A comparison of the composition of silk proteins produced by spiders and insects.

Proteins that are highly expressed and composed of amino acids that are costly to synthesize are likely to place a greater drain on an organism's energy resources than proteins that are composed of ingested amino acids or ones that are metabolically simple to produce. Silks are highly expressed proteins produced by all spiders and many insects. We compared the metabolic costs of silks spun by arthropods by calculating the amount of ATP required to produce their component amino acids. Although a definitive conclusion requires detailed information on the dietary pools of amino acids available to arthropods, on the basis of the central metabolic pathways, silks spun by herbivorous, Lepidoptera larvae require significantly less ATP to synthesize than the dragline silks spun by predatory spiders. While not enough data are available to draw a statistically based conclusion, comparison of homologous silks across ancestral and derived taxa of the Araneoidea seems to suggest an evolutionary trend towards reduced silk costs. However, comparison of the synthetic costs of dragline silks across all araneomorph spiders suggests a complicated evolutionary pattern that cannot be attributed to phylogenetic position alone. We propose that the diverse silk-producing systems of the araneoid spiders (including three types of protein glues and three types of silk fibroin), evolved through intra-organ competition and that taxon-specific differences in the composition of silks drawn from homologous glands may reflect limited or fluctuating amino acid availability. The different functional properties of spider silks may be a secondary result of selection acting on different polypeptide templates.     

The evolution of cryptic spider silk: a behavioral test

Phylogenetic patterns of change in spider silk coloration provideinsight into the selective pressures directing evolution ofsilks. Trends toward evolution of silks with low reflectanceof ultraviolet (UV) light suggest that reduced UV reflectancemay be an adaptation to reduce visibility of webs to insectprey. However, a test of the visibility of primitive and derivedspider silks is lacking. Several genera of orb-weaving spidersinclude conspicuous designs of silk, called "stabilimenta,"at the center of their webs. Due to their large size, stabilimentapresent signals that insects can use to avoid webs. Unlikeother silks in the orb web, which reflect little UV light,evolutionarily derived stabilimentum silk retains a bright UV reflectance. But, unlike primitive silks, stabilimentum silkalso reflects large amounts of blue and green light. We comparedthe visibility of primitive tarantula silks and derived stabilimentumsilks to insects by using the ability of honey bees to learnto forage at targets of spider silk. We found that the uniquespectral properties of stabilimentum silk render it crypticto insects and that primitive silks are more visible to bees.Our findings support a hypothesis that the coloration of stabilimentumsilk is an adaptation to reduce the ability of insects to avoidwebs and that ancient biases in the color vision of insectshave acted upon the evolution of spider silk coloration throughsensory drive. But our findings question the emphasis on UVreflectance alone for visibility of spider silks to insects.     

Strength and structure of spiders' silks.

Spider silks are composite materials with often complex microstructures. They are spun from liquid crystalline dope using a complicated spinning mechanism which gives the animal considerable control. The material properties of finished silk are modified by the effects of water and other solvents, and spiders make use of this to produce fibres with specific qualities. The surprising sophistication of spider silks and spinning technologies makes it imperative for us to understand both material and manufacturing in nature before embarking on the commercialization of biotechnologically modified silk dope.     

Strength and structure of spiders’ silks

Spider silks are composite materials with often complex microstructures. They are spun from liquid crystalline dope using a complicated spinning mechanism which gives the animal considerable control. The material properties of finished silk are modified by the effects of water and other solvents, and spiders make use of this to produce fibres with specific qualities. The surprising sophistication of spider silks and spinning technologies makes it imperative for us to understand both material and manufacturing in nature before embarking on the commercialization of biotechnologically modified silk dope.     

