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.
Citation
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.