Aphids have the gall… to sacrifice themselves in a pool of bodily fluids to save their home

Written by: Padmini S. Pillai

Original Article: Kutsukake et al. PNAS 2019
The Gist of It:
If you’ve had a paper cut recently, you may have noticed how quickly your body can repair the damage. Your immune system begins to heal the wound by forming a clot and protecting you from invading bacteria or viruses. The body uses cells called platelets to form a plug to stop the bleeding, and a mesh of fibrin stabilizes the clot to form a scab. Researchers have now demonstrated that insects called aphids do this on a societal level to repair their home and protect their colony from invaders! After an invading insect damages their plant-made nest, called a gall, an army of Nipponaphis monzeni, also called soldier nymphs, head to the breach and release most of their bodily fluids in a process called “body eruption”. Kutsukake and colleagues revealed that this fluid contains two components: cells filled with fat droplets and an enzyme called phenoloxidase (PO), and a liquid that contains the amino acid tyrosine and a protein called repeat-containing protein (RCP). When this fluid cocktail is released at the breach, the cells begin to form a soft plug, and PO turns tyrosine into reactive compounds called quinones. Quinones physically link RCPs together to form a strong mesh that reinforces and hardens the clot. In the process, these altruistic aphids die; some are buried in the forming clot, or get blocked out from the nest, while the rest succumb from the massive discharge of body fluid (even if they are able to crawl back in the gall). Over time, the plant tissue will regrow over the clot and the barrier will be restored. By developing a process akin to our body’s method of scab formation for wound healing, these aphids have evolved to use their immune cells and molecules to sacrifice themselves for the greater good of their colony! As Captain Spock once said, “the needs of the many outweigh the needs of the few.”

 

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Soldier nymphs sacrifice themselves by secreting a cocktail of fluid and coagulants to seal their damaged nest.

The Nitty Gritty:
Soldier nymphs were mechanically stimulated to secrete their body fluid, which was found to contain large globular cells (LGCs) and proteins. Immunoblotting revealed that a predominant protein was the pro-enzyme form of phenoloxidase (PO), which is involved in melanin synthesis. Sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting, and sequencing revealed a protein with repeats in glycine, serine, histidine, and glutamine, termed repeat-containing protein (RCP). In situ hybridization and immunohistochemistry showed that PO was localized inside LGCs and that RCP was extracellular. PO-related genes were upregulated in LGCs of soldier nymphs. Free tyrosine was abundant in the body fluid. Recombinant PO from Sf9 cells was purified and combined with PO and RCP, which resulted in RCP polymerization and cross-linking and darkening of the solution. Lipid droplets contained within LGCs mainly consisted of triglycerides.
Original Research Article: Kutsukake, M., et al.Exaggeration and cooption of innate immunity for social defense.” Proc Natl Acad Sci USA (2019): 201900917.

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