Darwin's bark spider shares a spidroin repertoire with <i>Caerostris extrusa</i> but achieves extraordinary silk toughness through gene expression

Abstract

Spider silk is a protein-based material whose toughness suggests possible novel applications. A particularly fascinating example of silk toughness is provided by Darwin's bark spider (<i>Caerostris darwini</i>) found in Madagascar. This spider produces extraordinarily tough silk, with an average toughness of 350 MJ m^-1 and over 50% extensibility, and can build river-bridging webs with a size of 2.8 m². Recent studies have suggested that specific spidroins expressed in <i>C. darwini</i> are responsible for the mechanical properties of its silk. Therefore, a more comprehensive investigation of spidroin sequences, silk thread protein contents and phylogenetic conservation among closely related species is required. Here, we conducted genomic, transcriptomic and proteomic analyses of <i>C. darwini</i> and its close relative <i>Caerostris extrusa</i>. A variety of spidroins and low-molecular-weight proteins were found in the dragline silk of these species; all of the genes encoding these proteins were conserved in both genomes, but their genes were more expressed in <i>C. darwini</i>. The potential to produce very tough silk is common in the genus <i>Caerostris</i>, and our results may suggest the existence of plasticity allowing silk mechanical properties to be changed by optimizing related gene expression in response to the environment.