Natural selection on floral traits of Caltha scaposa (Ranunculaceae), an alpine perennial with generalized pollination system from Northwest Yunnan

Floral traits, including those invisible to humans but visible to pollinators, that increase pollination efficiency may be selected by pollinators in plant species with pollen limitation of seed production, but the importance of pollinators as selective agents on different floral traits needs to be further quantified experimentally. In the present study, we examined selective strength on flower diameter, flower height,UV bulls-eye size, sepal size and UV proportion via female fitness in Caltha scaposa, based on openpollinated and hand-pollinated flowers, through which pollinator-mediated selection was calculated for each of floral traits. Our results suggest that seed production of C. scaposa is pollen limited in natural conditions. There was directional selection(△β_(pollinator)=-0.12) for larger flowers in open-pollinated flowers, while no significant selection was found in flower height, UV bulls-eye size, sepal size or UV proportion. Statistically significant selection was found in UV bulls-eye size, sepal size and UV proportion in hand-pollinated flowers, but interactions with pollinators contributed only to flower diameter. We conclude that in C. scaposa, floral traits that are subjected to selection might be driven by multiple selective agents, and suggest the importance of investigating floral traits that are invisible to human but visible to pollinators in measuring pollinator-mediated selection via male fitness.

The functional significance of a stigma color polymorphism in Acer pictum subsp. mono (aceraceae)

Aims Exploring the characteristics and function of a flower color poly-morphism contributes to our understanding of floral evolution in angiosperms.However,little information is available on stigma color polymorphisms in flowering plants despite their important functional role in plant reproduction.Methods We studied a stigma color polymorphism at the individual level in Acer pictum subsp.mono(aka A.mono)by investigating stigma-color morph proportion and comparing stigma performance and compo-nents of female fitness(pollen adhesion,pollen germination,fruit set,seed set and fruit weight)between the two morphs(red and greenish-yellow stigma)within natural populations at Dongling mountain in the north of China.in the flowering period,we conducted hand pol-lination in natural populations and then made microscopic observa-tions using the aniline blue fluorescence method.Important Findingsindividuals in the studied populations did not change their stigma color between years,and flowers produced by a single tree were uniform in their stigma color.This strongly suggests that stigma color variation is genetically controlled.The percentage of the red stigma flowers with germinated pollen grains was significantly higher than that of the greenish-yellow stigma flowers when hand pollination was conducted in the early flowering period,but not so in the late flowering period.There was no significant difference in the percentage of flowers with pollen adhesion to the stigma between the two morphs.Fruit set of the red morph was signifi-cantly higher than that of the greenish-yellow morph.our findings suggest that the red morph may be more advantageous for pollen germination and fruit set than the greenish-yellow morph,which may provide a functional explanation for the high incidence of the red stigma morph in the studied populations.alternative explana-tions for the stigma color polymorphism are discussed to stimulate further work.

Vegetation context modifies selection on flowering start and plant height in an orchid perennial herb

Aims Identifying the potential role of vegetation context(defined as the density,species identity/diversity and height of co-occurring plants)in modifying selection on floral traits is a critical step for clarifying and predicting the floral evolutionary trajectory in complex co-flowering species competition environments.It is also helpful to understand the variation in pollinator-mediated selection.Methods We experimentally reduced vegetation height around individual plants of Spiranthes sinensis(a bumblebee-pollinated perennial orchid herb)and estimated how vegetation context modified selection on four floral traits(flowering start,plant height,corolla size and number of flowers)through female function and pollen removal over two continuous years.We randomly selected independent plants in each year.Important Findings We demonstrated that vegetation context modified selection for earlier flowering start and shorter plant height of S.sinensis.The strength of selection differed between years.In addition,selection was stronger through female function than through pollen removal.Our findings indicate the potential role of vegetation context in shaping the differentiation and diversification of flowers in angiosperms.

The function of floral orientation in bluebells:interactions with pollinators and rain in two species of Mertensia(Boraginaceae)

Aims Pollinators are traditionally considered to be the primary agent of selection on floral traits.However,floral traits may also be under selection from abiotic agents(e.g.rain),which makes considering the relative importance of pollinators and abiotic selective agents on floral traits essential.The functional significance of floral orien-tation is often ascribed to pollinator attraction,but orientation can also protect reproductive structures from rain.Therefore,a study that incorporates both factors will enhance our understanding of the ecological roles of floral orientation in plant fitness.Mertensia brevistyla and M.fusiformis are herbaceous species that differ in their floral orientations.A series of field and laboratory experi-ments was used to investigate the adaptive function of floral orien-tation in these species,particularly with respect to pollinators and rain.Methods We measured and compared floral orientation and visitor assem-blages between M.brevistyla and M.fusiformis populations in west-ern Colorado,USA.We manipulated floral stems and conducted a choice experiment with floral visitors,and also compared orienta-tions of pollinator-visited stems with those of unvisited stems in a natural setting.We examined pollinator-and rain-mediated selec-tion on floral orientation by manipulating orientation,conducting supplemental pollinations,applying watering treatments and meas-uring subsequent seed set.We also experimentally tested the like-lihood of rain contact with anthers,and the effect of rainwater on pollen germinability.Important Findings Mertensia brevistyla had a significantly more upright floral orienta-tion than M.fusiformis,and seed set was highest in upright M.bre-vistyla and in horizontal/pendant M.fusiformis stems,supporting an adaptive function(via female fitness)of the interspecific difference in orientation.However,floral visitor assemblages did not differ significantly between the two species;visitors did not exhibit sig-nificant preference for either orientation;and pollinator-mediated selection on orientation was undetectable.Similarly,there was lit-tle effect of water on seed set in either species,regardless of floral orientation.However,pollen germinability was reduced in both species by immersion in water;and water was more likely to contact anthers in M.fusiformis than in M.brevistyla,due to interspecific differences in floral morphology.We conclude that pollinators are likely not the primary selective agent driving differences in orienta-tion in these Mertensia species.Instead,the negative effect of rain on pollen germinability helps explain the more pendant orientation of M.fusiformis,while short anthers in more upright M.brevistyla provide an alternative adaptation to rain.The selective agent driving effects of orientation on seed set remains unclear.This study illus-trates the necessity of considering male fitness and abiotic agents in interpreting the functional significance of inflorescence traits.