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Abstract Spiders amplify their physical capabilities by synthesizing multiple high performing silks. Renowned for its toughness, major ampullate (MA) silk composes the spiderweb frame, providing support and absorbing high-energy impacts. In ecribellate orb-weavers, proline-rich motifs in MaSp2 proteins of MA silk are linked to a range of mechanical properties, including extensibility, elasticity, stiffness, and supercontraction. We show a modification of these motifs outside of this clade in a spider that constructs a spring-loaded web. The triangle weaver spider Hyptiotes cavatus (family Uloboridae) stores energy in the support lines of its triangular web, then rapidly releases the tension to catapult forward, collapsing the web around prey. Hyptiotes has an expanded set of MaSp2 genes which encode proteins with far higher proline contents than typical MaSp2. The predominant GPGPQ motifs present in Hyptiotes spidroins also occur abundantly in MaSp sequences of distantly related spiders that produce the most extensible dragline, implying silk protein convergence. Proline-rich MaSp2 proteins constitute half of all MA gland expression in Hyptiotes, and we show that the resulting fibers are the most proline-rich spider silk measured to date. This unique silk composition suggests a functional importance that may facilitate the spring-loaded prey capture mechanism of this species' web and may inspire the design of novel biomaterials using protein engineering. 

Abstract Although spiders are one of the most diverse groups of arthropods, the genetic architecture of their evolutionary adaptations is largely unknown. Specifically, ancient genome-wide duplication occurring during arachnid evolution ~450 mya resulted in a vast assembly of gene families, yet the extent to which selection has shaped this variation is understudied. To aid in comparative genome sequence analyses, we provide a chromosome-level genome of the Western black widow spider (Latrodectus hesperus)—a focus due to its silk properties, venom applications, and as a model for urban adaptation. We used long-read and Hi-C sequencing data, combined with transcriptomes, to assemble 14 chromosomes in a 1.46 Gb genome, with 38,393 genes annotated, and a BUSCO score of 95.3%. Our analyses identified high repetitive gene content and heterozygosity, consistent with other spider genomes, which has led to challenges in genome characterization. Our comparative evolutionary analyses of eight genomes available for species within the Araneoidea group (orb weavers and their descendants) identified 1,827 single-copy orthologs. Of these, 155 exhibit significant positive selection primarily associated with developmental genes, and with traits linked to sensory perception. These results support the hypothesis that several traits unique to spiders emerged from the adaptive evolution of ohnologs—or retained ancestrally duplicated genes—from ancient genome-wide duplication. These comparative spider genome analyses can serve as a model to understand how positive selection continually shapes ancestral duplications in generating novel traits today within and between diverse taxonomic groups. 

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine–proline–glycine–glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin's bark spider ( Caerostris darwini ) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine–proline–glycine–proline motif and may explain C. darwini MA silk's extraordinary toughness. However, no direct correlation has been made between the silk's molecular structure and its mechanical properties for C. darwini . Here, we correlate the relative protein secondary structure composition of MA silk from C. darwini and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that C. darwini MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure–property correlation model.