Darwin's bark spider shares a spidroin repertoire with Caerostris extrusa but achieves extraordinary silk toughness through gene expression

Abstract

Spider silk is a protein-based material whose toughness suggests possible novel applications. A particularly fascinating example of silk toughness is provided by Darwin's bark spider ( Caerostris darwini ) found in Madagascar. This spider produces extraordinarily tough silk, with an average toughness of 350 MJ m −1 and over 50% extensibility, and can build river-bridging webs with a size of 2.8 m 2 . Recent studies have suggested that specific spidroins expressed in C. darwini are responsible for the mechanical properties of its silk. Therefore, a more comprehensive investigation of spidroin sequences, silk thread protein contents and phylogenetic conservation among closely related species is required. Here, we conducted genomic, transcriptomic and proteomic analyses of C. darwini and its close relative Caerostris extrusa . A variety of spidroins and low-molecular-weight proteins were found in the dragline silk of these species; all of the genes encoding these proteins were conserved in both genomes, but their genes were more expressed in C. darwini . The potential to produce very tough silk is common in the genus Caerostris , and our results may suggest the existence of plasticity allowing silk mechanical properties to be changed by optimizing related gene expression in response to the environment.

Characterization of the genome and silk-gland transcriptomes of Darwin’s bark spider (Caerostris darwini)

“…Very recent (unpublished) work corroborates our genomic findings and provides even further proteomic details about spidroins found in manually drawn dragline silk and its toughness relative to other, related bark spiders found in Madagascar [ 50 , 51 ]. Specifically, Kono et al report that a set of non-spidroin products they referred to as SpiCE proteins contributes to the composite nature of the fibers and are important to tensile properties.…”

Complexity of Spider Dragline Silk

“…Interestingly, a number of recent studies, spurred mostly by advances in proteomics and sequencing technologies, paint a more complex picture of dragline silk composition than is provided by a simple MaSp1/MaSp2 dichotomy. 13 − 20 A subtype termed MaSp3 was identified in several species via proteomics and genomic target capture techniques. 15 , 16 On the other hand, a major study by Babb et al reported at least 8 distinct subtypes of spidroins in the major ampullate silk of Trichonephila clavipes (denoted as MaSp-a to MaSp-h, plus other putative spidroins).…”

“… 17 The presence of MaSp4 has been linked to the extreme toughness of C. darwini dragline. 17 , 20 , 21 Kono et al used a combination of genomic, transcriptomic, and proteomic approaches to identify spidroin sequences from Araneus ventricosus , 19 which identified the MaSp3 subtype (later assigned to MaSp3A) as the third main component after MaSp1 and MaSp2. The same group likewise identified three main components in the dragline silk from the Nephilinae lineage: MaSp1, MaSp2, and MaSp3B, 18 with the latter being apparently synonymous with the previously identified Sp-74867 spidroin from T. clavipes .…”

“…MaSp4 (column iv) and MaSp5 (column v) have so far only been identified in bark spiders. 17 , 20 Figure 1 b shows the typical arrangement of the amino acid motifs in the repetitive regions of the three main MaSp subtypes (in this case, from T. clavata ). 18 …”

“…). 20 Several nonspidroin proteins with relatively low molecular weights have been detected in appreciable quantities in the dragline silk, to which the term SpiCE (for spider silk constituting element) has been collectively applied. Interestingly, a considerable subset of the SpiCE sequences also harbors a high abundance of cysteine residues, consistent with a CRP designation.…”