Evidence from Multiple Species that Spider Silk Glue Component ASG2 is a Spidroin

Revealing the Hidden Natural Ionic Liquids in Spider Glue: Insights from the Adhesion Process

The adhesive properties of aggregate glue droplets in spider orb webs are conferred by a complex composition of highly glycosylated and phosphorylated proteins, which dissolve in low molecular mass compounds. Although aggregate glue droplets exhibit heterogeneous structural distributions upon attachment to substrates, limited knowledge exists regarding alterations in the distribution of their chemical components before and after attachment. Understanding the spatial distribution of chemical components within these droplets before and after attachment is crucial to unraveling the underlying adhesion mechanisms. In this study, we employed in situ measurement methods to investigate the distribution of low molecular mass compounds and proteins within aggregate glue droplets from Neoscona nautica, thereby visualizing the role of specific low molecular mass compounds in facilitating glycoprotein modification within the aggregate glue. Our findings indicate that the constituents of aggregate glue droplets include at least one ionic liquid: hydrated choline dihydrogen phosphate, and the extent of glycoprotein modification within the aggregate glue is contingent upon the concentration of this ionic liquid.

From fibres to adhesives: evolution of spider capture threads from web anchors by radical changes in silk gland function

Spider webs that serve as snares are one of the most fascinating and abundant type of animal architectures. In many cases they include an adhesive coating of silk lines-so-called viscid silk-for prey capture. The evolutionary switch from silk secretions forming solid fibres to soft aqueous adhesives remains an open question in the understanding of spider silk evolution. Here we functionally and chemically characterized the secretions of two types of silk glands and their behavioural use in the cellar spider, Pholcus phalangioides. Both being derived from the same ancestral gland type that produces fibres with a solidifying glue coat, the two types produce respectively a quickly solidifying glue applied in thread anchorages and prey wraps, or a permanently tacky glue deployed in snares. We found that the latter is characterized by a high concentration of organic salts and reduced spidroin content, showing up a possible pathway for the evolution of viscid properties by hygroscopic-salt-mediated hydration of solidifying adhesives. Understanding the underlying molecular basis for such radical switches in material properties not only helps to better understand the evolutionary origins and versatility of ecologically impactful spider web architectures, but also informs the bioengineering of spider silk-based products with tailored properties.

Special Prey, Special Glue: NMR Spectroscopy on Aggregate Glue Components of Moth-Specialist Spiders, Cyrtarachninae

Orb-weaver spiders produce upwards of seven different types of silk, each with unique material properties. We focus on the adhesive within orb-weaving spider webs, aggregate glue silk. These droplets are composed of three main components: water, glycoproteins, and a wide range of low molecular mass compounds (LMMCs). These LMMCs are known to play a crucial role in maintaining the material properties of the glycoproteins, aid in water absorption from the environment, and increase surface adhesion. Orb-weavers within the Cyrtarachninae subfamily are moth specialists and have evolved glue droplets with novel material properties. This study investigated the biochemical composition and diversity of the LMMCs present in the aggregate glue of eight moth-specialist species and compared them with five generalist orb-weavers using nuclear magnetic resonance (NMR) spectroscopy. We hypothesized that the novel drying ability of moth-specialist glue was accompanied by novel LMMCs and lower overall percentages by silk weight of LMMCs. We measured no difference in LMMC weight by the type of prey specialization, but observed novel compositions in the glue of all eight moth-catching species. Further, we quantified the presence of a previously reported but unidentified compound that appears in the glue of all moth specialists. These silks can provide insight into the functions of bioadhesives and inform our own synthetic adhesives.

A review on complete silk gene sequencing and de novo assembly of artificial silk

Silk, especially spider and insect silk, is a highly versatile biomaterial with potential applications in biomedicine, materials science, and biomimetic engineering. The primary structure of silk proteins is the basis for the mechanical properties of silk fibers. Biotechnologies such as single-molecule sequencing have facilitated an increasing number of reports on new silk genes and assembled silk proteins. Therefore, this review aims to provide a comprehensive overview of the recent advances in representative spider and insect silk proteins, focusing on identification methods, sequence characteristics, and de novo design and assembly. The review discusses three identification methods for silk genes: polymerase chain reaction (PCR)-based sequencing, PCR-free cloning and sequencing, and whole-genome sequencing. Moreover, it reveals the main spider and insect silk proteins and their sequences. Subsequent de novo assembly of artificial silk is covered and future research directions in the field of silk proteins, including new silk genes, customizable artificial silk, and the expansion of silk production and applications are discussed. This review provides a basis for the genetic aspects of silk production and the potential applications of artificial silk in material science and biomedical engineering.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

The tissue-specific expression of silkworm cuticle protein gene ASSCP2 is mediated by the Sox-2 transcription factor

The silk gland of silkworm is a unique organ in which silk proteins are synthesized, secreted, and transformed into fibers. The anterior silk gland (ASG) is located at the end of the silk gland, and is thought to be involved in silk fibrosis. In our previous study, a cuticle protein, ASSCP2, was identified. This protein is specifically and highly expressed in the ASG. In this work, the transcriptional regulation mechanism of ASSCP2 gene was studied by a transgenic route. The ASSCP2 promoter was analyzed, truncated sequentially, and used to initiate the expression of EGFP gene in silkworm larvae. After egg injection, seven transgenic silkworm lines were isolated. Molecular analysis revealed that the green fluorescent signal could not be detected when the promoter was truncated to -257 bp, suggesting that the -357 to -257 sequence is the key region responsible for the transcriptional regulation of the ASSCP2 gene. Furthermore, an ASG specific transcription factor Sox-2 was identified. EMSA assays showed that Sox-2 binds with the -357 to -257 sequence, and thus regulates the tissue-specific expression of ASSCP2. This study on the transcriptional regulation of ASSCP2 gene provides theoretical and experimental basis for further studies of the regulatory mechanism of tissue-specific genes.