Evidence for diet effects on the composition of silk proteins produced by spiders

Silks are highly expressed, secreted proteins that represent a substantial metabolic cost to the insects and spiders that produce them. Female spiders in the superfamily Araneoidea (the orb-spinning spiders and their close relatives) spin six different kinds of silk (three fibroins and three fibrous protein glues) that differ in amino acid content and protein structure. In addition to this diversity in silks produced by different glands, we found that individual spiders of the same species can spin dragline silks (drawn from the spider's ampullate gland) that vary in content as well. Freely foraging ARGIOPE: argentata (Araneae: Araneoidea), collected from 13 Caribbean islands, produced dragline silk that showed an inverse relationship between the amount of serine and glycine they contained. X-ray microdiffraction of the silks localized these differences to the amorphous regions of the protein that are thought to lend silks their elasticity. The crystalline regions of the proteins, which lend silks their strength, were unaffected. Laboratory experiments with ARGIOPE: keyserlingi suggested that variation in silk composition reflects the type of prey the spiders were fed but not the total amount of prey they received. Hence, it may be that the amino acid content (and perhaps the mechanical properties) of dragline silk spun by ARGIOPE: directly reflect the spiders' diet. The ability to vary silk composition and, possibly, function is particularly important for organisms that disperse broadly, such as Argiope, and that occupy diverse habitats with diverse populations of prey.     

The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders

Spider orb webs are dynamic, energy absorbing nets whose ability to intercept prey is dependent on both the mechnical properties of web design and the material properties of web silks. Variation in web designs reflects variation in spider web spinning behaviours and variation in web silks reflects variation in spider metabolic processes. Therefore, natural selection may affect web function (or prey capture) through two independent and alternative pathways. In this paper, I examine the ways in which architectural and material properties, singly and in concert, influence the ability of webs to absorb insect impact energy. These findings are evaluated in the context of the evolution of diverse aerial webs. Orb webs range along a continuum from high to low energy absorbing. No single feature of web architecture characterizes the amount of energy webs can absorb, but suites of characters indicate web function. In general, webs that intercept heavy and fast flying prey (high energy absorbing webs) are large, built by large spiders, suspended under high tension and characterized by a ratio of radii to spiral turns per web greater than one. In contrast, webs that intercept light and slow flying prey (low energy absorbing webs) are suspended under low tension, are small and are characterized by radial to spiral turn ratios that are less than one. The data suggest that for spiders building high energy absorbing webs, the orb architecture contributes much to web energy absorption. In contrast, for spiders that build low energy absorbing webs, orb architecture contributes little to enhance web energy absorption. Small or slow flying insects can be intercepted by web silks regardless of web design. Although there exists variation in the material properties of silk collected from high and low energy absorbing webs, only the diameter of web fibres varies predictably with silk energy absorption. Web fibre diameter and hence the amount of energy absorbed by web silks is an isometric function of spider size. The significance of these results lies in the apparent absence of selective advantage of orb architecture to low energy absorbing webs and the evolutionary trend to small spiders that build them. Where high energy absorption is not an exacting feature of web design, web architecture should not be tightly constrained to the orb. Assuming the primitive araneoid web design is the orb web, I propose that the evolution of alternative web building behaviours is a consequence of the general, phyletic trend to small size among araneoids. Araneoids that build webs of other than orb designs are able to use new habitats and resources not available to their ancestors.     

Aerial web-weaving spiders: Linking molecular and organismal processes in evolution

Aerial web-weaving spiders display a wide variety of foraging behaviors that can be tied to the evolution of one family of proteins, the silks. In some cases, the physical structure and mechanical properties of silks alone determine the ecology of spiders: the habitats in which they forage, the prey they capture and their subsequent reproductive success. Future studies that integrate research on the physical structure of silks, the molecular genetics of silk synthesis and the foraging ecology of spiders in primitive and derived phylogenetic groups could reveal how molecular and organismal processes interact in evolution.     

