Spidroins and Silk Fibers of Aquatic Spiders

Abstract

Spiders are commonly found in terrestrial environments and many rely heavily on their silks for fitness related tasks such as reproduction and dispersal. Although rare, a few species occupy aquatic or semi-aquatic habitats and for them, silk-related specializations are also essential to survive in aquatic environments. Most spider silks studied to date are from cob-web and orb-web weaving species, leaving the silks from many other terrestrial spiders as well as water-associated spiders largely undescribed. Here, we characterize silks from three Dictynoidea species: the aquatic spiders Argyroneta aquatica and Desis marina as well as the terrestrial Badumna longinqua. From silk gland RNA-Seq libraries, we report a total of 47 different homologs of the spidroin (spider fibroin) gene family. Some of these 47 spidroins correspond to known spidroin types (aciniform, ampullate, cribellar, pyriform, and tubuliform), while other spidroins represent novel branches of the spidroin gene family. We also report a hydrophobic amino acid motif (GV) that, to date, is found only in the spidroins of aquatic and semi-aquatic spiders. Comparison of spider silk sequences to the silks from other water-associated arthropods, shows that there is a diversity of strategies to function in aquatic environments.

Figure 1

Focal spider species. Top view of Desis marina (Desidae) on sandstone at katikati, New Zealand (A). Top view of Badumna longinqua (Desidae) in its web (B). Side view of Argyroneta aquatica (Dictynidae) ventral side up inside its diving bell with a prey attached to its mouthparts (chelicerae) (C).

Figure 2

Phylogeny of spidroin N-terminal regions. Argyroneta aquatica, Badumna longinqua, and Desis marina spidroin paralogs highlighted in brown, green, and purple respectively. Tree is rooted with the Bothriocyrtum californicum fibroin 1 (not shown). Spidroin names are abbreviated as in Table S1. Bootstrap support percentages >50% are shown. The scale bar indicates 0.7 substitutions per site. See Supplementary Fig. S7 for complete tree with all sequence names.

Figure 3

Phylogeny of spidroin C-terminal regions. Sequence names highlighted as in Fig. 2. Tree is rooted with the Bothriocyrtum californicum fibroin 1 (not shown). Spidroin names are abbreviated as in Table S1. Bootstrap support percentages >50% are shown. The scale bar indicates 0.6 substitutions per site. See Supplementary Fig. S8 for complete tree with all sequence names.

Figure 4

Comparison of exemplar repeats of sequences from the “Other Spidroins” clade shown in Fig. 2. Names for Sp spidroins from Argyroneta aquatica, Badumna longinqua, Desis marina, Stegodyphus mimosarum, and Dolomedes triton abbreviated as in Supplementary Tables S1 and S5. Abundant amino acids colored in red (alanine), blue (valine), green (glycine), and orange (leucine).

Figure 5

Relative expression levels of spidroin genes. Stacked bar graphs of gene expression levels in Argyroneta aquatica females (left) and males (right), Desis marina females (left) and males (right), and Badumna longinqua females. TuSp (purple), PySp (green), AcSp (orange), AmSp (blue), and Sp (grey) are shown. Letters indicate different variants for each spidroin type (e.g. AcSp_vA and AcSp_vB are indicated by the A and B in orange). Percentages show reads per kilobase of transcript per million mapped reads (RPKM) of average expression of male and female reads mapped to species-specific transcriptomes. Total RPKM of spidroins for A. aquatica: ♀ 2,883, ♂ 5,041; D. marina: ♀ 11,260, ♂ 8,155; and B. longinqua ♀ 43,284.