Spider-capture-silk mimicking fibers with high-performance fog collection derived from superhydrophilicity and volume-swelling of gelatin knots

Spider-capture-silk (SCS) can directionally capture and transport water from humid air relying on the unique geometrical structure. Although there have been adequate reports on the fabrication of artificial SCSs from petroleum-based materials, it remains a big challenge to innovate bio-based SCS mimicking fibers with high-performance fog collection ability and efficiency simultaneously. Herein, we report an eco-friendly and economical fiber system for water collection by coating gelatin on degummed silk. Compared to the previously reported fibers with the best fog collection ability (~ 13.10 μL), Gelatin on silk fiber 10 (GSF10) can collect larger water droplet (~ 16.70 μL in 330 s) with ~ 98% less mass. Meanwhile, the water collection efficiency of GSF10 demonstrates ~ 72% and ~ 48% enhancement to the existing best water collection polymer coated SCS fibers and spidroin eMaSp2 coated degummed silk respectively in terms of volume-to-TCL (vapor–liquid-solid three-phase contact line) index. The simultaneous function of superhydrophilicity, surface energy gradient, and ~ 65% water-induced volume swelling of the gelatin knots are the key factors in advancing the water collection performance. Abundant availability of feedstocks and ~ 75% improved space utilization guarantee the scalability and practical application of such bio-based fiber.

Graphic Abstract

The evolutionary history of cribellate orb-weaver capture thread spidroins

Background

Spiders 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-weaver Uloborus diversus.

Results

We 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.

Conclusions

Our 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.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02042-5.

Change of mechanical characteristics in spider silk capture threads after contact with prey

Most spiders rely on specialized capture threads to subdue prey. Cribellate spiders use capture threads, whose adhesion is based on thousands of nanofibers instead of specialized glue. The nanofibers adhere due to van der Waals and hygroscopic forces, but the adhesion is strengthened by an interaction with the cuticular hydrocarbons (CHCs) covering almost all insects. The interaction between CHCs and cribellate threads becomes visible through migration of the CHCs into the thread even far beyond the point of contact. In this study, we were able to show that the migrated CHCs not only influence adhesion but also change the mechanical characteristics of the thread. While adhesion, extensibility and total energy decreased in threads treated with CHCs from different insects, we observed an increasing force required to break threads. Such mechanical changes could be beneficial for the spider: Upon the first impact of the insect in the web, it is important to absorb all the energy without breaking. Afterwards, a reduction in extensibility could cause the insect to stay closer to the web and thus become additionally entangled in neighboring threads. An increased tensile force would additionally ensure that for insects already in the web, it is even harder to free themselves. Taken together, all these changes make it unlikely that cribellate spiders reuse their capture threads, if not reacting rapidly and removing the prey insect before the CHCs can spread across the thread. STATEMENT OF SIGNIFICANCE: Cribellate spiders use capture threads that, unlike other spiders, consist of nanofibers and do not rely glue. Instead, prey adheres mainly because their surface compounds, so-called cuticular hydrocarbons (CHCs), interact with the thread, this way generating strong adhesion forces. Previous studies on biomechanics and adhesion of cribellate threads only dealt with artificial surfaces, neglecting any interaction with surface compounds. This study examines the dramatical mechanical changes of a cribellate thread after interaction with prey CHCs, showing modifications of the thread's extensibility, tensile force and total energy. Our results highlight the importance of studying mechanical properties of silk not only in an artificial context, but also in real life.

Adhesive contact and protein elastic modulus tune orb weaving spider glue droplet biomechanics to habitat humidity

Tiny glue droplets along the viscous capture threads of spider orb webs prevent insects from escaping. Each droplet is formed of a protein core surrounded by a hygroscopic aqueous layer, which cause the droplet's adhesion to change with humidity. As an insect struggles to escape the web, a thread's viscoelastic core proteins extend, transferring adhesive forces to the thread's support fibers. Maximum adhesive force is achieved when absorbed atmospheric moisture allows a flattened droplet to establish sufficient adhesive contact while maintaining the core protein cohesion necessary for force transfer. We examined the relationship between these droplet properties and adhesive force and the work of extending droplets at five relative humidities in twelve species that occupy habitats which have different humidities. A regression analysis that included both flattened droplet area and core protein elastic modulus described droplet adhesion, but the model was degraded when core protein area was substituted for droplet. Species from low humidity habitats expressed greater adhesion at lower humidities, whereas species from high humidity habitats expressed greater adhesion at high humidities. Our results suggest a general model of droplet adhesion with two adhesion peaks, one for low humidity species, which occurs when increasing droplet area and decreasing protein cohesion intersect, and another for high humidity species, which occurs when area and cohesion have diverged maximally. These dual peaks in adhesive force explain why some species from intermediate and high humidity habitats express high adhesion at several humidities. STATEMENT OF SIGNIFICANCE: We characterized the effect of humidity on the adhesion of twelve orb weaving spider species' glue droplets and showed how humidity-mediated changes in the contact area of a droplet's outer, hygroscopic aqueous layer and the stiffness of its protein core affect droplet performance. This revealed how droplet adhesion has been tuned to the humidity of a species' habitat and allowed us to revise a model that describes the environmental determinants of droplet biomechanics.

Molecular Changes in Spider Viscid Glue As a Function of Relative Humidity Revealed Using Infrared Spectroscopy

Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.

Solution Structure of Tubuliform Spidroin N-Terminal Domain and Implications for pH Dependent Dimerization

The spidroin N-terminal domain (NT) is responsible for high solubility and pH-dependent assembly of spider silk proteins during storage and fiber formation, respectively. It forms a monomeric five-helix bundle at neutral pH and dimerizes at lowered pH, thereby firmly interconnecting the spidroins. Mechanistic studies with the NTs from major ampullate, minor ampullate, and flagelliform spidroins (MaSp, MiSp, and FlSp) have shown that the pH dependency is conserved between different silk types, although the residues that mediate this process can differ. Here we study the tubuliform spidroin (TuSp) NT from Argiope argentata, which lacks several well conserved residues involved in the dimerization of other NTs. We solve its structure at low pH revealing an antiparallel dimer of two five-α-helix bundles, which contrasts with a previously determined Nephila antipodiana TuSp NT monomer structure. Further, we study a set of mutants and find that the residues participating in the protonation events during dimerization are different from MaSp and MiSp NT. Charge reversal of one of these residues (R117 in TuSp) results in significantly altered electrostatic interactions between monomer subunits. Altogether, the structure and mutant studies suggest that TuSp NT monomers assemble by elimination of intramolecular repulsive charge interactions, which could lead to slight tilting of α-helices.

