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1. Hierarchical spidroin micellar nanoparticles as the fundamental precursors of spider silks

   https://doi.org/10.1073/pnas.1810203115  

   Parent, Lucas R.; Onofrei, David; Xu, Dian; Stengel, Dillan; Roehling, John D.; Addison, J. Bennett; Forman, Christopher; Amin, Samrat A.; Cherry, Brian R.; Yarger, Jeffery L.; et al (October 2018, Proceedings of the National Academy of Sciences)

   Many natural silks produced by spiders and insects are unique materials in their exceptional toughness and tensile strength, while being lightweight and biodegradable–properties that are currently unparalleled in synthetic materials. Myriad approaches have been attempted to prepare artificial silks from recombinant spider silk spidroins but have each failed to achieve the advantageous properties of the natural material. This is because of an incomplete understanding of the in vivo spidroin-to-fiber spinning process and, particularly, because of a lack of knowledge of the true morphological nature of spidroin nanostructures in the precursor dope solution and the mechanisms by which these nanostructures transform into micrometer-scale silk fibers. Herein we determine the physical form of the natural spidroin precursor nanostructures stored within spider glands that seed the formation of their silks and reveal the fundamental structural transformations that occur during the initial stages of extrusion en route to fiber formation. Using a combination of solution phase diffusion NMR and cryogenic transmission electron microscopy (cryo-TEM), we reveal direct evidence that the concentrated spidroin proteins are stored in the silk glands of black widow spiders as complex, hierarchical nanoassemblies (∼300 nm diameter) that are composed of micellar subdomains, substructures that themselves are engaged in the initial nanoscale transformations that occur in response to shear. We find that the established micelle theory of silk fiber precursor storage is incomplete and that the first steps toward liquid crystalline organization during silk spinning involve the fibrillization of nanoscale hierarchical micelle subdomains. 

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2. The common house spider, Parasteatoda tepidariorum, maintains silk gene expression on sub-optimal diet

   https://doi.org/10.1371/journal.pone.0237286  

   Miller, Jeremy; Vienneau-Hathaway, Jannelle; Dendev, Enkhbileg; Lan, Merrina; Ayoub, Nadia A. (December 2020, PLOS ONE)

   Signore, Giovanni (Ed.)

   Cobweb weaving spiders and their relatives spin multiple task-specific fiber types. The unique material properties of each silk type result from differences in amino acid sequence and structure of their component proteins, primarily spidroins (spider fibrous proteins). Amino acid content and gene expression measurements of spider silks suggest some spiders change expression patterns of individual protein components in response to environmental cues. We quantified mRNA abundance of three spidroin encoding genes involved in prey capture in the common house spider, Parasteatoda tepidariorum (Theridiidae), fed different diets. After 10 days of acclimation to the lab on a diet of mealworms, spiders were split into three groups: (1) individuals were immediately dissected, (2) spiders were fed high-energy crickets, or (3) spiders were fed low-energy flies, for 1 month. All spiders gained mass during the acclimation period and cricket-fed spiders continued to gain mass, while fly-fed spiders either maintained or lost mass. Using quantitative PCR, we found no significant differences in the absolute or relative abundance of dragline gene transcripts, major ampullate spidroin 1 ( MaSp1 ) and major ampullate spidroin 2 ( MaSp2 ), among groups. In contrast, prey-wrapping minor ampullate spidroin ( MiSp) gene transcripts were significantly less abundant in fly-fed than lab-acclimated spiders. However, when measured relative to Actin , cricket-fed spiders showed the lowest expression of MiSp . Our results suggest that house spiders are able to maintain silk production, even in the face of a low-quality diet. 

