A Study Of Evolution And Adaptive Significance Of Distyly In Fagopyrum Species

Distyly is characterized by reciprocal herkogamy in Angiosperm,with the reciprocal stigma and anther position in two morphs.Since Darwin proposed that distyly could promote cross pollination between the two morphs,the adaptive significance of distyly v has been studied in many taxa,but mostly used most tubular flowers.Fagopyrum is a genus of Polygonaceae,including about 15 species,widely distributing in Asia and Europe especially in Southwestern China.In our study,distyly and homostyly coexist in Fagopyrum,and the flowers are open dished flower and different with other heterostylous flowers.We study on the evolution and adaptive significance of distylous Fagopyrum,and our main results are:1.Based on the investigation of the distribution in the nine speices,we observed that seven species are distylous and the other two are homostylous species.By measuring the floral traits in buckwheat,distylous buckwheat are characterized by reciprocal herkogamy(the stigmas and anthers of the two morphs are positioned at different(matching)heights),accompanied with the differences of style length,pollen number and pollen size.The homostylous species are characterized by the similar stigma height and anther height.Our quantitative analysis of the stigmatic pollen loads showed asymmetric pollen flow in Fagopyrum which incompatible pollen load on stigmas of long-styled morph was higher than that of short-styled morph and percentage of compatible pollen on stigma in short-styled morph was higher than long-stylled morph.This asymmetry is commonly found in other distylous taxa.The correlation between floral traits and stigmatic pollen load showed that stigma height,anther height and the stigma-anther distance had effects on the stigmatic pollen loads.The short-styled morph with higher reciprocial index received higher percentage of compatible pollen.2.Variations in stigmatic pollen load and reproductive success were studied in two populations of Fagopyrum dibotrys,a closely related wild species to F.esculentum.The stigmatic pollen load was different in the two populations,stigmatic pollen load in Chengjiang population did not differ between long-and short-styled morph.While in Dali population,the total pollen and incompatible pollen on stigmas of short-styled morph was higher than that of long-styled morph,percentage of compatible pollen load on stigmas of short-styled morph was significantly lower than that of long-styled morph.This may be related to the morph ratio in populations.The morph ratio was 1:1 in Chengjiang population,but was short-styled biased in Dali population population(more short-styled morph).Natural seed set in Dali population was significantly higher than in Chengjiang population,likely due to more short-styled individuals in Dali population,a situation benefitical to the female reproductive success.The artificial pollination experiments showed that seed set did not increase after sufficient hand pollination.The low seed set(average 27.3%)in this species may be limited by resource rather than pollen,given that there were more than 50 open flowers per plant simultaneously.In addition,most of the pollinators are general pollinators,floral visit frequency of the functional groups was not significantly different between the two populations.The results showed that stigmatic pollen loads and reproductive success were affected by the population morph ratios in Fagopyrum dibotrys.3.Using one nuclear gene(nrITS)and three chloroplast genes or intergenic regions(matK,atpB-rbcL and psbA-trnH),we constructed a phylogenetic tree consisting of 13 Fagopyrum species.This phylogeny shows that the homostyly morphs all have been derived from distyly,and evolved at least three times in the genus Fagopyrum.During the transitions from distyly to homostyly,floral traits were changed,especially the sexual organs:flower size,pollen production and stigma-anther distance became smaller but stigma depth and pollen size did not.We observed that pollen size changed with stigma depth rather than style length,supporting the functional hypothesis proposed by Charles Darwin that pollen size may functionally correlate with a stigma feature(depth and surface)rather than style length.4.To compare stigmatic pollen load and quantify pollen sources on stigmas between distylous and homostylous buckwheat,a combination of one-flower and whole-plant emasculation treatments was involved in the distylous Fagopyrum esculentum and homostylous F.tataricum.Our results showed that intra-flower self pollen deposited on stigmas was significantly higher in homostylous(71.3%)than in distylous(average of two morphs 37.5%)buckwheat.While inter-plant pollination(accounting for 48.8%)in distyly was significantly higher than in homostyly(17.9%),intra-plant pollination(geitonogamy)was not significantly different,indicating that distyly might help of decreasing self-pollen deposition and promoting cross pollination.Compared with natural pollination,seed set was significantly decreased by the mixed pollination with compatible and incompatible pollen simultaneously in the distylous Fagopyrum esculentum,indicating that incompatible pollen significantly affected female reproductive success.Our observation of pollen performance in pistils indicated that incompatible pollen could germinate in styles but pollen tube growth was constrained before reaching the ovaries,indicating that self-pollen deposition not only reduces the opportunity of pollen delivery to other plants but also occupy positions on stigmas that could otherwise receive outcross pollen.Our results indicated that the maintenance of distyly(the reciprocal herkogamy)could be attributable to reducing self-pollen deposition and promoting cross-pollination,compared to homostyly.