of species, including neotropical dipsadid and colubrid snakes, narrowly range into the United States from Mexico. Rear-fanged snakes are an understudied group of species in terms of venom composition when compared to front-fanged species such as atractaspidids, elapids, and vipers. Here I describe the composition of venoms of three rear-fanged species, Coniophanes imperialis, Drymobius margaritiferus, and Leptodeira septentrionalis, via enzymatic, electrophoretic, toxinological and proteomic analyses. Two of these species (Coniophanes, Leptodeira) are dipsadid snakes, and the venom of these species is relatively complex, with five major protein families represented (P-III snake venom metalloproteinase (SVMP), cysteine-rich secretory protein (CRiSP), phospholipase A2 (PLA2), three-finger toxins (3FTx), and acetylcholinesterase (AChase). For both species, the venoms are largely dominated by metalloproteinases. Venoms from these dipsadid species are moderately toxic to mice, but much less toxic to Anolis lizards. Also present in the same geographical location, the colubrid Drymobius margaritiferus venom proteome is relatively simple, being dominated by 3FTx(s), with apparent minor levels of CRiSP, P-III SVMP, PDE, and very low levels of serine proteases. Drymobius venom is moderately toxic to non-Swiss albino (NSA) mice (similar to Coniophanes and Leptodeira) but nearly three times as toxic to Anolis lizards. Differences in toxicities between anoles and mice suggest taxon-specific toxins that target iv specific prey species. These differences in composition between the dipsadid and colubrid species are also apparent in the pathology of envenomated prey, as mice administered Coniophanes or Leptodeira venom present with considerable edema/hemorrhaging, while anoles show no signs of hemorrhage. Mice and lizards injected with Drymobius venom exhibit no hemorrhagic pathology, but present with symptoms suggestive of neurotoxicity (ptosis, lethargy, paralysis, agonal breathing). Venoms from the three species analyzed, with samples from the extreme northern extension of their ranges, show basic compositional patterns similar to those that have been observed in other colubrid and dipsadid rear-fanged snakes, with venoms dominated by three-finger toxins (several New World colubrids) or by metalloproteinases (numerous dipsadids). Results presented here indicate that the broad compositional patterns of venoms typical in many front-fanged snake species (viper type I/type II venoms, elapid vs viperid venoms) are also reflected in the dichotomous patterns observed in rear-fanged snakes.