Left and right sides are needed: seed development improves when both stigma lobes of Habenaria limprichtii...

Handedness, or behavioural lateralization, during foraging is widespread in animals. Although floral foraging by left- or right-biased pollinators has been reported, the effects of handedness on pollination rates and reproductive success have not been tested. We used the orchid Habenaria limprichtii to investigate handedness effects. This is an appropriate model system because the zygomorphic flowers have two separate left and right pollinia and stigma lobes. We surveyed 14 populations (755 plants; 6,518 flowers), checking natural rates of pollinia removal and deposition by moths according to three handedness levels: (1) left, (2) right, and (3) both sides of flowers. We conducted experimental cross-pollinations based on this same design to compare seed production. Roughly equal proportions of pollinia removal and deposition from both sides are the most common pattern across populations, with fewer removal/deposition on either single left or right side. When compared to fruits in which only one stigma lobe was pollinated (either left or right), seeds from pollen deposition onto both sides of the same flower show more embryos developed, with fewer embryo abortions, and fewer embryos absent (i.e., empty seeds). Our results suggest there are no handedness biases in pollinia removal and deposition in this species. Pollination of both stigma lobes by moths is consistent and probably increases auxin production, leading to ovule maturation and subsequent seed formation. Maintaining two receptive stigma lobes in the same flower may be selectively advantageous since it increases seed production in orchids with sectile pollinia.