Single-compound floral signaling stabilizes cooperation in an obligate brood-site pollination mutualism

How conflict-prone brood-site pollination mutualisms persist remains a central question in evolutionary ecology.^1,2,3,4 We demonstrate that a single, stereochemically specific volatile signal, together with sex-specific daily timing of anthesis, aligns gall midge behavior with plant reproductive success in the brood-site pollination mutualism between the dioecious climbing plant Smilax insularis and the gall midge Dasineura heterosmilacicola.⁵ Field observations together with pollination experiments showed that fruit set in S. insularis depends entirely on this midge. Because male flowers open earlier each day than female flowers, female midges typically acquire pollen on male flowers before visiting female flowers. This temporal offset enhances pollination while directing most oviposition and larval development to male flowers, which abscise within days. In contrast, female flowers receive fewer eggs and support lower larval survival, and we detected no larval feeding on developing seeds. Headspace analyses revealed that both floral sexes emit near-monocomponent floral scent profiles dominated by the rare apocarotenoid dihydroedulan I.⁶ Field bioassays with synthetic stereoisomers showed that (+)-dihydroedulan I elicited stereotypical zigzag odor-tracking flights by female D. heterosmilacicola that closely matched natural orientation toward S. insularis flowers, whereas the diastereomer (-)-dihydroedulan II did not attract the midge. No other insects responded to the baits under field conditions. Together, these findings indicate that a single, stereochemically specific signal shared by both floral sexes reinforces partner fidelity and stabilizes cooperation despite asymmetric larval performance.