Venomous protease of aphid soldier for colony defense

In social aphids, morphological, behavioral, and physiological differences between soldiers and normal insects are attributed to differences in gene expression between them, because they are clonal offspring parthenogenetically produced by the same mothers. By using cDNA subtraction, we identified a soldier-specific cysteine protease of the family cathepsin B in a social aphid, Tuberaphis styraci, with a second-instar soldier caste. The cathepsin B gene was specifically expressed in soldiers and first-instar nymphs destined to be soldiers. The cathepsin B protein was preferentially produced in soldiers and showed a protease activity typical of cathepsin B. The cathepsin B mRNA and protein were localized in the midgut of soldiers. For colony defense, soldiers attack enemies with their stylet, which causes paralysis and death of the victims. Notably, after soldiers attacked moth larvae, the cathepsin B protein was detected from the paralyzed larvae. Injection of purified recombinant cathepsin B protein certainly killed the recipient moth larvae. From these results, we concluded that the cathepsin B protein is a major component of the aphid venom produced by soldiers of T. styraci. Soldier-specific expression of the cathepsin B gene was found in other social aphids of the genus Tuberaphis. The soldier-specific cathepsin B gene showed an accelerated molecular evolution probably caused by the action of positive selection, which had been also known from venomous proteins of other animals.