Chitin and cuticle proteins form the cuticular layer in the spinning duct of silkworm

Chitin is found in the exoskeleton and peritrophic matrix of arthropods, but recent studies have also identified chitin in the spinning duct of silk-spinning arthropods. Here, we report the presence and function of chitin and cuticle proteins ASSCP1 and ASSCP2 in the spinning duct of silkworm. We show that chitin and these proteins are co-located in the cuticular layer of the spinning duct. Ultrastructural analysis indicates that the cuticular layer has a multilayer structure by layered stacking of the chitin laminae. After knocking down ASSCP1 and ASSCP2, the fine structure of this layer was disrupted, which had negative impacts on the mechanical properties of silk. This work clarifies the function of chitin in the spinning duct of silkworm. Chitin and cuticle proteins are the main components of the cuticular layer, providing the shearing stress during silk fibrillogenesis and regulating the final mechanical properties of silk. STATEMENT OF SIGNIFICANCE: Recent studies have identified chitin in the spinning duct of silk-spinning arthropods. However, the role of chitin in this specific organ remains unclear. This study reports that chitin and cuticle proteins form the cuticular layer, a unique structure of the spinning duct of silkworm. This layer with a precise laminate structure gives the spinning duct flexible properties, provides shearing forces for silk fibrillogenesis, and contributes to silk final mechanical properties. Our work clarifies the component, ultrastructure, and biological significance of the silkworm cuticular layer, describes the specific process of silk fiber formation, and proposes new molecular targets (chitin and cuticle proteins) for the improvement of animal silks.

Recombinant Spider Silk: Promises and Bottlenecks

Spider silk threads have exceptional mechanical properties such as toughness, elasticity and low density, which reach maximum values compared to other fibre materials. They are superior even compared to Kevlar and steel. These extraordinary properties stem from long length and specific protein structures. Spider silk proteins can consist of more than 20,000 amino acids. Polypeptide stretches account for more than 90% of the whole protein, and these domains can be repeated more than a hundred times. Each repeat unit has a specific function resulting in the final properties of the silk. These properties make them attractive for innovative material development for medical or technical products as well as cosmetics. However, with livestock breeding of spiders it is not possible to reach high volumes of silk due to the cannibalistic behaviour of these animals. In order to obtain spider silk proteins (spidroins) on a large scale, recombinant production is attempted in various expression systems such as plants, bacteria, yeasts, insects, silkworms, mammalian cells and animals. For viable large-scale production, cost-effective and efficient production systems are needed. This review describes the different types of spider silk, their proteins and structures and discusses the production of these difficult-to-express proteins in different host organisms with an emphasis on plant systems.

Bio-based and bio-inspired adhesives from animals and plants for biomedical applications

With the "many-headed" slime mold Physarum polycelphalum having been voted the unicellular organism of the year 2021 by the German Society of Protozoology, we are reminded that a large part of nature's huge variety of life forms is easily overlooked - both by the general public and researchers alike. Indeed, whereas several animals such as mussels or spiders have already inspired many scientists to create novel materials with glue-like properties, there is much more to discover in the flora and fauna. Here, we provide an overview of naturally occurring slimy substances with adhesive properties and categorize them in terms of the main chemical motifs that convey their stickiness, i.e., carbohydrate-, protein-, and glycoprotein-based biological glues. Furthermore, we highlight selected recent developments in the area of material design and functionalization that aim at making use of such biological compounds for novel applications in medicine - either by conjugating adhesive motifs found in nature to biological or synthetic macromolecules or by synthetically creating (multi-)functional materials, which combine adhesive properties with additional, problem-specific (and sometimes tunable) features.

Sum frequency generation spectroscopy of the attachment disc of a spider

The pyriform silk of the attachment disc of a spider was studied using infrared-visible vibrational sum frequency generation (SFG) spectroscopy. The spider can attach dragline and radial lines to many kinds of substrates in nature (concrete, alloy, metal, glass, plant branches, leaves, etc.) with the attachment disc. The adhesion can bear the spider's own weight, and resist the wind on its orb web. From our SFG spectroscopy study, the NH group of arginine side chain and/or NH₂ group of arginine and glutamine side chain in the amino acid sequence of the attachment silk proteins are suggested to be oriented in the disc. It was inferred from the observed doublet SFG peaks at around 3300 cm^-1 that the oriented peptide contains two kinds of structures.

High mechanical property silk produced by transgenic silkworms expressing the spidroins PySp1 and ASG1

Spider silk is one of the best natural fibers with excellent mechanical properties; however, due to the visual awareness, biting behavior and territory consciousness of spiders, we cannot obtain spider silk by large-scale breeding. Silkworms have a spinning system similar to that of spiders, and the use of transgenic technology in Bombyx mori, which is an ideal reactor for producing spider silk, is routine. In this study, the piggyBac transposon technique was used to achieve specific expression of two putative spider silk genes in the posterior silk glands of silkworms: aggregate spider glue 1 (ASG1) of Trichonephila clavipes (approximately 1.2 kb) and two repetitive units of pyriform spidroin 1 (PySp1) of Argiope argentata (approximately 1.4 kb). Then, two reconstituted spider silk-producing strains, the AG and PA strains, were obtained. Finally, the toughness of the silk fiber was increased by up to 91.5% and the maximum stress was enhanced by 36.9% in PA, and the respective properties in AG were increased by 21.0% and 34.2%. In summary, these two spider genes significantly enhanced the mechanical properties of silk fiber, which can provide a basis for spidroin silk production.

Humidity mediated performance and material properties of orb weaving spider adhesive droplets

Capture thread glue droplets retain insects that strike an orb web and are key to the success of over 4,600 described spider species. Each droplet is a self-assembling adhesive system whose emergent biomechanical properties are centered on its viscoelastic, protein core. This bioadhesive is dependent on its surrounding hygroscopic aqueous layer for hydration and chemical conditioning. Consequently, a droplet's water content and adhesive performance track environmental humidity. We tested the hypothesis that natural selection has tuned a droplet's adhesive performance and material properties to a species' foraging humidity. At 55% relative humidity (RH) the adhesive properties of 12 species ranged from that of PEG-based hydrogels to that of silicone rubber, exhibiting a 1088-fold inter-specific difference in stiffness (0.02-21.76 MPa) and a 147-fold difference in toughness (0.14-20.51 MJ/m³). When tested over a 70% RH range, droplet extension lengths per protein core volume peaked at lower humidities in species from exposed, low humidity habitats, and at higher humidities in nocturnal species and those found in humid habitats. However, at the RH's where these species' maximum extension per protein volume indices were observed, the stiffness of most species' adhesive did not differ, documenting that selection has tuned elastic modulus by adjusting droplet hygroscopicity. This inverse relationship between droplet hygroscopicity and a species' foraging humidity ensures optimal adhesive stiffness. By characterizing the humidity responsiveness and properties of orb spider glue droplets, our study also profiles the range of its biomimetic potential. STATEMENT OF SIGNIFICANCE: Over 4,600 described species of orb weaving spider rely on tiny glue droplets in their webs to retain insect that the web intercepts. The aqueous layer that covers each droplet's core allows this adhesive to remain pliable and to stretch as an insect struggles to escape. The aqueous solution also attracts water from the air, causing the glue droplet's performance to change with humidity. By characterizing the droplet extensions and adhesive material properties of twelve species at relative humidities between of 20 and 90%, this study examined how this unique adhesive system responds to its environment and how it is tuned to the humidity of a species' habitat.

