The Eastern Diamondback Rattlesnake, Crotalus adamanteus, is the largest rattlesnake species and has an exclusively endothermic diet. Although native to seven states in the southeastern Coastal Plain, the species has been extirpated from Louisiana, is listed as endangered in North Carolina, and is currently under consideration for listing as threatened under the Endangered Species Act.
The question of how all of the parts of an organism work together with environmental factors, even when they are changing during an individual’s life is fascinating. In venomous snakes, ontogenetic changes in diet and intraspecific variation in venoms have been documented. However, the timing of such changes in a life history context and a comparison of the extent of ontogenetic and geographical variation in natural populations have not been investigated.
In an early on-line version of a new paper by Margres et al. (2015), the authors examine the feeding system of the Eastern Diamondback Rattlesnake. They combine venom, morphology (head shape and fang length) and ontogeny over the various environments and geography the snake inhabits. Using a genotype-phenotype map approach, protein expression data, and morphological data they found: ontogenetic effects explained more of the variation in toxin expression variation than geographic effects; both juveniles and adults vary geographically; variation in toxin expression was a result of directional selection; and different venom phenotypes co-varied with morphological traits also are associated with feeding in temporal (ontogenetic) and geographic (functional) contexts.
Venom is ultimately responsible for knocking down prey, and a suite of morphological traits such as gape and fang length should be equally important to the feeding ecology of venomous species. Phenotypic integration is the dependent relationship between different traits that collectively produce a complex phenotype. In venomous snakes, phenotypic integration includes characters as diverse as venom, head shape, and fang length. The optimal depth of venom injection (i.e., fang length) may depend on venom composition which, along with the head shape, may vary with prey size. Morphological differences are associated with variation in venom composition, and phenotypic integration of the complete feeding system, have not been investigated at any level.
This appears to be the first demonstration of phenotypic integration between multiple morphological characters and a biochemical phenotype across populations and age classes. The authors identified copy number variation as the mechanism driving the differences in the venom phenotypes associated with these morphological differences. They also found parallel mitochondrial, venom, and morphological divergence between northern and southern clades suggests that each clade may warrant classification as a separate evolutionarily significant unit.
Sampling sites for Crotalus adamanteus. The authors collected venom and blood samples from 123 C. adamanteus from seven putative populations; 127 preserved C. adamanteus specimens were used for morphological analyses. Phylogenetic analyses identified two distinct clades, one north of the Suwannee River and one south of the Suwannee River, with dating estimates placing the split at approximately 1.27 Ma. Abbreviations: AR, Apalachicola River; Ca, Crotalus adamanteus; SMR, Saint Mary's River; SR, Suwannee River.
Margres MJ, Wray KP, Seavy M, McGivern JJ, Sanader D, Rokyta DR. (2015). Phenotypic integration in the feeding system of the eastern diamondback rattlesnake (Crotalus adamanteus). Molecular Ecology.