Web building and silk properties functionally covary among species of wolf spider

Although phylogenetic studies have shown covariation between the properties of spider major ampullate (MA) silk and web building, both spider webs and silks are highly plastic so we cannot be sure whether these traits functionally covary or just vary across environments that the spiders occupy. As MaSp2‐like proteins provide MA silk with greater extensibility, their presence is considered necessary for spider webs to effectively capture prey. Wolf spiders (Lycosidae) are predominantly non‐web building, but a select few species build webs. We accordingly collected MA silk from two web‐building and six non‐web‐building species found in semirural ecosystems in Uruguay to test whether the presence of MaSp2‐like proteins (indicated by amino acid composition, silk mechanical properties and silk nanostructures) was associated with web building across the group. The web‐building and non‐web‐building species were from disparate subfamilies so we estimated a genetic phylogeny to perform appropriate comparisons. For all of the properties measured, we found differences between web‐building and non‐web‐building species. A phylogenetic regression model confirmed that web building and not phylogenetic inertia influences silk properties. Our study definitively showed an ecological influence over spider silk properties. We expect that the presence of the MaSp2‐like proteins and the subsequent nanostructures improves the mechanical performance of silks within the webs. Our study furthers our understanding of spider web and silk co‐evolution and the ecological implications of spider silk properties.     

Environmentally induced post‐spin property changes in spider silks: influences of web type,spidroin composition and ecology

Many spiders use silk to construct webs that must function for days at a time, whereas many other species renew their webs daily. The mechanical properties of spider silk can change after spinning under environmental stress, which could influence web function. We hypothesize that spiders spinning longer‐lasting webs produce silks composed of proteins that are more resistant to environmental stresses. The major ampullate (MA) silks of orb web spiders are principally composed of a combination of two proteins (spidroins) called MaSp1 and MaSp2. We expected spider MA silks dominated by MaSp1 to have the greatest resistance to post‐spin property change because they have high concentrations of stable crystalline β‐sheets. Some orb web spiders that spin three‐dimensional orb webs, such as Cyrtophora, have MA silks that are predominantly composed of MaSp1. Hence, we expected that the construction of three‐dimensional orb webs might also coincide with MA silk resistance to post‐spin property change. Alternatively, the degree of post‐spin mechanical property changes in different spider silks may be explained by factors within the spider's ecosystem, such as exposure to solar radiation. We exposed the MA silks of ten spider species from five genera (Nephila, Cyclosa, Leucauge, Cyrtophora, and Argiope) to ecologically high temperatures and low humidity for 4 weeks, and compared the mechanical properties of these silks with unexposed silks. Using species pairs enabled us to assess the influence of web dimensionality and MaSp composition both with and without phylogenetic influences being accounted for. We found neither the MaSp composition nor the three‐dimensionality of the orb web to be associated with the degree of post‐spin mechanical property changes in MA silk. The MA silks in Leucauge spp. are dominated by MaSp2, which we found to have the least resistance to post‐spin property change. The MA silk in Argiope spp. is also dominated by MaSp2, but has high resistance to post‐spin property change. The ancestry of Argiope is unresolved, but it is largely a tropical genus inhabiting hot, open regions that present similar stressors to silk as those of our experiment. Ecological factors thus appear to influence the vulnerability of orb web spider MA silks to post‐spin property change. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 580–588.     