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3. The transcriptome of Darwin’s bark spider silk glands predicts proteins contributing to dragline silk toughness

   https://doi.org/10.1038/s42003-019-0496-1  

   Garb, Jessica E.; Haney, Robert A.; Schwager, Evelyn E.; Gregorič, Matjaž; Kuntner, Matjaž; Agnarsson, Ingi; Blackledge, Todd A. (July 2019, Communications Biology)

   Abstract Darwin’s bark spider (Caerostris darwini) produces giant orb webs from dragline silk that can be twice as tough as other silks, making it the toughest biological material. This extreme toughness comes from increased extensibility relative to other draglines. We showC. darwinidragline-producing major ampullate (MA) glands highly express a novel silk gene transcript (MaSp4) encoding a protein that diverges markedly from closely related proteins and contains abundant proline, known to confer silk extensibility, in a unique GPGPQ amino acid motif. This suggestsC. darwinievolved distinct proteins that may have increased its dragline’s toughness, enabling giant webs.Caerostris darwini’sMA spinning ducts also appear unusually long, potentially facilitating alignment of silk proteins into extremely tough fibers. Thus, a suite of novel traits from the level of genes to spinning physiology to silk biomechanics are associated with the unique ecology of Darwin’s bark spider, presenting innovative designs for engineering biomaterials. 

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4. The evolutionary history of cribellate orb-weaver capture thread spidroins

   https://doi.org/10.1186/s12862-022-02042-5  

   Correa-Garhwal, Sandra_M; Baker, Richard_H; Clarke, Thomas_H; Ayoub, Nadia_A; Hayashi, Cheryl_Y (July 2022, BMC Ecology and Evolution)

   Abstract BackgroundSpiders have evolved two types of sticky capture threads: one with wet adhesive spun by ecribellate orb-weavers and another with dry adhesive spun by cribellate spiders. The evolutionary history of cribellate capture threads is especially poorly understood. Here, we use genomic approaches to catalog the spider-specific silk gene family (spidroins) for the cribellate orb-weaverUloborus diversus. ResultsWe show that the cribellar spidroin, which forms the puffy fibrils of cribellate threads, has three distinct repeat units, one of which is conserved across cribellate taxa separated by ~ 250 Mya. We also propose candidates for a new silk type, paracribellar spidroins, which connect the puffy fibrils to pseudoflagelliform support lines. Moreover, we describe the complete repeat architecture for the pseudoflagelliform spidroin (Pflag), which contributes to extensibility of pseudoflagelliform axial fibers. ConclusionsOur finding that Pflag is closely related to Flag, supports homology of the support lines of cribellate and ecribellate capture threads. It further suggests an evolutionary phase following gene duplication, in which both Flag and Pflag were incorporated into the axial lines, with subsequent loss of Flag in uloborids, and increase in expression of Flag in ecribellate orb-weavers, explaining the distinct mechanical properties of the axial lines of these two groups. 

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5. Allelic resolution of insect and spider silk genes reveals hidden genetic diversity

   https://doi.org/10.1073/pnas.2221528120  

   Frandsen, Paul B.; Hotaling, Scott; Powell, Ashlyn; Heckenhauer, Jacqueline; Kawahara, Akito Y.; Baker, Richard H.; Hayashi, Cheryl Y.; Ríos-Touma, Blanca; Holzenthal, Ralph; Pauls, Steffen U.; et al (May 2023, Proceedings of the National Academy of Sciences)

   Arthropod silk is vital to the evolutionary success of hundreds of thousands of species. The primary proteins in silks are often encoded by long, repetitive gene sequences. Until recently, sequencing and assembling these complex gene sequences has proven intractable given their repetitive structure. Here, using high-quality long-read sequencing, we show that there is extensive variation—both in terms of length and repeat motif order—between alleles of silk genes within individual arthropods. Further, this variation exists across two deep, independent origins of silk which diverged more than 500 Mya: the insect clade containing caddisflies and butterflies and spiders. This remarkable convergence in previously overlooked patterns of allelic variation across multiple origins of silk suggests common mechanisms for the generation and maintenance of structural protein-coding genes. Future genomic efforts to connect genotypes to phenotypes should account for such allelic variation. 

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