Spider Silks: An Overview of Their Component Proteins for Hydrophobicity and Biomedical Applications

Spider silks have received extensive attention from scientists and industries around the world because of their remarkable mechanical properties, which include high tensile strength and extensibility. It is a leading-edge biomaterial resource, with a wide range of potential applications. Spider silks are composed of silk proteins, which are usually very large molecules, yet many silk proteins still remain largely underexplored. While there are numerous reviews on spider silks from diverse perspectives, here we provide a most up-to-date overview of the spider silk component protein family in terms of its molecular structure, evolution, hydrophobicity, and biomedical applications. Given the confusion regarding spidroin naming, we emphasize the need for coherent and consistent nomenclature for spidroins and provide recommendations for pre-existing spidroin names that are inconsistent with nomenclature. We then review recent advances in the components, identification, and structures of spidroin genes. We next discuss the hydrophobicity of spidroins, with particular attention on the unique aquatic spider silks. Aquatic spider silks are less known but may inspire innovation in biomaterials. Furthermore, we provide new insights into antimicrobial peptides from spider silk glands. Finally, we present possibilities for future uses of spider silks.

Repeat variation resolves a complete aggregate silk sequence of bolas spider Mastophora phrynosoma

Many species of spider use a modified silk adhesive, called aggregate glue, to aid in prey capture. Aggregate spidroins (spider fibroins) are modified members of the spider silk family, however they are not spun into fibers as are their solid silk relatives. The genes that encode for aggregate spidroins are the largest of the known spidroin genes and are similarly highly repetitive. In this study, we used long read sequencing to discover the aggregate spidroin genes of the toad-like bolas spider, Mastophora phrynosoma, which employs the glue in a unique way, using only a single, large droplet to capture moths. While Aggregate Spidroin 1 (AgSp1) remains incomplete, AgSp2 is more than an extraordinary 62 kilobases of coding sequence, 20 kb longer than the longest spidroin on record. The structure of repeats from both aggregate silk proteins follows a similar pattern seen in other species, with the same strict conservation of amino acid residue number for much of the repeats' lengths. Interestingly, AgSp2 lacks the elevated number and groupings of glutamine residues seen in the other reported AgSp2 of a classic orb weaving species. The role of gene length in glue functionality remains a mystery, and thus discovering length differences across species will allow understanding and harnessing of this attribute for the next generation of bio-inspired adhesives.

Shifts in morphology, gene expression, and selection underlie web loss in Hawaiian Tetragnatha spiders

BACKGROUND

A striking aspect of evolution is that it often converges on similar trajectories. Evolutionary convergence can occur in deep time or over short time scales, and is associated with the imposition of similar selective pressures. Repeated convergent events provide a framework to infer the genetic basis of adaptive traits. The current study examines the genetic basis of secondary web loss within web-building spiders (Araneoidea). Specifically, we use a lineage of spiders in the genus Tetragnatha (Tetragnathidae) that has diverged into two clades associated with the relatively recent (5 mya) colonization of, and subsequent adaptive radiation within, the Hawaiian Islands. One clade has adopted a cursorial lifestyle, and the other has retained the ancestral behavior of capturing prey with sticky orb webs. We explore how these behavioral phenotypes are reflected in the morphology of the spinning apparatus and internal silk glands, and the expression of silk genes. Several sister families to the Tetragnathidae have undergone similar web loss, so we also ask whether convergent patterns of selection can be detected in these lineages.

RESULTS

The cursorial clade has lost spigots associated with the sticky spiral of the orb web. This appears to have been accompanied by loss of silk glands themselves. We generated phylogenies of silk proteins (spidroins), which showed that the transcriptomes of cursorial Tetragnatha contain all major spidroins except for flagelliform. We also found an uncharacterized spidroin that has higher expression in cursorial species. We found evidence for convergent selection acting on this spidroin, as well as genes involved in protein metabolism, in the cursorial Tetragnatha and divergent cursorial lineages in the families Malkaridae and Mimetidae.

CONCLUSIONS

Our results provide strong evidence that independent web loss events and the associated adoption of a cursorial lifestyle are based on similar genetic mechanisms. Many genes we identified as having evolved convergently are associated with protein synthesis, degradation, and processing, which are processes that play important roles in silk production. This study demonstrates, in the case of independent evolution of web loss, that similar selective pressures act on many of the same genes to produce the same phenotypes and behaviors.

Long-read transcriptomic analysis of orb-weaving spider Araneus ventricosus indicates transcriptional diversity of spidroins

Spider silk, which is composed of diverse silk proteins (spidroin), is a kind of natural high-mass biomaterial with great potential. However, due to the complexity of both the structure and the composition of the spidroins in natural spider silk, application of this valuable biomass is still limited to date. There are diverse kinds of spider silk in the orb-weaving spider with different mechanical and structural characteristics. In order to systematically illustrate the landscape of all the different spidrons, here we chose Araneus ventricosus, an orb-weaving spider with superior silk mechanical features and genome information, to generate a long-read whole body transcriptome. We deciphered the repeat arrangements of each kind of spidroin, based on which we found that there are substantially transcriptional diversity of each spidroin gene. Some repeat motifs are not documented before. Specifically, we discovered novel full-lengh MaSp transcript as well as a relatively small full-length AcSp isoforms, which are potential promising materials for bioengineering of recombinant spidroin. Our study provided a batch of new spidron resources with detail sequential information. The finding of transcriptional diversity may provide cues in understanding of within-species variation of the mechanical properties of the natural spider silk and further molecular designing of recombinant spidroin.

