Host's guardian protein counters degenerative symbiont evolution.

Area of Science:

• Evolutionary biology
• Microbial symbiosis
• Insect-microbe interactions

Background:

• Long-term microbial symbioses often lead to symbiont genome erosion and loss of function.
• Maintaining stable symbiotic partnerships despite symbiont vulnerability is an evolutionary puzzle.
• Plataspid stinkbugs rely on an uncultivable, genome-reduced gut symbiont, *Ishikawaella*, for survival.

Purpose of the Study:

• To identify host factors essential for sustaining the obligate symbiosis in plataspid stinkbugs.
• To elucidate the mechanisms ensuring symbiont survival and vertical transmission between host generations.

Main Methods:

• Identification and characterization of a dominant protein within symbiont-containing capsules.
• Gene knockdown experiments to assess the protein's function in symbiont survival and host development.
• Analysis of the protein's origin and the role of symbiont-derived molecules (GroEL) in the capsules.

Main Results:

• A novel host protein, essential for symbiont survival and capsule formation, was identified.
• Knockdown of this protein led to symbiont degeneration, failed transmission, and impaired nymphal growth.
• The host protein originates from an odorant-binding protein-like family, and symbiont GroEL compensates for lost functions.

Conclusions:

• A specific host protein acts as a guardian, protecting and facilitating the transmission of a vulnerable gut symbiont.
• This mechanism highlights how stable symbiotic relationships can persist despite symbiont genome reduction.
• The findings reveal a substantial cost of symbiosis for the host and a dual strategy involving host and symbiont factors for maintaining the partnership.