Yellow protein co-opted to sustain obligate symbiosis in leaf beetles.
Yellow proteins are best known for their roles in pigmentation, behavior, and development across insects. Here, we uncover their striking evolutionary co-option for a wholly distinct function: sustaining a Paleocene-aged digestive symbiosis in tortoise beetles. We show that a female-specific Yellow forms the gelatinous spheres that encapsulate the bacterium Stammera during vertical transmission, allowing it to subsist extracellularly despite its drastically reduced genome (0.24 Mb) and limited metabolic capacity. Yellow expression is highly localized to symbiont-harboring glands in the ovaries, where the protein is assembled into a matrix and secreted during egg-laying. Functional knockdown of yellow disrupts sphere integrity and compromises symbiont viability under dry conditions, underscoring the protein's embedding properties and protective role for Stammera. These findings reveal a novel function for an ancient gene family and demonstrate how tortoise beetles have repurposed Yellows to overcome the extreme metabolic constraints faced by their symbionts during extracellular transmission.