Behavioural and biomaterial coevolution in spider orb webs

Mechanical performance of biological structures, such as tendons, byssal threads, muscles, and spider webs, is determined by a complex interplay between material quality (intrinsic material properties, larger scale morphology) and proximate behaviour. Spider orb webs are a system in which fibrous biomaterials—silks—are arranged in a complex design resulting from stereotypical behavioural patterns, to produce effective energy absorbing traps for flying prey. Orb webs show an impressive range of designs, some effective at capturing tiny insects such as midges, others that can occasionally stop even small birds. Here, we test whether material quality and behaviour (web design) co‐evolve to fine‐tune web function. We quantify the intrinsic material properties of the sticky capture silk and radial support threads, as well as their architectural arrangement in webs, across diverse species of orb‐weaving spiders to estimate the maximum potential performance of orb webs as energy absorbing traps. We find a dominant pattern of material and behavioural coevolution where evolutionary shifts to larger body sizes, a common result of fecundity selection in spiders, is repeatedly accompanied by improved web performance because of changes in both silk material and web spinning behaviours. Large spiders produce silk with improved material properties, and also use more silk, to make webs with superior stopping potential. After controlling for spider size, spiders spinning higher quality silk used it more sparsely in webs. This implies that improvements in silk quality enable ‘sparser’ architectural designs, or alternatively that spiders spinning lower quality silk compensate architecturally for the inferior material quality of their silk. In summary, spider silk material properties are fine‐tuned to the architectures of webs across millions of years of diversification, a coevolutionary pattern not yet clearly demonstrated for other important biomaterials such as tendon, mollusc byssal threads, and keratin.     

Consequences of forced silking

The forced silking of a spider to obtain major ampullate (MA) silk for experiments is a standard practice; however, this method may have profound effects on the resulting silk's properties. Experiments were performed to determine the magnitude of the difference in the forces required to draw silk from the MA gland between unrestrained spiders descending on their draglines and restrained spiders from which MA silk was drawn with a motor. The results show that freely falling spiders can spool silk with as little as 0.1 body weights of force, which generates a stress that is about 2% of the silk's tensile strength. In contrast, forcibly silked spiders apply as much as 4 body weights of force with an internal braking mechanism, and this force creates silk stresses in excess of 50% of the silk's tensile strength. The large forces observed in forced silking should strongly affect the draw alignment of the polymer network in the newly spun fibers, and this may account for the differences in material properties observed between naturally spun and forcibly spun MA silks. In addition, the heat produced by the internal friction brake during forced silking may set the upper limit of forced silking speed.     

Gumfooted lines in black widow cobwebs and the mechanical properties of spider capture silk

Orb-weaving spiders produce webs using two types of silk that have radically different mechanical properties. The dragline silk used to construct the supporting frame and radii of the web is stiff and as strong as steel, while the capture spiral is much weaker but more than ten times as extensible. This remarkable divergence in mechanical properties has been attributed to the aqueous glue that coats the capture spiral, which is thought to decrease capture spiral stiffness and increase its extensibility. However, discerning the effect of the aqueous glue on fiber performance is complicated because dragline silk and the capture spiral are assembled from different proteins, which may also affect mechanical performance. Here, we use the sticky gumfooted lines of black widow cobwebs to test the effect of the addition of aqueous glue on the mechanical properties of dragline silk. We also surveyed orb-webs spun by a broad range of species for bundles of looped silk. Such bundles, termed windlasses, have been thought to increase capture spiral extensibility by "paying out" additional lengths of silk. Our results suggest that neither plasticization of silk by aqueous glue nor excess silk in windlasses can by themselves account for the remarkable extensibility of orb-weaver capture silk compared to other spider silks. This argues that the unique amino acid motifs of the flagelliform fibroins that constitute the core of the capture spiral play an essential role in capture silk's extreme extensibility.     

悦目金蛛拖牵丝力学性能的变异

蜘蛛丝作为一种具有优良机械性能的天然动物蛋白纤维,其特有的结构和机械性能与其生物学功能密切相关。由大壶状腺纺出的拖牵丝在蜘蛛的行走、建网、捕食、逃生、繁殖等多种生命活动中均发挥了重要的功能,其机械性能会受到多种内外因素相互作用的影响。本文对在不同体重、不同猎物饲养和不同营养状态3种条件下人工抽出的悦目金蛛(Argiope amoena)拖牵丝与其不同单丝间的力学性能进行了比较研究。结果表明,悦目金蛛拖牵丝的力学性能在组间、组内不同个体,以及同一个体不同丝纤维间变异都较大。随着蜘蛛个体的增大,蛛丝横截面直径逐渐增大,这会使得蛛丝的力学性能更好,便于作为救命索的拖牵丝在遇到危险时承受蜘蛛体重;蜘蛛在经过1个月的饥饿后,蛛丝在屈服点附近的力学性能并未发生显著变化,而断裂点应变和断裂能均显著减小,同时也表明无论对于作为救命索还是网丝,拖牵丝的弹性形变性能在与蛛丝相关的微观进化中要优先于塑性形变。这是蜘蛛在能量摄入受到限制时对拖牵丝的投入权衡的结果。     

