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Early events in the evolution of spider silk genes.

ABSTRACT: Silk spinning is essential to spider ecology and has had a key role in the expansive diversification of spiders. Silk is composed primarily of proteins called spidroins, which are encoded by a multi-gene family. Spidroins have been studied extensively in the derived clade, Orbiculariae (orb-weavers), from the suborder Araneomorphae ('true spiders'). Orbicularians produce a suite of different silks, and underlying this repertoire is a history of duplication and spidroin gene divergence. A second class of silk proteins, Egg Case Proteins (ECPs), is known only from the orbicularian species, Lactrodectus hesperus (Western black widow). In L. hesperus, ECPs bond with tubuliform spidroins to form egg case silk fibers. Because most of the phylogenetic diversity of spiders has not been sampled for their silk genes, there is limited understanding of spidroin gene family history and the prevalence of ECPs. Silk genes have not been reported from the suborder Mesothelae (segmented spiders), which diverged from all other spiders >380 million years ago, and sampling from Mygalomorphae (tarantulas, trapdoor spiders) and basal araneomorph lineages is sparse. In comparison to orbicularians, mesotheles and mygalomorphs have a simpler silk biology and thus are hypothesized to have less diversity of silk genes. Here, we present cDNAs synthesized from the silk glands of six mygalomorph species, a mesothele, and a non-orbicularian araneomorph, and uncover a surprisingly rich silk gene diversity. In particular, we find ECP homologs in the mesothele, suggesting that ECPs were present in the common ancestor of extant spiders, and originally were not specialized to complex with tubuliform spidroins. Furthermore, gene-tree/species-tree reconciliation analysis reveals that numerous spidroin gene duplications occurred after the split between Mesothelae and Opisthothelae (Mygalomorphae plus Araneomorphae). We use the spidroin gene tree to reconstruct the evolution of amino acid compositions of spidroins that perform different ecological functions.

SUBMITTER: Starrett J

PROVIDER: S-EPMC3382249 | biostudies-literature | 2012

REPOSITORIES: biostudies-literature

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Early events in the evolution of spider silk genes.

Starrett James J Garb Jessica E JE Kuelbs Amanda A Azubuike Ugochi O UO Hayashi Cheryl Y CY

PloS one 20120622 6

Silk spinning is essential to spider ecology and has had a key role in the expansive diversification of spiders. Silk is composed primarily of proteins called spidroins, which are encoded by a multi-gene family. Spidroins have been studied extensively in the derived clade, Orbiculariae (orb-weavers), from the suborder Araneomorphae ('true spiders'). Orbicularians produce a suite of different silks, and underlying this repertoire is a history of duplication and spidroin gene divergence. A second ...[more]

PMID: 22761664

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Project description:BACKGROUND: Spidroins are a unique family of large, structural proteins that make up the bulk of spider silk fibers. Due to the highly variable nature of their repetitive sequences, spidroin evolutionary relationships have principally been determined from their non-repetitive carboxy (C)-terminal domains, though they offer limited character data. The few known spidroin amino (N)-terminal domains have been difficult to obtain, but potentially contain critical phylogenetic information for reconstructing the diversification of spider silks. Here we used silk gland expression data (ESTs) from highly divergent species to evaluate the functional significance and phylogenetic utility of spidroin N-terminal domains. RESULTS: We report 11 additional spidroin N-termini found by sequencing approximately 1,900 silk gland cDNAs from nine spider species that shared a common ancestor > 240 million years ago. In contrast to their hyper-variable repetitive regions, spidroin N-terminal domains have retained striking similarities in sequence identity, predicted secondary structure, and hydrophobicity. Through separate and combined phylogenetic analyses of N-terminal domains and their corresponding C-termini, we find that combined analysis produces the most resolved trees and that N-termini contribute more support and less conflict than the C-termini. These analyses show that paralogs largely group by silk gland type, except for the major ampullate spidroins. Moreover, spidroin structural motifs associated with superior tensile strength arose early in the history of this gene family, whereas a motif conferring greater extensibility convergently evolved in two distantly related paralogs. CONCLUSIONS: A non-repetitive N-terminal domain appears to be a universal attribute of spidroin proteins, likely retained from the origin of spider silk production. Since this time, spidroin N-termini have maintained several features, consistent with this domain playing a key role in silk assembly. Phylogenetic analyses of the conserved N- and C-terminal domains illustrate dramatic radiation of the spidroin gene family, involving extensive duplications, shifts in expression patterns and extreme diversification of repetitive structural sequences that endow spider silks with an unparalleled range of mechanical properties.

Project description:Spiders are a highly diverse group of arthropods that occur in most habitats on land. Notably, spiders have significant ecological impact as predators because of their extraordinary prey capture adaptations, venom and silk. Spider venom is among the most heterogeneous animal venoms and has pharmacological applications, while spider silk is characterized by great toughness with potential for biomaterial application. We describe the genome sequences of two spiders representing two major taxonomic groups, the social velvet spider Stegodyphus mimosarum (Araneomorphae), and the Brazilian white-knee tarantula Acanthoscurria geniculata (Mygalomorphae). We annotate genes using a combination of transcriptomic and in-depth proteomic analyses. The genomes are large (2.6 Gb and 6 Gb, respectively) with short exons and long introns and approximately 50% repeats, reminiscent of typical mammalian genomes. Phylogenetic analyses show that spiders and ticks are sister groups outgrouped by mites, and phylogenetic dating using a molecular clock dates separation of velvet spider and tarantula at 270 my. Based on the genomes and proteomes, we characterize the genetic basis of venom and silk production of both species in detail. Venom protein composition differs markedly between the two spiders, with lipases as the most abundant protein in the velvet spider and present only at low concentration in tarantula. Venom in both spiders contains proteolytic enzymes, and our analyses suggest that these enzymes target and process precursor peptides that subsequently mediate the toxic effects of venom. Complete analysis of silk genes reveal a diverse suite of silk proteins in the velvet spider including novel types of spidroins, and dynamic evolution of major ampullate spidroin genes, whereas silk protein diversity in tarantula is far less complex. The difference in silk proteins between species is consistent with a more complex silk gland morpholgy and use of three-dimentional capture webs consisting of multiple silk types in aranomorph spiders.

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Early events in the evolution of spider silk genes.

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Early events in the evolution of spider silk genes.

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