Spidroin profiling of cribellate spiders provides insight into the evolution of spider prey capture strategies

Orb-weaving spiders have two main methods of prey capture: cribellate spiders use dry, sticky capture threads, and ecribellate spiders use viscid glue droplets. Predation behaviour is a major evolutionary driving force, and it is important on spider phylogeny whether the cribellate and ecribellate spiders each evolved the orb architecture independently or both strategies were derived from an ancient orb web. These hypotheses have been discussed based on behavioural and morphological characteristics, with little discussion on this subject from the perspective of molecular materials of orb web, since there is little information about cribellate spider-associated spidroin genes. Here, we present in detail a spidroin catalogue of six uloborid species of cribellate orb-weaving spiders, including cribellate and pseudoflagelliform spidroins, with transcriptome assembly complemented with long read sequencing, where silk composition is confirmed by proteomics. Comparative analysis across families (Araneidae and Uloboridae) shows that the gene architecture, repetitive domains, and amino acid frequencies of the orb web constituting silk proteins are similar among orb-weaving spiders regardless of the prey capture strategy. Notably, the fact that there is a difference only in the prey capture thread proteins strongly supports the monophyletic origin of the orb web.

Wet-Spinning Synthetic Fibers from Aggregate Glue: Aggregate Spidroin 1 (AgSp1)

Spidroin has the potential of wide applications in the biomedicine field as a natural biomaterial. Various synthetic fibers with outstanding mechanical properties have been produced from different spidroins. However, studies on the structural analysis or biomimetic exploration of aggregate spidroin (AgSp) remain scarce. Here, three recombinant AgSp1 spidroins (1RP, 1RC, 3RP) were constructed and expressed in Escherichia coli, followed by purification via coupling heating and ammonium sulfate precipitation. Circular dichroism (CD) spectrum-based secondary structural analysis shows that 1RP and 3RP have similar structures (mainly random coil) in water and PB buffer, while 1RC is mainly composed of α-helix structure and HFIP can change all of the recombinant AgSp1 into helix structure. Through the wet-spinning method, six types of synthetic fibers were produced from these three recombinant AgSp1 spidroins. Subsequently, the properties and structures of synthetic fibers were characterized by mechanical testing and ATR-FTIR. Synthetic fibers spun from 3RP have considerable tensile strength and extensibility (∼37.56 MPa and ∼4.5%, respectively). To the best of our knowledge, this is the first synthetic fiber obtained from AgSp spidroin. Our results demonstrated that AgSp1 can be regarded as an available source of spidroin for silklike fiber production and may provide valuable perspectives on the AgSp1 biomimetic process for certain applications.

Properties of orb weaving spider glycoprotein glue change during Argiope trifasciata web construction

An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. Each droplet’s viscoelastic glycoprotein adhesive core extends to dissipate the forces of prey struggle as it transfers force to stiffer, support line flagelliform fibers. In large orb webs, switchback capture thread turns are placed at the bottom of the web before a continuous capture spiral progresses from the web’s periphery to its interior. To determine if the properties of capture thread droplets change during web spinning, we characterized droplet and glycoprotein volumes and material properties from the bottom, top, middle, and inner regions of webs. Both droplet and glycoprotein volume decreased during web construction, but there was a progressive increase in the glycoprotein’s Young’s modulus and toughness. Increases in the percentage of droplet aqueous material indicated that these increases in material properties are not due to reduced glycoprotein viscosity resulting from lower droplet hygroscopicity. Instead, they may result from changes in aqueous layer compounds that condition the glycoprotein. A 6-fold difference in glycoprotein toughness and a 70-fold difference in Young’s modulus across a web documents the phenotypic plasticity of this natural adhesive and its potential to inspire new materials.

Loss of Phosphate Determines the Versatility of a Spider Orb-web Glue Ball

Spiders capture their prey by weaving an "invisible" orb-web that has both adhesive and fixed properties. Different types of silk in the orb-web have different functions, wherein the key to capturing a prey is the ball-like glue (glue ball), which coats the silk strands. This glue ball has highly versatile properties, but the mechanisms leading to its versatility remain unclear. The salts found in the web have been previously suggested to play an important role in terms of viscosity, not water. However, the distribution of salt and water in the glue ball has not yet been directly observed. Here, we mapped the salts in different states using a homemade time-of-flight secondary ion mass spectrometer (TOF-SIMS) with a high lateral resolution. To our surprise, the glue ball was found to contain little water. The functional transformation of the glue ball from a viscous glycoprotein (capturing prey) to a hardened protein (retaining prey) relies solely on the stimulation of mechanical forces. The phosphate is a key factor for its versatility.

Toward Spider Glue: Long Read Scaffolding for Extreme Length and Repetitious Silk Family Genes AgSp1 and AgSp2 with Insights into Functional Adaptation

An individual orb weaving spider can spin up to seven different types of silk, each with unique functions and material properties. The capture spiral silk of classic two-dimensional aerial orb webs is coated with an amorphous glue that functions to retain prey that get caught in a web. This unique modified silk is partially comprised of spidroins (spider fibroins) encoded by two members of the silk gene family. The glue differs from solid silk fibers as it is a viscoelastic, amorphic, wet material that is responsive to environmental conditions. Most spidroins are encoded by extremely large, highly repetitive genes that cannot be sequenced using short read technology alone, as the repetitive regions are longer than read length. We sequenced for the first time the complete genomic Aggregate Spidroin 1 (AgSp1) and Aggregate Spidroin 2 (AgSp2) glue genes of orb weaving spider Argiope trifasciata using error-prone long reads to scaffold for high accuracy short reads. The massive coding sequences are 42,270 bp (AgSp1) and 20,526 bp (AgSp2) in length, the largest silk genes currently described. The majority of the predicted amino acid sequence of AgSp1 consists of two similar but distinct motifs that are repeated ∼40 times each, while AgSp2 contains ∼48 repetitions of an AgSp1-similar motif, interspersed by regions high in glutamine. Comparisons of AgSp repetitive motifs from orb web and cobweb spiders show regions of strict conservation followed by striking diversification. Glues from these two spider families have evolved contrasting material properties in adhesion (stickiness), extensibility (stretchiness), and elasticity (the ability of the material to resume its native shape), which we link to mechanisms established for related silk genes in the same family. Full-length aggregate spidroin sequences from diverse species with differing material characteristics will provide insights for designing tunable bio-inspired adhesives for a variety of unique purposes.

