The researchers have uncovered the molecular and cellular mechanisms of “self-sacrificing gall repair” whereby soldier nymphs release large amounts of coagulating body fluid from their own bodies to plaster up the hole when a social aphid gall (nest) formed on a host plant is damaged by an enemy, in collaboration with the National Institute for Basic Biology, The Open University of Japan, and University of Tsukuba.

Gall repair by Nipponaphis monzeni.

(A) A soldier nymph and an adult. (B) A gall formed on the host tree. (C) An inside view of a gall. (D) A gall-repairing soldier nymph discharging body fluid. (E) Scanning electron microscope image of a soldier nymph (ventral view). Arrows indicate droplets of the discharged body fluid. (F) A gall with a naturally repaired hole (arrow). (C)-(F) were cited from Kutsukake et al. (2019) PNAS 116: 8950-8959 (doi:10.1073/pnas.1900917116).

Nipponaphis monzeni is a social aphid that forms galls on the host tree, Distylium racemosum. Since the plant tissue of growing galls is still thin and soft, the galls are often attacked and invaded by enemies like moth larvae. When the enemy breaches the gall wall, soldier nymphs immediately gather around the hole, release large amounts of whitish body fluid from the posterior parts of the body, and stir the fluid with their legs, whereby the gall breach is completely plastered with the coagulated body fluid. A series of these behaviors is called self-sacrificing gall repair, which has been reported as an interesting social behavior of the aphid. However, molecular mechanisms of the gall repair, especially those underlying the body fluid coagulation, were not investigated.

The researchers demonstrated that the body cavity of the soldiers is filled with peculiar soldier-specific “large globular cells (LGCs)” that accumulate a large amount of phenoloxidase, a key enzyme involved in melanization and scab formation in insects, and lipids (triglycerides), whereas their hemolymph contains high levels of tyrosine, a substrate of phenoloxidase, and a unique repeat-containing protein (RCP). After discharge of the body fluid, the LGCs rupture and release phenoloxidase and lipids to the hemolymph. The lipids form a soft clot immediately, whereas the activated phenoloxidase oxidizes tyrosine, which is followed by generation of highly-reactive quinone molecules through the melanin synthesis pathway. The reactive quinones crosslink surrounding proteins including RCP, whereby the clot is solidified and hardened.

Tyrosine is generally an insoluble amino acid. The solubilization mechanism by which a large amount of tyrosine is accumulated in the body fluid of N. monzeni soldiers will be investigated.