Intragenic homogenization and multiple copies of prey-wrapping silk genes in Argiope garden spiders

Abstract

BackgroundSpider silks are spectacular examples of phenotypic diversity arising from adaptive molecular evolution. An individual spider can produce an array of specialized silks, with the majority of constituent silk proteins encoded by members of the spidroin gene family. Spidroins are dominated by tandem repeats flanked by short, non-repetitive N- and C-terminal coding regions. The remarkable mechanical properties of spider silks have been largely attributed to the repeat sequences. However, the molecular evolutionary processes acting on spidroin terminal and repetitive regions remain unclear due to a paucity of complete gene sequences and sampling of genetic variation among individuals. To better understand spider silk evolution, we characterize a complete aciniform spidroin gene from an Argiope orb-weaving spider and survey aciniform gene fragments from congeneric individuals.ResultsWe present the complete aciniform spidroin (AcSp1) gene from the silver garden spider Argiope argentata (Aar_AcSp1), and document multiple AcSp1 loci in individual genomes of A. argentata and the congeneric A. trifasciata and A. aurantia. We find that Aar_AcSp1 repeats have >98% pairwise nucleotide identity. By comparing AcSp1 repeat amino acid sequences between Argiope species and with other genera, we identify regions of conservation over vast amounts of evolutionary time. Through a PCR survey of individual A. argentata, A. trifasciata, and A. aurantia genomes, we ascertain that AcSp1 repeats show limited variation between species whereas terminal regions are more divergent. We also find that average dN/dS across codons in the N-terminal, repetitive, and C-terminal encoding regions indicate purifying selection that is strongest in the N-terminal region.ConclusionsUsing the complete A. argentata AcSp1 gene and spidroin genetic variation between individuals, this study clarifies some of the molecular evolutionary processes underlying the spectacular mechanical attributes of aciniform silk. It is likely that intragenic concerted evolution and functional constraints on A. argentata AcSp1 repeats result in extreme repeat homogeneity. The maintenance of multiple AcSp1 encoding loci in Argiope genomes supports the hypothesis that Argiope spiders require rapid and efficient protein production to support their prolific use of aciniform silk for prey-wrapping and web-decorating. In addition, multiple gene copies may represent the early stages of spidroin diversification.

The Nephila clavipes genome highlights the diversity of spider silk genes and their complex expression

“…First, we saw evidence of new spidroin genes originating from A r t i c l e s tandem-duplication events, as hypothesized in earlier studies 17,26,29,[58][59][60] . We identified pairs of tandem spidroins on two genomic scaffolds (scaffold_16392,…”

“…Many spider species produce just a subset of these silk classes, and some produce yet other silk types, including cribellate silk 25 . Each species possesses an assortment of specialized gland types that are thought to produce distinct classes of silks to fit specific needs 9,26,27 .Spider silks are composed primarily of spidroin proteins (where a 'spidroin' is a spider fibroin [28][29][30][31] ) that, by convention, have been named and classified according to the specific silk gland in which they were first discovered. Spidroin proteins have conserved N-and C-terminal domains that flank long runs of repeated motifs 32-34 , the composition and number of which confer specific physical properties to silks 27 .…”

“…Many spider species produce just a subset of these silk classes, and some produce yet other silk types, including cribellate silk 25 . Each species possesses an assortment of specialized gland types that are thought to produce distinct classes of silks to fit specific needs 9,26,27 .…”

Identification of Wet-Spinning and Post-Spin Stretching Methods Amenable to Recombinant Spider Aciniform Silk

“…Most notably, native aciniform silk exhibits the greatest toughness of the silks 17,22 . As with the MA spidroins, the conformational transformation from α-helix to β-sheet is believed to be a key component in providing strength to silk fibers 35 .…”

Advances in understanding the properties of spider silk

“…Quite surprisingly, the AcSp1 internal block repeats of orb weavers are about half the length, consisting of 200-215 residues (Hayashi et al, 2004;Ayoub et al, 2013;Chaw et al, 2014). Similar to the DNA architecture of the MaSps from L. hesperus, the AcSp1 gene consists of a single large exon.…”