Evidence for diet effects on the composition of silk proteins produced by spiders

Silks are highly expressed, secreted proteins that represent a substantial metabolic cost to the insects and spiders that produce them. Female spiders in the superfamily Araneoidea (the orb-spinning spiders and their close re latives) spin six different kinds of silk (three fibroins and three fibrous protein glues) that differ in amino acid content and protein structure. In addition to this diversity in silks produced by different glands, we found that individual spiders of the same species can spin dragline silks (drawn from the spider's ampullate gland) that vary in content as well. Freely fo raging Argiope argentata (Araneae: Araneoidea), collected from 13 Caribbean islands, produced dragline silk that showed an inverse relationship betwee n the amount of serine and glycine they contained. X-ray microdiffraction o f the silks localized these differences to the amorphous regions of the pro tein that are thought to lend silks their elasticity. The crystalline regio ns of the proteins, which lend silks their strength, were unaffected. Labor atory experiments with Argiope keyserlingi suggested that variation in silk composition reflects the type of prey the spiders were fed but not the tot al amount of prey they received. Hence, it may be that the amino acid conte nt land perhaps the mechanical properties) of dragline silk spun by Argiope directly reflect the spiders' diet. The ability to vary silk composition a nd, possibly, function is particularly important for organisms that dispers e broadly, such as Argiope, and that occupy diverse habitats with diverse p opulations of prey.