From generalization to pollination syndromes: filtering and dependency on functional-group of pollinators in two cassava wild relatives

Abstract

The reproductive biology of the cassava genus (Manihot, Euphorbiaceae) remains unknown, despite its relevance to humanity. Here, we investigated the pollination system of two cassava wild relatives concerning the filtering of visitors in unisexual flowers, the specialization–generalization degree, and verified the dependency on pollinators for seed production. We studied two monoecious species M. violacea and M. oligantha on floral traits differences and visitors during flowering periods in natural conditions. For specialization–generalization degree analysis, we grouped pollinators into functional groups. We performed hand pollination treatments to verify the mating system. About half of floral visitors visited a floral type exclusively. The sexually dimorphic floral display contributed to filtering abundant pollinators, whereas nectar traits acted as attractant filter. In addition to polysaccharides, a remarkable presence of alkaloids and terpenes was observed in the nectary tissue of both male and female flowers. The species exhibited specialization associated with distinct functional groups, M. violacea with bees and M. oligantha with flies. The two Manihot species presented self-compatibility and depend on pollinators to produce seeds and the threatened species M. oligantha manifested pollen limitation. Our findings evidence conspicuous filtering of visitors and consequently, a specialized pollination system emerged for the two Manihot species despite the apparent generalist flowers. The reproductive success relying on groups of abundant pollinators suggests a threatening scenario to cassava wild relatives due to the worldwide decline of insect abundance.