Pollination biology of the hexaploid self-compatible species Turnera velutina (Passifloraceae)

The evolution of monomorphisms from heterostylous ancestors has been related to the presence of homostyly and the loss of self-incompatibility, allowing the occurrence of selfing, which could be advantageous under pollinator limitation. However, flowers of some monomorphic species show herkogamy, attraction and rewarding traits that presumably favour cross-pollination and/or a mixed mating system. This study evaluated the contributions of pollinators, breeding system and floral traits to the reproduction of Turnera velutina, a herkogamous monomorphic species. Floral visitors and frequency of visits were recorded, controlled hand cross-pollinations were conducted under greenhouse and natural conditions, and individual variation in floral traits was characterised to determine their contribution to seed production. Apis mellifera was the most frequent floral visitor. Flowers presented approach herkogamy, high variation in nectar features, and a positive correlation of floral length with nectar volume and sugar concentration. Seed production did not differ between manual self- and cross-pollinations, controls or open cross-pollinations, but autonomous self-pollination produced, on average, 82.74% fewer seeds than the other forms, irrespective of the level of herkogamy. Differences in seed production among autonomous self-pollination and other treatments showed that T. velutina flowers depend on insect pollination for reproduction, and that approach herkogamy drastically reduced seed production in the absence of pollen vectors. The lack of differences in seed production from manual cross- and self-pollinations suggests the possible presence of a mixed mating system in the studied population. Overall, this species was possibly derived from a distylous ancestor but appears fully capable of outcrossing despite being monomorphic.