Spider silks: recombinant synthesis,assembly, spinning,and engineering of synthetic proteins

Since thousands of years humans have utilized insect silks for their own benefit and comfort. The most famous example is the use of reeled silkworm silk from Bombyx mori to produce textiles. In contrast, despite the more promising properties of their silk, spiders have not been domesticated for large-scale or even industrial applications, since farming the spiders is not commercially viable due to their highly territorial and cannibalistic nature. Before spider silks can be copied or mimicked, not only the sequence of the underlying proteins but also their functions have to be resolved. Several attempts to recombinantly produce spider silks or spider silk mimics in various expression hosts have been reported previously. A new protein engineering approach, which combines synthetic repetitive silk sequences with authentic silk domains, reveals proteins that closely resemble silk proteins and that can be produced at high yields, which provides a basis for cost-efficient large scale production of spider silk-like proteins.     

Signal conflict in spider webs driven by predators and prey

Variation in the sensory physiologies of organisms can bias the receptions of signals, driving the direction of signal evolution. Sensory drive in the evolution of signals may be particularly important for organisms that confront trade-offs in signal design between the need for conspicuousness to allow effective transfer of information and the need for crypsis of the signal to unintended receivers. Several genera of orb-weaving spiders include conspicuous silk designs, stabilimenta, in the centre of their webs. Stabilimenta can be highly visible signals to predators, warning them of the presence of a noxious, sticky silk web. However, stabilimenta can also be used by prey as a signal in avoidance of webs, creating a trade-off in signal visibility. I argue that the derived spectral properties of stabilimentum silk have resulted in part from this conflict. The innate colour preferences of insects, their ability to learn colours, and the spectral properties of flowers all suggest that the reflectance spectra of stabilimenta renders them relatively cryptic to many insect prey, while maintaining their visibility to vertebrate predators.     

Proteinfasern als Hochleistungsmaterial

Evolution, properties and applications of spider silk The evolutionary success of spiders (Araneae) is closely linked to the development and multiple purposes of their silks. The fibrous material is used to protect their offspring, for distribution and orientation, and especially for prey catching. About half of the approx. 48,000 known species build webs, the variability of which is considered an example of co-evolution with insects and their habitats. In the course of evolution, adaptation to prey ecology and changes at the molecular level led to high-performance materials such as the silks of the Major Ampullate gland (MA silk), with mechanical toughness surpassing that of most technical materials. The establishment of recombinant production on an industrial scale has enabled the use of biocompatible, wound-healing and bacteriostatic silks as green sustainable biopolymers in a wide range of applications such as cosmetics, biomedicine, special textiles, filter materials, and nanobiotechnology.     

Biomechanical variation of silk links spinning plasticity to spider web function

Spider silk is renowned for its high tensile strength, extensibility and toughness. However, the variability of these material properties has largely been ignored, especially at the intra-specific level. Yet, this variation could help us understand the function of spider webs. It may also point to the mechanisms used by spiders to control their silk production, which could be exploited to expand the potential range of applications for silk. In this study, we focus on variation of silk properties within different regions of cobwebs spun by the common house spider, Achaearanea tepidariorum. The cobweb is composed of supporting threads that function to maintain the web shape and hold spiders and prey, and of sticky gumfooted threads that adhere to insects during prey capture. Overall, structural properties, especially thread diameter, are more variable than intrinsic material properties, which may reflect past directional selection on certain silk performance. Supporting threads are thicker and able to bear higher loads, both before deforming permanently and before breaking, compared with sticky gumfooted threads. This may facilitate the function of supporting threads through sustained periods of time. In contrast, sticky gumfooted threads are more elastic, which may reduce the forces that prey apply to webs and allow them to contact multiple sticky capture threads. Therefore, our study suggests that spiders actively modify silk material properties during spinning in ways that enhance web function.     

