Salivary proteins in spider mites: dual roles in feeding and silk fiber coating

Spider mites are plant cell-sucking herbivores that spin nanoscale adhesive silk fibers released from their mouthpart appendages. Previous whole-genome sequencing predicted 17 silk genes in the two-spotted spider mite, Tetranychus urticae, while silk proteomics detected two candidates, Fibroin-1 and Fibroin-2. However, saliva proteomics also detected Fibroin-1, Fibroin-2, and sFibroin-1 (with high sequence similarity to Fibroin-1). We investigated whether these three proteins function in silk, saliva, or both. We found Fibroin-1, sFibroin-1, and Fibroin-2 genes expressed in the salivary glands but found no direct internal connection between salivary and silk glands. Our silk proteomics detected 8 silk proteins previously predicted and 66 salivary proteins, including Fibroin-1, sFibroin-1, and Fibroin-2. We detected a bead-on-a-string pattern formed by fluid (hypothesized to be saliva) on the silk fiber and a fluid patch that remains at the piercing site on the leaf surface after mite feeding. RNAi-mediated silencing of Fibroin-1 and sFibroin-1 genes reduced mite feeding duration, survival, and fecundity, and silencing of Fibroin-2 gene showed a tendency to reduce the thickness of individual silk fibers. These results suggest that Fibroin-1, sFibroin-1, and Fibroin-2 are secreted from the salivary glands and then adhere to silk fibers. Fibroin-1 and sFibroin-1 may assist mites to adhere the tip of the mouthparts to the leaf surface when sucking with their stylets, while Fibroin-2 may coat silk fibers. This study reveals the dual role of saliva in plant cell-sucking herbivorous mites, namely, stabilizing the mouthparts during feeding and coating silk fibers to provide them with adhesive properties. HighlightsThree salivary proteins of spider mites are abundantly detected in their silk fibers. Though the salivary glands and silk glands are not connected, saliva appears to adhere to silk fibers after secretion. Knockdown of two of the three salivary protein genes suppressed the feeding behavior of spider mites, resulting in reduced survival and fecundity. Knockdown of the remaining salivary protein gene showed a tendency to reduce the thickness of the silk filament.