Sunday, March 13, 2011

Venom Composition in a Litter of Death Adders


Photo Credit: 

Mohammad Al-Saleh
Knowledge of venoms is incomplete and sometimes experiments produce conflicting results. At least one study suggested that juvenile venom was more toxic than adult venom, giving rise to the commonly held, but misleading, belief that neonates snakes were more poisonous than the adults. The primary functions of venom are immobilization of prey, initiation of digestion, and to deter potential predators. Thus, venom has evolved numerous  proteins with different roles to produce substantial variation in venom composition between genera and species as well as within species. Remarkably venom can vary within a single litter and even within an individual over its life time.Genetics determine venom composition but variation during the life of an individuals suggest plasticity in the expression of the venom genes. The metabolism of a snake increases after venom has been spent. In a new study Anna Pintor and colleagues examine the dependence of increases in metabolic rate following venom expenditure on the stage of venom replenishment that the venom producing tissue is in at the time of venom extraction in the Common Death Adder, Acanthophis antarcticus. They found that venom expenditure is followed by a sudden increase in metabolic rate when snakes have previously not expended venom for at least two days, suggesting that repetitive venom expenditure does not further increase the activity of venom gland tissue during this initial time period but that a second upregulation occurs when the tissue is past the initial activation stage. In addition, venom composition appears to remain constant during replenishment within an individual, but they observed  substantial variations between siblings. Thus, venom composition does not appear to change during replenishment in individuals of the Death Adder, A. antarcticus, but variation between individuals is surprisingly high. The authors suggest that venom components are most likely replenished at a similar rate with metabolic costs being related to slow and extended rates of synthesis. They report an apparent absence of additional increases in metabolism after venom expenditure during the early stages of replenishment and suggest the physiological response to venom expenditure is not additive, but is initiated only while the venom glands are past a certain stage of replenishment. The mechanism that activates venom gland tissue is not known and further research in this direction would be of interest. Also see this related post.

Citation
Pintor A. F., K. L.Winter, A. K. Krockenberger, and J. E. Seymour. 2011. Venom physiology and composition in a litter of Death Adders (Acanthophis antarcticus) and their parents. Toxicon 57:68-75.

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