Silk elasticity as a potential constraint on spider body size

Silk is known for its strength and extensibility and has played a key role in the radiation of spiders. Individual spiders use different glands to produce silk types with unique sets of proteins. Most research has studied the properties of major ampullate and capture spiral silks and their ecological implications, while little is known about minor ampullate silk, the type used by those spider species studied to date for bridging displacements. A biomechanical model parameterised with available data shows that the minimum radius of silk filaments required for efficient bridging grows with the square root of the spider’s body mass, faster than the radius of minor ampullate silk filaments actually produced by spiders. Because the morphology of spiders adapted to walking along or under silk threads is ill suited for moving on a solid surface, for these species there is a negative relationship between body mass and displacement ability. As it stands, the model suggests that spiders that use silk for their displacements are prevented from attaining a large body size if they must track their resources in space. In particular, silk elasticity would favour sexual size dimorphism because males that must use bridging lines to search for females cannot grow large.     

Spinning an elastic ribbon of spider silk

The Sicarid spider Loxosceles laeta spins broad but very thin ribbons of elastic silk that it uses to form a retreat and to capture prey. A structural investigation into this spider's silk and spinning apparatus shows that these ribbons are spun from a gland homologous to the major ampullate gland of orb web spiders. The Loxosceles gland is constructed from the same basic parts (separate transverse zones in the gland, a duct and spigot) as other spider silk glands but construction details are highly specialized. These differences are thought to relate to different ways of spinning silk in the two groups of spiders. Loxosceles uses conventional die extrusion, feeding a liquid dope (spinning solution) to the slit-like die to form a flat ribbon, while orb web spiders use an extrusion process in which the silk dope is processed in an elongated duct to produce a cylindrical thread. This is achieved by the combination of an initial internal draw down, well inside the duct, and a final draw down, after the silk has left the spigot. The spinning mechanism in Loxosceles may be more ancestral.     

Conserved C-terminal domain of spider tubuliform spidroin 1 contributes to extensibility in synthetic fibers

Spider silk is renowned for its extraordinary mechanical properties, having a balance of high tensile strength and extensibility. To date, the majority of studies have focused on the production of dragline silks from synthetic spider silk gene products. Here we report the first mechanical analysis of synthetic egg case silk fibers spun from the Latrodectus hesperus tubuliform silk proteins, TuSp1 and ECP-2. We provide evidence that recombinant ECP-2 proteins can be spun into fibers that display mechanical properties similar to other synthetic spider silks. We also demonstrate that silks spun from recombinant thioredoxin-TuSp1 fusion proteins that contain the conserved C-terminal domain exhibit increased extensibility and toughness when compared to the identical fibers spun from fusion proteins lacking the C-terminus. Mechanical analyses reveal that the properties of synthetic tubuliform silks can be modulated by altering the postspin draw ratios of the fibers. Fibers subject to increased draw ratios showed elevated tensile strength and decreased extensibility but maintained constant toughness. Wide-angle X-ray diffraction studies indicate that postdrawn fibers containing the C-terminal domain of TuSp1 have more amorphous content when compared to fibers lacking the C-terminus. Taken together, these studies demonstrate that recombinant tubuliform spidroins that contain the conserved C-terminal domain with embedded protein tags can be effectively spun into fibers, resulting in similar tensile strength but increased extensibility relative to nontagged recombinant dragline silk proteins spun from equivalently sized proteins.