An Exploration Of The Evolutionary Pathway Of Distyly In Fagopyrum Esculentum

Within population of a typically distylous plant species,heights of the stigma and anther are reciprocally herkogamous on each hermaphrodite flower,presenting two floral morphs on two types of plants:long-styled(L-)morph and short-style(S-)morph.In distylous flowers,stigma height of L-morph is significantly higher than that of S-morph,while anther height of L-morph is significantly lower than that of S-morph.Anther-stigma distance,corolla diameter and pollen size per flower of S-morph are usually larger than those of L-morph,but pollen production is smaller.To investigate the potential adaptation of inter-morph pollination involved by distylous plants,preliminary discussion on the evolutionary path of distylous plants,we used a crop the sweet buckwheat Fagopyrum esculentum as an experimental model system.The nectar volume of Fagopyrum esculentum is about 0.2 μL,the sugar concentrations range from 33%to 37%.There is no significant difference in the nectar volume and sugar concentration between two floral morphs,indicating the two morphs provide identical nectar reward for pollinators and gain equally reproductive success.High performance liquid chromatography(HPLC)was used to analyze the sugar composition of Fagopyrum esculentum of two floral morphs.Nectar of two floral morphs both are rich in fructose and glucose,and their production are similar.In addition,a small amount of sucrose(1.31%)is existed in L-morph.According to Percival’s classification Fagopyrum esculentum nectar should be classified as a type rich in hexose which is related to be pollinated by flies and short tongue bee.Correlation analysis floral,plant traits and seed set of Fagopyrum esculentum showed that there was a positive correlation between corolla diameter and seed set.The larger the corolla,the higher the seed set.As predicted,larger flower can attract more pollinators,bring more legitimate pollen and increase production.No significant difference was detected between natural and inter-morph seed set,indicating no pollen limitation.Self-pollination and intra-morph could set small amount of seeds,showing that sweet buckwheat was not strict self-incompatibility.The seed set of intra-morph,inter-intra and natural pollination did not significantly differ suggesting that the sequence of deposition of either intra-morph pollen or inter-morph pollen did not affect seed set.The natural seed set of short-styled flower was higher than that of longstyled flower,consistent with the fact that the legitimate pollen received from the S-morph was higher than that from the L-morph.To examine the effects of morph ratios on intra-and inter-morph pollination,we counted and identified stigmatic pollen loads and natural seed set in seven combinations of two morph plants in experimental plots.Each experimental plot had 24 individuals.The ratios of L-to S-morph in these plots were manipulated to be 1:0,3:1,2:1,1:1,1:2,1:3 and 0:1 respectively.Data analysis was divided into 5 categories:monomorphic L(L:S=1:0);more biased L(L:S=3:1,2:1);L:S=1:1;more biased S(L:S=1:2,1:3);monomorphic S(L:S=0:1).The results from the seven plots showed that either only L-or only S-morph plot was deleterious to the survival of sweet buckwheat,whereas the coexistence of two morphs was obviously more beneficial to both morphs.In the experiment,we concluded that under the condition of S-morph in predominance,the number and proportion of compatible pollen loads on stigmas were the highest in population while incompatible pollen loads were the lowest,and the seed set was the highest.Therefore,this morph ratio was the best for the development of population.From this point,if distyly of sweet buckwheat was evolved from only S-morph to dimorphic style,then it would be optimized,under the condition of S-morph in predominance and L-morph individuals will no longer increase.On the other hand,the dimorphic style is more likely to be evolved from L-morph.Our results support the hypothesis proposed by Barrett(1990)and Lloyd and Webb(1992):the ancestor of distyly was L-morph with higher level of stigma rather than the anther.