Ontogenetic Variation in Venom Composition and Diet of Crotalus oreganus concolor: A Case of Venom Paedomorphosis?
Ontogenetic shifts in diet are common for snakes, and such shifts in diet for venomous snakes may be associated with changes in venom composition. The present study investigated whether an ontogenetic shift in diet and venom composition, as observed for Crotalus oreganus helleri and Crotalus oreganus oreganus, occurs in Crotalus oreganus concolor. Like C. o. helleri and C. o. oreganus, and at similar body sizes, C. o. concolor show an ontogenetic shift in diet. Juvenile snakes primarily feed on small lizards, whereas adults typically consume small rodents. However, C. o. concolor do not show the same pattern of venom ontogeny as do C. o. helleri and C. o. oreganus. Because of the presence of a phospholipase A2-based β-neurotoxin (concolor toxin) and several myotoxins, C. o. concolor venom is particularly toxic, but mouse LD50 assays demonstrated no significant difference in toxicity between adult (0.38 μg/g) and juvenile (0.45 μg/g) venoms. Metalloprotease activity (correlated with extensive tissue damage and prey predigestion) was extremely low in both juvenile and adult venoms. Levels of peptide myotoxins and several serine proteases that interfere with hemostasis (specifically thrombin-like and plasmin-like activities) showed a positive correlation with size. Human envenomations recorded during this study showed symptoms consistent with biochemical analyses, with numbness associated with the bite, coagulation abnormalities and essentially no tissue damage. Results suggest that the occurrence of potent neurotoxic component(s) in a venom minimizes predigestive components (metalloproteases). Further, concurrence of these functional components in the venom of an individual may be selected against, and highly toxic venom in both juvenile and adult C. o. concolor may represent a form of venom paedomorphosis.Abstract
Diet as a function of length in Crotalus oreganus concolor. There is a strong dependency of neonate and young snakes on lizards, followed by a switch to mammalian prey as snakes mature
(A) Relationship of snake mass to snake total length. (B) Relationship of venom volume to snake total length. (C) Relationship of venom volume to venom mass. Solid lines are regression lines, and dashed lines in C indicate 95% confidence intervals
Enzyme activities of crude venoms of Crotalus oreganus concolor as a function of snake total length. All activities are normalized to total protein content of venoms and are expressed as specific activities. (A) Plasmin-like activity; (B) thrombin-like activity; (C) kallikrein-like activity; (D) phosphodiesterase activity; (E) azocasein metalloprotease activity; (F) hide powder azure metalloprotease activity; (G) L-amino acid oxidase activity; (H) phospholipase A2 activity. Solid lines are regression lines, and dashed lines indicate 95% confidence intervals
SDS-PAGE analysis of Crotalus oreganus concolor crude venoms under reducing conditions. (A) Lanes 1–4, 6–9: adult venoms; lane 5: juvenile venom. (B) Lanes 10–14: adult venoms; lanes 15–20: neonate venoms. Note that the prominent myotoxin band (apparent mass = 6.0 kD) at the bottom of each adult venom lane is very faint in neonate and juvenile venoms. Mr, Invitrogen Mark 12 protein standards; mass in kilodaltons
Zymogram analysis of Crotalus oreganus concolor crude venom metalloprotease activity. (A) Lanes 1–9: neonate venoms; lanes A1–D1: adult venoms. (B) All samples are adult venoms. Bands containing activity are seen as a clear band on the dark background; no clear differences between neonate and adult venoms are apparent. Mr, Invitrogen Mark 12 protein standards; mass in kilodaltons
Size exclusion HPLC fractionation of crude venom from (A) neonate and (B) adult Crotalus oreganus concolor. SDS-PAGE analysis of peak fractions was used to identify proteins present in each sample. The concolor toxin peak in A fractionated as two peaks with identical SDS-PAGE migration patterns; these may represent isoforms. Note that the myotoxin peak is greatly diminished in neonate venom (A), despite the greater total protein load. PDE, venom phosphodiesterase; LAAO, L-amino acid oxidase
Comparison of metalloprotease activity (toward hide powder azure) of crude venoms from Crotalus oreganus concolor (this study) and Crotalus oreganus helleri and Crotalus oreganus oreganus (data from Mackessy, 1988); spacing for the two smallest size classes is not linear with respect to the other size classes. Note that activities for samples of C. o. concolor venom for all size classes are below the lowest levels seen in Pacific Rattlesnake venoms
Contributor Notes
(Spm, Kw) Venom Analysis Laboratory, Department of Biological Sciences, University of Northern Colorado, Cb 92, 501 20th Street, Greeley, Colorado 80639–0017; and (Kga) Department of E.P.O. Biology, University of Colorado, Cb 334, Boulder, Colorado 80309–0334. Present address: (Kga) Archbold Biological Station, 123 Main Drive, Venus, Florida 33960. (SPM)stephen.mackessy@unco.edu Send reprint requests to SPM.