The expansion of genes encoding soluble silk components in the greater wax moth, Galleria mellonella

Lepidopteran silk is a complex assembly of proteins produced by a pair of highly specialized labial glands called silk glands. Silk composition has been examined only in a handful of species. Here we report on the analysis of silk gland-specific transcriptomes from three developmental stages of the greater wax moth, Galleria mellonella, combined with proteomics, Edman microsequencing and northern blot analysis. In addition to the genes known earlier, we identified twenty seven candidate cDNAs predicted to encode secretory proteins, which may represent novel silk components. Eight were verified by proteomic analysis or microsequencing, and several others were confirmed by similarity with known silk genes and their expression patterns. Our results revealed that most candidates encode abundant secreted proteins produced by middle silk glands including ten sericins, two seroins, one or more mucins, and several sequences without apparent similarity to known proteins. We did not detect any novel PSG-specific protein, confirming that there are only three fibroin subunits. Our data not only show that the number of sericin genes in the greater wax moth is higher than in other species thus far examined, but also the total content of soluble proteins in silk is twice as high in G. mellonella than in B. mori or A. yamamai. Our data will serve as a foundation for future identification and evolutionary analysis of silk proteins in the Lepidoptera.

Major ampullate silk gland transcriptomes and fibre proteomes of the golden orb-weavers, Nephila plumipes and Nephila pilipes (Araneae: Nephilidae)

Natural spider silk is one of the world’s toughest proteinaceous materials, yet a truly biomimetic spider silk is elusive even after several decades of intense focus. In this study, Next-Generation Sequencing was utilised to produce transcriptomes of the major ampullate gland of two Australian golden orb-weavers, Nephila plumipes and Nephila pilipes, in order to identify highly expressed predicted proteins that may co-factor in the construction of the final polymer. Furthermore, proteomics was performed by liquid chromatography tandem-mass spectroscopy to analyse the natural solid silk fibre of each species to confirm highly expressed predicted proteins within the silk gland are present in the final silk product. We assembled the silk gland transcriptomes of N. plumipes and N. pilipes into 69,812 and 70,123 contigs, respectively. Gene expression analysis revealed that silk gene sequences were among the most highly expressed and we were able to procure silk sequences from both species in excess of 1,300 amino acids. However, some of the genes with the highest expression values were not able to be identified from our proteomic analysis. Proteome analysis of “reeled” silk fibres of N. plumipes and N. pilipes revealed 29 and 18 proteins, respectively, most of which were identified as silk fibre proteins. This study is the first silk gland specific transcriptome and proteome analysis for these species and will assist in the future development of a biomimetic spider silk.

Silk genes and silk gene expression in the spider Tengella perfuga (Zoropsidae), including a potential cribellar spidroin (CrSp)

Most spiders spin multiple types of silk, including silks for reproduction, prey capture, and draglines. Spiders are a megadiverse group and the majority of spider silks remain uncharacterized. For example, nothing is known about the silk molecules of Tengella perfuga, a spider that spins sheet webs lined with cribellar silk. Cribellar silk is a type of adhesive capture thread composed of numerous fibrils that originate from a specialized plate-like spinning organ called the cribellum. The predominant components of spider silks are spidroins, members of a protein family synthesized in silk glands. Here, we use silk gland RNA-Seq and cDNA libraries to infer T. perfuga silks at the protein level. We show that T. perfuga spiders express 13 silk transcripts representing at least five categories of spider silk proteins (spidroins). One category is a candidate for cribellar silk and is thus named cribellar spidroin (CrSp). Studies of ontogenetic changes in web construction and spigot morphology in T. perfuga have documented that after sexual maturation, T. perfuga females continue to make capture webs but males halt web maintenance and cease spinning cribellar silk. Consistent with these observations, our candidate CrSp was expressed only in females. The other four spidroin categories correspond to paralogs of aciniform, ampullate, pyriform, and tubuliform spidroins. These spidroins are associated with egg sac and web construction. Except for the tubuliform spidroin, the spidroins from T. perfuga contain novel combinations of amino acid sequence motifs that have not been observed before in these spidroin types. Characterization of T. perfuga silk genes, particularly CrSp, expand the diversity of the spidroin family and inspire new structure/function hypotheses.

Elastic modulus and toughness of orb spider glycoprotein glue

An orb web's prey capture thread features tiny glue droplets, each formed of an adhesive glycoprotein core surrounded by an aqueous layer. Small molecules in the aqueous layer confer droplet hygroscopicity and maintain glycoprotein viscoelasticity, causing droplet volume and glycoprotein performance to track changes in environmental humidity. Droplet extension combines with that of a thread's supporting flagelliform fibers to sum the adhesive forces of multiple droplets, creating an effective adhesive system. We combined measurements of the force on an extending droplet, as gauged by the deflection of its support line, with measurements of glycoprotein volume and droplet extension to determine the Young's modulus (E) and toughness of three species' glycoproteins. We did this at five relative humidities between 20-90% to assess the effect of humidity on these properties. When droplets of a thread span extend, their extensions are constrained and their glycoprotein filaments remain covered by aqueous material. This was also the case during the first extension phase of the individual droplets that we examined. However, as extension progressed, the aqueous layer was progresses disrupted, exposing the glycoprotein. During the first extension phase E ranged from 0.00003 GPa, a value similar to that of fibronectin, a glycoprotein that anchors cells in the extracellular matrix, to 0.00292 GPa, a value similar to that of resilin in insect ligaments. Second phase E increased 4.7-19.4-fold. When compared at the same humidity the E of each species' glycoprotein was less than 5% of the value reported for its flagelliform fibers. This difference may facilitate the coordinated extension of these two capture thread components that is responsible for summing the thread's adhesive forces.

Hygroscopic compounds in spider aggregate glue remove interfacial water to maintain adhesion in humid conditions

Adhesion in humid environments is fundamentally challenging because of the presence of interfacial bound water. Spiders often hunt in wet habitats and overcome this challenge using sticky aggregate glue droplets whose adhesion is resistant to interfacial failure under humid conditions. The mechanism by which spider aggregate glue avoids interfacial failure in humid environments is still unknown. Here, we investigate the mechanism of aggregate glue adhesion by using interface-sensitive spectroscopy in conjunction with infrared spectroscopy. We demonstrate that glycoproteins act as primary binding agents at the interface. As humidity increases, we observe reversible changes in the interfacial secondary structure of glycoproteins. Surprisingly, we do not observe liquid-like water at the interface, even though liquid-like water increases inside the bulk with increasing humidity. We hypothesize that the hygroscopic compounds in aggregate glue sequester interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design principle for developing water-resistant synthetic adhesives.

