Diversification of spider venoms: Patterns and correlates of variation within individuals and across populations and phylogenies

Abstract

Spider venoms are complex mixtures of chemicals whose primary function is to immobilize prey. The chemical composition of venoms and the role of venoms in prey immobilization vary widely among spiders. Yet, little is known about patterns of venom diversity and evolutionary processes that influence venom diversification. I used a comparative approach to (1) investigate phenotypic and evolutionary plasticity in whole venom chemical composition by comparing venoms among taxa that differ in time since divergence; and (2) determine whether evolutionary changes in venom are predictable based on changes in behavior or ecology. Non-genetic variation in venoms of a funnel-web spider, Agelenopsis aperta, was investigated by determining the effect on adult venom expression that results from rearing broodmates on different diets. This analysis provided no evidence of an effect of rearing diet on the chemical composition of adult venoms. Variation within species in venoms among sexes and populations was investigated in two spider groups. Tegenaria agrestis, a species suspected of interpopulational differences in venom pharmacology, had limited differences in venom composition and potency between same-sex individuals from isolated populations. However, within populations, sexes differed in concentrations of shared components, and female venoms were more potent on insects. Similarly, venom sexual dimorphism was detected in species of Tetragnatha. Male venoms had large concentrations of high molecular weight proteins that were absent in females. The dimorphism in Tetragnatha is likely not a function of sexual niche differences. Finally, ecological and behavioral correlates of differences in venom chemistry among clades of spiders that differ in feeding behavior were investigated in Hawaiian Tetragnatha. Associated with loss of web-use in prey capture in a lineage of this genus, was a reduction in the concentration of low molecular weight components, and an increase in the concentration of components between 35 and 80 kDa. Coincident with this change was an increase in the taxonomic range of prey and a decrease in the rate of onset of paralysis in prey after bites. However, there were no detectable differences in the role of venom in prey immobilization, and the general physiological effects of bites on prey.