Wolff Jonas O, Michalik Peter, Ravelo Alexandra M, Herberstein Mariella E, Ramírez Martín J
Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia.
Zoologisches Institut und Museum, Ernst-Moritz-Arndt-Universität, Greifswald, 17489, Germany.
Integr Comp Biol. 2021 Oct 14;61(4):1411-1431. doi: 10.1093/icb/icab003.
Spider web anchors are attachment structures composed of the bi-phasic glue-fiber secretion from the piriform silk glands. The mechanical performance of the anchors strongly correlates with the structural assembly of the silk lines, which makes spider silk anchors an ideal system to study the biomechanical function of extended phenotypes and its evolution. It was proposed that silk anchor function guided the evolution of spider web architectures, but its fine-structural variation and whether its evolution was rather determined by changes of the shape of the spinneret tip or in the innate spinning choreography remained unresolved. Here, we comparatively studied the micro-structure of silk anchors across the spider tree of life, and set it in relation to spinneret morphology, spinning behavior and the ecology of the spider. We identified a number of apomorphies in the structure of silk anchors that may positively affect anchor function: (1) bundled dragline, (2) dragline envelope, and (3) dragline suspension ("bridge"). All these characters were apomorphic and evolved repeatedly in multiple lineages, supporting the notion that they are adaptive. The occurrence of these structural features can be explained with changes in the shape and mobility of the spinneret tip, the spinning behavior, or both. Spinneret shapes generally varied less than their fine-tuned movements, indicating that changes in construction behavior play a more important role in the evolution of silk anchor assembly. However, the morphology of the spinning apparatus is also a major constraint to the evolution of the spinning choreography. These results highlight the changes in behavior as the proximate and in morphology as the ultimate causes of extended phenotype evolution. Further, this research provides a roadmap for future bioprospecting research to design high-performance instant line anchors.
蛛网锚是由梨形丝腺分泌的双相胶-纤维组成的附着结构。锚的力学性能与丝线的结构组装密切相关，这使得蜘蛛丝锚成为研究扩展表型的生物力学功能及其进化的理想系统。有人提出丝锚功能引导了蛛网结构的进化，但其精细结构变化以及其进化是由纺丝器尖端形状的变化还是先天的纺丝编排决定的仍未得到解决。在这里，我们比较研究了整个蜘蛛生命树中丝锚的微观结构，并将其与纺丝器形态、纺丝行为和蜘蛛的生态联系起来。我们在丝锚结构中发现了一些可能对锚功能有积极影响的衍征：（1）成束的拖牵丝，（2）拖牵丝包膜，以及（3）拖牵丝悬带（“桥”）。所有这些特征都是衍征，并且在多个谱系中反复进化，支持了它们具有适应性的观点。这些结构特征的出现可以用纺丝器尖端的形状和移动性变化、纺丝行为或两者的变化来解释。纺丝器形状的变化通常小于其微调运动，这表明构建行为的变化在丝锚组装的进化中起更重要的作用。然而，纺丝装置的形态也是纺丝编排进化的一个主要限制因素。这些结果突出了行为变化是扩展表型进化的直接原因，形态变化是最终原因。此外，这项研究为未来设计高性能即时线锚的生物勘探研究提供了路线图。
