The Secondary Chemistry and Complex Morphology of Galls Formed by the Cynipinae (Hymenoptera): Why and How?

Oak galls formed by cynipine wasps are extremely complex and diverse structurally, histologically and chemically. Evidence suggests that the need for enemyfree space provides the major selective force molding this rich diversity, which in turn allows many species to co-occur locally in spite of highly successful attacks by natural enemies. It is proposed that the Cynipinae form mutualisms with virus or virus-like particles which work in concert with larval salivary secretions to produce galls. This ecological alliance may have engendered the adaptive shift to gall-forming from the seed-eating habit of secondarily phytophagous Hymenoptera. The role of mutualism as a driving force in evolutionary innovation may be considerable.