Coevolution of diet and prey-specific venom activity supports the role of selection in snake venom evolution

The processes that drive the evolution of snake venom variability, particularly the role of diet, have been a topic of intense recent research interest. Here, we test whether extensive variation in venom composition in the medically important viper genus Echis is associated with shifts in diet. Examination of stomach and hindgut contents revealed extreme variation between the major clades of Echis in the proportion of arthropod prey consumed. The toxicity (median lethal dose, LD50) of representative Echis venoms to a natural scorpion prey species was found to be strongly associated with the degree of arthropod feeding. Mapping the results onto a novel Echis phylogeny generated from nuclear and mitochondrial sequence data revealed two independent instances of coevolution of venom toxicity and diet. Unlike venom LD50, the speed with which venoms incapacitated and killed scorpions was not associated with the degree of arthropod feeding. The prey-specific venom toxicity of arthropod-feeding Echis may thus be adaptive primarily by reducing venom expenditure. Overall, our results provide strong evidence that variation in snake venom composition results from adaptive evolution driven by natural selection for different diets, and underscores the need for a multi-faceted, integrative approach to the study of the causes of venom evolution

Snakes across the Strait: trans-Torresian phylogeographic relationships in three genera of Australasian snakes (Serpentes: Elapidae: Acanthophis, Oxyuranus, and Pseudechis)

We analyze the phylogeny of three genera of Australasian elapid snakes (Acanthophis—death adders; Oxyuranus—taipans; Pseudechis—blacksnakes), using parsimony, maximum likelihood, and Bayesian analysis of sequences of the mitochondrial cytochrome b and ND4 genes. In Acanthophis and Pseudechis, we find evidence of multiple trans-Torresian sister-group relationships. Analyses of the timing of cladogenic events suggest crossings of the Torres Strait on several occasions between the late Miocene and the Pleistocene. These results support a hypothesis of repeated land connections between Australia and New Guinea in the late Cenozoic. Additionally, our results reveal undocumented genetic diversity in Acanthophis and Pseudechis, supporting the existence of more species than previously believed, and provide a phylogenetic framework for a reinterpretation of the systematics of these genera. In contrast, our Oxyuranus scutellatus samples from Queensland and two localities in New Guinea share a single haplotype, suggesting very recent (late Pleistocene) genetic exchange between New Guinean and Australian populations

Origin of the eastern brownsnake, Pseudonaja textilis (Duméril, Bibron and Duméril) (Serpentes: Elapidae: Hydrophiinae) in New Guinea: evidence of multiple dispersals from Australia, and comments on the status of Pseudonaja textilis pughi Hoser 2003

Pseudonaja textilis is a widespread and common snake in eastern parts of Australia, but its distribution in New Guinea is poorly understood, and the origin of the New Guinea populations and its timing have been the subject of much speculation. Phylogenetic analysis of mitochondrial DNA sequences from three New Guinea populations of P. textilis indicates that New Guinea was colonised from two independent eastern and western migration routes most likely in the Pleistocene. One dispersal event from northern Queensland led to the populations in eastern New Guinea (Milne Bay, Oro and Central Provinces, Papua New Guinea), whereas another, from Arnhem Land to central southern New Guinea, led to the populations from the Merauke area, Indonesian Papua. The results are consistent with the effects of Pleistocene sea level changes on the physical geography of Australasia, and are thus suggestive of a natural rather than anthropogenic origin of the New Guinea populations. The taxonomic status of the New Guinean populations is discussed. Copyright © 2008 - Magnolia Press

Data from: When one phenotype is not enough - divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species

Understanding the origin and maintenance of phenotypic variation, particularly across a continuous spatial distribution, represents a key challenge in evolutionary biology. For this, animal venoms represent ideal study systems: they are complex, variable, yet easily quantifiable molecular phenotypes with a clear function. Rattlesnakes display tremendous variation in their venom composition, mostly through strongly dichotomous venom strategies, which may even coexist within single species. Here, through dense, widespread population-level sampling of the Mojave rattlesnake, Crotalus scutulatus, we show that genomic structural variation at multiple loci underlies extreme geographic variation in venom composition, which is maintained despite extensive gene flow. Unexpectedly, neither diet composition nor neutral population structure explain venom variation. Instead, venom divergence is strongly correlated with environmental conditions. Individual toxin genes correlate with distinct environmental factors, suggesting that different selective pressures can act on individual loci independently of their co-expression patterns or genomic proximity. Our results challenge common assumptions about diet composition as the key selective driver of snake venom evolution and emphasise how the interplay between genomic architecture and local-scale spatial heterogeneity in selective pressures may facilitate the retention of adaptive functional polymorphisms across a continuous space