Most diverse, most neglected: weevils (Coleoptera: Curculionoidea) are ubiquitous specialized brood-site pollinators of tropical flora

In tropical environments, and especially tropical rainforests, a major part of pollination servicesis provided by diverse insect lineages. Unbeknownst to most, beetles, and more specificallyhyperdiverse weevils (Coleoptera: Curculionoidea), play a substantial role there as specialistmutualist brood pollinators. The latter contrasts with a common view where they are onlyregarded as plant antagonists. This study aims at giving a comprehensive understanding ofwhat is known on plant-weevil mutualist interactions, through a review of the known reciprocalbehavioral, morphological and physiological adaptations found in plant-weevil systems, andthe identification of potential knowledge gaps to fill. To date, plant-weevil associations havebeen described or indicated in no less than 600 instances. Representatives of all major plantlineages (gymnosperms, angiosperms monocots and dicots) are involved in theseinteractions, which have emerged independently at least a dozen times. Strikingly, thesemutualistic interactions have led to a range of multiple convergent adaptations in plants andweevils. Plants engaged in weevil-mediated pollination are generally of typicalcantharophilous type and they also show specific structures to host the larval stages of theirspecialist pollinators. Another characteristic feature is that flowers often performthermogenesis and exhibit a range of strategies to separate sexual phases, either physicallyfor chronologically. Conversely, lineages of brood-site weevil pollinators present numerousbehavioral and physiological adaptations, and often form multispecific assemblages of closelyrelated species on a single host; recent studies also revealed that they generally display ahigh degree of phylogenetic niche conservatism. This pollination mutualism occurs in alltropical regions, and the contrasts between the known and expected diversity of these systemssuggests that a wide range of interactions remain to be described globally. Our early estimatesof the species richness of the corresponding weevil clades and the marked pattern ofphylogenetic niche conservatism of host use further suggest that weevil-based pollination farexceeds the diversity of other brood-site mutualistic systems, which are generally restricted to one or a few groups of plants. As such, weevil pollinators constitute a relevant model to explore the emergence and evolution of specialized brood-site pollination systems in the tropics