Spider aggregate glue avoids failure in humid environments but the fundamental mechanism behind it is still unknown. Here, the authors demonstrate that humidity-dependent structural changes of glycoproteins and sequestering of liquid water by low molecular mass compounds prevents adhesion failure of the glue in humid environments.

Tuning orb spider glycoprotein glue performance to habitat humidity

Orb-weaving spiders use adhesive threads to delay the escape of insects from their webs until the spiders can locate and subdue the insects. These viscous threads are spun as paired flagelliform axial fibers coated by a cylinder of solution derived from the aggregate glands. As low molecular mass compounds (LMMCs) in the aggregate solution attract atmospheric moisture, the enlarging cylinder becomes unstable and divides into droplets. Within each droplet an adhesive glycoprotein core condenses. The plasticity and axial line extensibility of the glycoproteins are maintained by hygroscopic LMMCs. These compounds cause droplet volume to track changes in humidity and glycoprotein viscosity to vary approximately 1000-fold over the course of a day. Natural selection has tuned the performance of glycoprotein cores to the humidity of a species' foraging environment by altering the composition of its LMMCs. Thus, species from low-humidity habits have more hygroscopic threads than those from humid forests. However, at their respective foraging humidities, these species' glycoproteins have remarkably similar viscosities, ensuring optimal droplet adhesion by balancing glycoprotein adhesion and cohesion. Optimal viscosity is also essential for integrating the adhesion force of multiple droplets. As force is transferred to a thread's support line, extending droplets draw it into a parabolic configuration, implementing a suspension bridge mechanism that sums the adhesive force generated over the thread span. Thus, viscous capture threads extend an orb spider's phenotype as a highly integrated complex of large proteins and small molecules that function as a self-assembling, highly tuned, environmentally responsive, adhesive biomaterial. Understanding the synergistic role of chemistry and design in spider adhesives, particularly the ability to stick in wet conditions, provides insight in designing synthetic adhesives for biomedical applications.

Conservation of a pH-sensitive structure in the C-terminal region of spider silk extends across the entire silk gene family

Spiders produce multiple silks with different physical properties that allow them to occupy a diverse range of ecological niches, including the underwater environment. Despite this functional diversity, past molecular analyses show a high degree of amino acid sequence similarity between C-terminal regions of silk genes that appear to be independent of the physical properties of the resulting silks; instead, this domain is crucial to the formation of silk fibers. Here, we present an analysis of the C-terminal domain of all known types of spider silk and include silk sequences from the spider Argyroneta aquatica, which spins the majority of its silk underwater. Our work indicates that spiders have retained a highly conserved mechanism of silk assembly, despite the extraordinary diversification of species, silk types and applications of silk over 350 million years. Sequence analysis of the silk C-terminal domain across the entire gene family shows the conservation of two uncommon amino acids that are implicated in the formation of a salt bridge, a functional bond essential to protein assembly. This conservation extends to the novel sequences isolated from A. aquatica. This finding is relevant to research regarding the artificial synthesis of spider silk, suggesting that synthesis of all silk types will be possible using a single process.

Adhesion enhancement of cribellate capture threads by epicuticular waxes of the insect prey sheds new light on spider web evolution

To survive, web-building spiders rely on their capture threads to restrain prey. Many species use special adhesives for this task, and again the majority of those species cover their threads with viscoelastic glue droplets. Cribellate spiders, by contrast, use a wool of nanofibres as adhesive. Previous studies hypothesized that prey is restrained by van der Waals' forces and entrapment in the nanofibres. A large discrepancy when comparing the adhesive force on artificial surfaces versus prey implied that the real mechanism was still elusive. We observed that insect prey's epicuticular waxes infiltrate the wool of nanofibres, probably induced by capillary forces. The fibre-reinforced composite thus formed led to an adhesion between prey and thread eight times stronger than that between thread and wax-free surfaces. Thus, cribellate spiders employ the originally protective coating of their insect prey as a fatal component of their adhesive and the insect promotes its own capture. We suggest an evolutionary arms race with prey changing the properties of their cuticular waxes to escape the cribellate capture threads that eventually favoured spider threads with viscous glue.

Treatment of pediatric black widow spider envenomation: A national poison center's experience

BACKGROUND

Black widow species (Latrodectus species) envenomation can produce a syndrome characterized by painful muscle rigidity and autonomic disturbances. Symptoms tend to be more severe in young children and adults. We describe black widow spider exposures and treatment in the pediatric age group, and investigate reasons for not using antivenom in severe cases.

METHODS

All black widow exposures reported to the Rocky Mountain Poison Center between January 1, 2012, and December 31, 2015, were reviewed. Demographic data were recorded. Patients were divided into 2 groups. Group 1: contact through families from their place of residence, public schools and/or cases where patients were not referred to healthcare facilities. Group 2: patient contact through healthcare facilities.

RESULTS

93 patients were included. Forty (43%) calls were in Group 1 and 53 (57%) in Group 2. Symptoms were evident in all victims; 43 (46.2%) were grade 1, 16 (17.2%) grade 2 and 34 (36.5%) grade 3, but only 14 patients (41.1%) of this group received antivenom. Antivenom use was associated with improvement of symptoms within minutes, and all treated patients were discharged within hours, without an analgesic requirement or any complications. Reasons for not receiving antivenom included: skin test positive (2/20), strong history of asthma or allergies (2/20), physician preference (2/20), non-availability of the antivenom at the health care facility (14/20).

CONCLUSION

In our study, most symptomatic black widow envenomations were minor. Relatively few patients received antivenom, but antivenom use was associated with shorter symptom duration among moderate and major outcome groups.

