Pollinator-mediated self-pollination and reproductive assurance in an isolated tree of Magnolia grandiflora L.

Background:The ability to produce seeds when pollinators or potential mates are scarce is one of the principal advantages of self-pollination in flowering plants.However,the role of pollinators mediating self-pollination to ensure seed set when pollen or potential mates are limited has received less attention.This study examined the reproductive consequences and involvement of pollinators in assuring seed set in an isolated tree of Magnolia grandiflora,a predominantly outcrossing species.Methods:We tested various aspects of reproduction such as flower density,floral rewards,stigma receptivity,pollinator abundance and behaviour,fruit set and seed set,in two successive reproductive years(2015-2016).Results:Flowers of M.grandiflora possess a suite of traits facilitating pollinator-mediated self-pollination(PMS),although the chances of autonomous self-pollination are reduced due to herkogamy,spatial separation of anther and stigma.The mean pollen production/flower was 5,152,289±285,094 with a pollen-to-ovule ratio of 39430±164.We found a significant positive correlation between number of visits and seed production for bees(r=0.5099,p=0.0007)and beetles(r=0.7159,p=0.00001),indicating these are effective at PMS.There was a significant negative correlation for thrips(r=-0.3206,p=0.044)and no correlation for flies or spiders.The percent fruit set was 100%and the seed set per ovule ranged between 19 and 20%.Conclusions:PMS will guarantee reproductive assurance and mitigate the effect of reduced mates or pollen limitation which is expected to increase the fecundity and establishment of individual trees in geographically isolated locations.

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.

Do different measures of maternal fitness affect estimation of natural selection on floral traits? A lesson from Linum pubescens (Linaceae)

Aims Floral traits in animal-pollinated plants are proposed to be selected by the behavior of pollinators,which create differential fitness.However,fitness estimation is not trivial and there is no agreed or universal measure of fitness.in plants,fitness can be measured in various ways,including the number of fruits or seeds,probability of survival,or growth rate.Methods Because each fitness measure can be associated with different pol-linator behavior,estimating pollinator-mediated selection on floral traits can differ among various fitness measures.We used the annual Mediterranean plant,Linum pubescens,as a model to assess the relative role of various fitness measures in estimating pollinator-mediated selection on floral traits.We compared four maternal fit-ness measures and estimated their relative advantage in assessing natural selection on floral traits of L.pubescens.To identify whether each trait is under pollinator-mediated selection,we used media-tional analysis.Important Findings We found that each floral trait was under natural selection assessed by a different fitness measure.The color combination of floral tube throat and stamen color was under selection when using seed mass as a fitness measure,and floral diameter was under positive direc-tional selection when the number of seeds or number of fruits was used as a fitness measure.Selection on floral color was not mediated by pollinators’visitation rates,while flower diameter showed par-tial pollinator-mediated selection.We conclude that using several fitness measures gives an insight into the mechanisms underlying phenotypic selection on a floral trait,and facilitates the understand-ing of pollinator-mediated selection.