Abstract
A long-standing question in evolutionary studies of snake venoms is the extent to which phylogenetic divergence and diet can account for between-species differences in venom composition. Here we apply phylogeny-based comparative methods to address this question. We use data on venom variation generated using proteomic techniques for all members of a small clade of rattlesnakes (Sistrurus sp.) and two outgroups for which phylogenetic and diet information is available. We first complete the characterization of venom variation for all members of this 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. We then test whether there is 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. We 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. Our 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.
Highlights
Snake venom proteins are one of the most widely studied types of animal toxins
The venom proteins of S. m. streckeri and S. m. miliarius were initially separated by reverse-phase HPLC (Figs. 2 and 3)
Phylogenetic Signal in Venom Composition and Diet For all subsequent analysis we focused on 7 protein families each of which had abundances of .1% and were present in at least 6 of 8 taxa (Table 3)
Summary
Snake venom proteins are one of the most widely studied types of animal toxins (for general reviews see [1,2]) These proteins largely belong to a few major protein families, including enzymes (serine proteinases, Zn2+-metalloproteinases, L-amino acid oxidase, group II PLA2s) and proteins without enzymatic activity (disintegrins, C-type lectin-like proteins, vasoactive peptides, myotoxins, CRISP, nerve and vascular endothelium growth factors, cystatin and Kunitz-type protease inhibitors) [3,4,5,6,7,8]. Interspecific differences in venom composition should be closely related to the degree of phylogenetic divergence between species
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