Activity of two key toxin groups in Australian elapid venoms show a strong correlation to phylogeny but not to diet

Abstract

Background

The relative influence of diet and phylogeny on snake venom activity is a poorly understood aspect of snake venom evolution. We measured the activity of two enzyme toxin groups – phospholipase A₂ (PLA₂), and L-amino acid oxidase (LAAO) – in the venom of 39 species of Australian elapids (40% of terrestrial species diversity) and used linear parsimony and BayesTraits to investigate any correlation between enzyme activity and phylogeny or diet.

Results

PLA₂ activity ranged from 0 to 481 nmol/min/mg of venom, and LAAO activity ranged from 0 to 351 nmol/min/mg. Phylogenetic comparative methods, implemented in BayesTraits showed that enzyme activity was strongly correlated with phylogeny, more so for LAAO activity. For example, LAAO activity was absent in both the Vermicella and Pseudonaja/Oxyuranus clade, supporting previously proposed relationships among these disparate taxa. There was no association between broad dietary categories and either enzyme activity. There was strong evidence for faster initial rates of change over evolutionary time for LAAO (delta parameter mean 0.2), but no such pattern in PLA₂ (delta parameter mean 0.64). There were some exceptions to the phylogenetic patterns of enzyme activity: different PLA₂ activity in the ecologically similar sister-species Denisonia devisi and D. maculata; large inter-specific differences in PLA₂ activity in Hoplocephalus and Austrelaps.

Conclusions

We have shown that phylogeny is a stronger influence on venom enzyme activity than diet for two of the four major enzyme families present in snake venoms. PLA₂ and LAAO activities had contrasting evolutionary dynamics with the higher delta value for PLA₂ Some species/individuals lacked activity in one protein family suggesting that the loss of single protein family may not incur a significant fitness cost.