Environmentally induced post-spin property changes in spider silks: influences of web type, spidroin composition and ecology

Abstract

Many spiders use silk to construct webs that must function for days at a time, whereas many other species renew their webs daily. The mechanical properties of spider silk can change after spinning under environmental stress, which could influence web function. We hypothesize that spiders spinning longer-lasting webs produce silks composed of proteins that are more resistant to environmental stresses. The major ampullate (MA) silks of orb web spiders are principally composed of a combination of two proteins (spidroins) called MaSp1 and MaSp2. We expected spider MA silks dominated by MaSp1 to have the greatest resistance to post-spin property change because they have high concentrations of stable crystalline β-sheets. Some orb web spiders that spin three-dimensional orb webs, such as Cyrtophora, have MA silks that are predominantly composed of MaSp1. Hence, we expected that the construction of three-dimensional orb webs might also coincide with MA silk resistance to post-spin property change. Alternatively, the degree of post-spin mechanical property changes in different spider silks may be explained by factors within the spider's ecosystem, such as exposure to solar radiation. We exposed the MA silks of ten spider species from five genera (Nephila, Cyclosa, Leucauge, Cyrtophora, and Argiope) to ecologically high temperatures and low humidity for 4 weeks, and compared the mechanical properties of these silks with unexposed silks. Using species pairs enabled us to assess the influence of web dimensionality and MaSp composition both with and without phylogenetic influences being accounted for. We found neither the MaSp composition nor the three-dimensionality of the orb web to be associated with the degree of post-spin mechanical property changes in MA silk. The MA silks in Leucauge spp. are dominated by MaSp2, which we found to have the least resistance to post-spin property change. The MA silk in Argiope spp. is also dominated by MaSp2, but has high resistance to post-spin property change. The ancestry of Argiope is unresolved, but it is largely a tropical genus inhabiting hot, open regions that present similar stressors to silk as those of our experiment. Ecological factors thus appear to influence the vulnerability of orb web spider MA silks to post-spin property change.