Dual roles of salivary proteins in feeding and silk fiber coating in the spider mite Tetranychus urticae

The two-spotted spider mite (Tetranychus urticae), a plant cell-sucking herbivore, produces nanoscale adhesive silk fibers. Previous silk proteomics identifies Fibroin-1 and Fibroin-2, differing from 17 silk proteins predicted by genome sequencing. Here we demonstrate that these candidate fibroins, along with sFibroin-1 (highly similar to Fibroin-1), function as salivary proteins rather than silk proteins. The three candidate fibroins are expressed in salivary glands, which lack direct connection to silk glands. Fluid beads-on-a-string patterns on silk fibers and fluid patches at feeding sites, likely saliva, are observed. Consistent with this, silk proteomics detects the eight predicted silk proteins and 66 salivary proteins including the three candidate fibroins. RNAi of Fibroin-1 and sFibroin-1 decreases feeding duration, survival, and fecundity, while RNAi of Fibroin-2 reduces silk fiber thickness. The results suggest that Fibroin-1 and sFibroin-1 are involved during feeding, while Fibroin-2 provides adhesive coating to silk fibers, demonstrating saliva's dual role in this species.