From EST to novel spider silk gene identification for production of spidroin-based biomaterials

A cDNA library from a pool of all the seven silk glands from a tropical spider species was constructed. More than 1000 expressed sequence tag (EST) clones were created. Almost 65% of the EST clones were identified and around 50% were annotated. The cellular and functional distribution of the EST clones indicated high protein synthesis activity in spider silk glands. Novel clones with repetitive amino acid sequences, which is one of the most important characteristics of spider silk genes, were isolated. One of these clones, namely TuSp2 in current research, contains two almost identical fragments with one short C-terminal domain. Reverse transcription (RT) PCR and expression analysis showed that it is expressed in the tubuliform gland and involved in eggcase silk formation. Furthermore, its single repetitive domain can be induced to form various types of materials, including macroscopic fibers, transparent film and translucent hydrogel. This study implies promising potentials for future identification of novel spidroins and development of new spidroin-based biomaterials.

Evolutionary shifts in gene expression decoupled from gene duplication across functionally distinct spider silk glands

Spider silk synthesis is an emerging model for the evolution of tissue-specific gene expression and the role of gene duplication in functional novelty, but its potential has not been fully realized. Accordingly, we quantified transcript (mRNA) abundance in seven silk gland types and three non-silk gland tissues for three cobweb-weaving spider species. Evolutionary analyses based on expression levels of thousands of homologous transcripts and phylogenetic reconstruction of 605 gene families demonstrated conservation of expression for each gland type among species. Despite serial homology of all silk glands, the expression profiles of the glue-forming aggregate glands were divergent from fiber-forming glands. Also surprising was our finding that shifts in gene expression among silk gland types were not necessarily coupled with gene duplication, even though silk-specific genes belong to multi-paralog gene families. Our results challenge widely accepted models of tissue specialization and significantly advance efforts to replicate silk-based high-performance biomaterials.

Comparative transcriptomics of Entelegyne spiders (Araneae, Entelegynae), with emphasis on molecular evolution of orphan genes

Next-generation sequencing technology is rapidly transforming the landscape of evolutionary biology, and has become a cost-effective and efficient means of collecting exome information for non-model organisms. Due to their taxonomic diversity, production of interesting venom and silk proteins, and the relative scarcity of existing genomic resources, spiders in particular are excellent targets for next-generation sequencing (NGS) methods. In this study, the transcriptomes of six entelegyne spider species from three genera (Cicurina travisae, C. vibora, Habronattus signatus, H. ustulatus, Nesticus bishopi, and N. cooperi) were sequenced and de novo assembled. Each assembly was assessed for quality and completeness and functionally annotated using gene ontology information. Approximately 100 transcripts with evidence of homology to venom proteins were discovered. After identifying more than 3,000 putatively orthologous genes across all six taxa, we used comparative analyses to identify 24 instances of positively selected genes. In addition, between ~ 550 and 1,100 unique orphan genes were found in each genus. These unique, uncharacterized genes exhibited elevated rates of amino acid substitution, potentially consistent with lineage-specific adaptive evolution. The data generated for this study represent a valuable resource for future phylogenetic and molecular evolutionary research, and our results provide new insight into the forces driving genome evolution in taxa that span the root of entelegyne spider phylogeny.

Molecular Structural Analysis of Spider's Capture Thread and Viscid Droplets Studied by Microscopic FT-IR Spectroscopy

The molecular structural analysis of capture thread, including its viscid droplets of oriental golden orb-web spider Nephila clavata, has been performed by microscopic FT-IR spectroscopy. The obtained spectra of capture threads with and without viscid droplets indicate that the features in the region of 1400 - 1000 cm^-1 will be useful as marker bands for the degree of the dissolving of viscid droplet; further, the bands at 1395 and 1335 cm^-1 are attributable to the components of anchoring granules located at the inner side of viscid droplets. By recrystallization and its infrared measurements, the main chemical component of viscid droplets is assignable to glycosylated proline. It has also been demonstrated that the components of the anchoring granule of a viscid droplet are decomposed by UV irradiation, and that the molecular conformation of silk fiber protein of a capture thread is denatured at over 60°C, whereas the viscid droplets on a capture thread retain their structure.

Candidate egg case silk genes for the spider Argiope argentata from differential gene expression analyses

Orb-web weaving spiders produce a variety of task-specific silks from specialized silk glands. The genetics underlying the synthesis of specific silk types are largely unknown, and transcriptome analysis could be a powerful approach for identifying candidate genes. However, de novo assembly and expression profiling of silk glands with RNA-sequencing (RNAseq) are problematic because the few known gene transcripts for silk proteins are extremely long and highly repetitive. To identify candidate genes for tubuliform (egg case) silk synthesis by the orb-weaver Argiope argentata (Araneidae), we estimated transcript abundance using two sequencing methods: RNAseq reads from throughout the length of mRNA molecules, and 3' digital gene expression reads from the 3' region of mRNA molecules. Both analyses identified similar sets of genes as differentially expressed when comparing tubuliform and nonsilk gland tissue. However, incompletely assembled silk gene transcripts were identified as differentially expressed because of RNAseq read alignments to highly repetitive regions, confounding interpretation of RNAseq results. Homologues of egg case silk protein (ECP) genes were upregulated in tubuliform glands. This discovery is the first description of ECP homologues in an araneid. We also propose additional candidate genes involved in synthesis of tubuliform or other silk types.

Diet-induced covariation between architectural and physicochemical plasticity in an extended phenotype

The adaptive benefits of extended phenotypic plasticity are imprecisely defined due to a paucity of experiments examining traits that are manipulable and measurable across environments. Spider webs are often used as models to explore the adaptive benefits of variations in extended phenotypes across environments. Nonetheless, our understanding of the adaptive nature of the plastic responses of spider webs is impeded when web architectures and silk physicochemical properties appear to co-vary. An opportunity to examine this co-variation is presented by modifying prey items while measuring web architectures and silk physiochemical properties. Here we performed two experiments to assess the nature of the association between web architectures and gluey silk properties when the orb web spider Argiope keyserlingi was fed a diet that varied in either mass and energy or prey size and feeding frequency. We found web architectures and gluey silk physicochemical properties to co-vary across treatments in both experiments. Specifically, web capture area co-varied with gluey droplet morphometrics, thread stickiness and salt concentrations when prey mass and energy were manipulated, and spiral spacing co-varied with gluey silk salt concentrations when prey size and feeding frequency were manipulated. We explained our results as A. keyserlingi plastically shifting its foraging strategy as multiple prey parameters simultaneously varied. We confirmed and extended previous work by showing that spiders use a variety of prey cues to concurrently adjust web and silk traits across different feeding regimes.