Variation in Sistrurus venom does not support specific status for subspecies


Sistrurus is a  clade of rattlesnakes containing two named species: the massasauga (S. catenatus) and pigmy rattlesnake (S. miliarius) rattlesnake. Each species consists of three named subspecies (S. c. catenatus; S. c. tergeminus, and S. c. edwardsii, and S. m. milarius, S. m. barbouri, and S. m. streckeri ). Recently,  a phylogenetic analysis of the genus based on multilocus data generated a species tree with branch lengths based on multiple gene trees, estimated dates of divergence, and conducted tests for the genetic distinctiveness of each subspecies. The results showed evidence for genetic distinctiveness of all subspecies. Further, all taxa surveyed to date show substantial differences in venom composition. Venom composition is a key trait involved in prey capture and digestion and differences have functional consequences in terms of the ability to subdue ecologically-diverse prey. In particular, toxicity to mammals appears to be a key axis along which venom function has evolved with whole venom from taxa showing unusually high (S. c. catenatus) and unusually low (S. m. barbouri) LD50 values for mice. These data suggest that the proportion of mammals in the diets of different Sistrurus may be related to venom composition and imply a role for natural selection in generating differences in venom composition between taxa. However, the extent that levels of phylogenetic divergence alone could account for interspecific differences in venom is unknown.

Gibbs et al (2013) characterized the variation in venom for all members of the Sistrurus clade with a “venomic” analysis of pooled venoms from two members of this genus, S. milarius streckeri and S. m. milarius. These venoms exhibit the same general classes of proteins as those found in other Sistrurus species but differ in their relative abundances of specific protein families. The authors tested for significant phylogenetic signal in the relative abundances of major venom proteins across species and if diet (measured as percent mammals and lizards among all prey consumed) covaries with venom composition after phylogenetic divergence is accounted for. They found no evidence for significant phylogenetic signal in venom variation: K values for seven snake venom proteins and two composite venom variables [PC 1 and 2] were all nonsignificant and lower (mean = 0.11+0.06 sd) than mean K values (>0.35) previously reported for a wide range of morphological, life history, physiological and behavioral traits from other species. Finally, analyses based on Phylogenetic Generalized Least Squares (PGLS) methods reveal that variation in abundance of some venom proteins, most strongly CRISP is significantly related to snake diet. Their results demonstrate that venom variation in these snakes is evolutionarily a highly labile trait even among very closely-related taxa and that natural selection acting through diet variation may play a role in molding the relative abundance of specific venom proteins.

Citation
Gibbs HL, Sanz L, Sovic MG, Calvete JJ (2013) Phylogeny-Based Comparative Analysis of Venom Proteome Variation in a Clade of Rattlesnakes (Sistrurus sp.). PLoS ONE 8(6): e67220. doi:10.1371/journal.pone.0067220