Butterfly Pollination In A Lily Flower With Plastic Sexual Systems

In diverse sexual systems of flowering plants,hermaphrodite(cosexual)is predominant accounting for over 70%angiosperm species,but the separated sex such as androdioecy is much rare.The hypothesis of "One stone,Two birds" has been proposed to explain the benefit of hermaphrodites,in which pollinator once visit could bring pollen in and deliver pollen out from a perfect flower,achieving both female and male function.However,it remains unclear why many hermaphrodites produce both male(staminate)and hermaphroditic(perfect)flowers on the same individuals,i.e.,in andromonoecy.To understand adaptive strategies of andromonoecious plants,we examined effective pollinators and gender variation in a lily flower Lilium lankongense(Liliaceae)in the mountainous region of Shangri-La,southwestern China.Three aspects of reproductive ecology were studied.Below are main results.(1)To test theoretical predictions of size-dependent sex allocation(SDS)that relative allocation to female function increases with plant size in L.lankongense,plant height and each floral sex were recorded in five wild populations.We observed three types of individuals,including smallest plants that were pure males only producing staminate flowers,hermaphrodites and andromonoecious plants.The total flower numbers of andromonoecious and hermaphrodite plants increased significantly with plant size.Different from the prediction of SDS that large plants are female-biased,our data indicated large individuals were more likely to produce terminal staminate flowers,as there were more flowers in andromonoecious than in hermaphrodite individuals.Staminate(male)flowers were significantly smaller and its style was much shorter than perfect flowers.The production of flowers depended on plants attaining a certain size threshold.Male plants,which usually produced one flower,were significantly smaller than andromonoecious and hermaphrodite plants.(2)To investigate whether butterflies are effective pollinators in the lily flower,we examined the role of butterfly wings as pollen vector and foraging behavior of various floral visitors.Although flowering plants are pollinated by animal pollinators,pollination by butterfly wings is little studied.Butterflies were confirmed to be effective pollinators in that their wings touched both anthers and stigma,resulting in the highest pollination efficiency.By contrast,different bees and flies were nectar or pollen robbers as they were too small to mechanic fit with the flower sexual organ,as the stigma height in perfect flowers was over 50 mm.Butterflies could only bring pollen out from staminate flowers but not deposit pollen on the short stigmas.(3)To test the hypothesis of pollinator attraction for andromonoecy whether staminate flowers could enhance pollinator attraction at lower cost,we examined whether flower displays increased the visitation frequency of the butterfly pollinators.Flower number,flower size,plant height and pollinator visits were measured at three sites.Our results showed that two butterfly species(Palearctic species and Pieris brassicaw)preferred visiting large flower displays.In conclusion,our findings indicated that when plants were small,the less costly sex was favored.When plants were large,both female and male investments increased isometrically,as later-produced flowers tended to be functionally male.Our comparisons in pollination roles of different floral visitors indicated the butterfly wings contributed effective pollen vectors in L.lankongense.Large plants may benefit in both female and male functions in producing terminal staminate flowers,as it increased pollinator attraction.