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

Natural silks crafted by spiders comprise some of the most versatile materials known. Artificial silks-based on the sequences of their natural brethren-replicate some desirable biophysical properties and are increasingly utilized in commercial and medical applications today. To characterize the repertoire of protein sequences giving silks their biophysical properties and to determine the set of expressed genes across each unique silk gland contributing to the formation of natural silks, we report here draft genomic and transcriptomic assemblies of Darwin's bark spider, Caerostris darwini, an orb-weaving spider whose dragline is one of the toughest known biomaterials on Earth. We identify at least 31 putative spidroin genes, with expansion of multiple spidroin gene classes relative to the golden orb-weaver, Trichonephila clavipes. We observed substantial sharing of spidroin repetitive sequence motifs between species as well as new motifs unique to C. darwini. Comparative gene expression analyses across six silk gland isolates in females plus a composite isolate of all silk glands in males demonstrated gland and sex-specific expression of spidroins, facilitating putative assignment of novel spidroin genes to classes. Broad expression of spidroins across silk gland types suggests that silks emanating from a given gland represent composite materials to a greater extent than previously appreciated. We hypothesize that the extraordinary toughness of C. darwini major ampullate dragline silk may relate to the unique protein composition of major ampullate spidroins, combined with the relatively high expression of stretchy flagelliform spidroins whose union into a single fiber may be aided by novel motifs and cassettes that act as molecule-binding helices. Our assemblies extend the catalog of sequences and sets of expressed genes that confer the unique biophysical properties observed in natural silks.