An Exploration Of The Evolution Of Distyly

The population of a distylous species consists of two genotypes,individual plants exhibiting reciprocal herkogamy between stigmas and anther of two floral morphs.In most distylous species,self-pollination and intra-morph pollination are incompatible,while inter-morph pollination is compatible.Since Darwin drew attention to heterostyly,the evolution of heterostyly has been an unsolved mystery in the field of sexual system and evolutionary issues.Although two main theoretical models have been proposed to explain the evolutionary process of distyly,there have few experimental evidence.Charlesworth and Charlesworth(1979)postulated that ancestors of distyly exhibited self-compatible flower with stigmas and anther at the same height.Under the pressure of inbreeding depression,diallelic self-incompatibility appeared to prevent self-pollination.Subsequently,stamen-or style-length dimorphisms evolved by the transfer of compatible pollen.Lloyd and Webb(1992)postulated that ancestors of distyly exhibited stigmas above and separated from anther.Style-length polymorphism occurred prior to the reciprocal herkogamy.Subsequently,establishment of dimorphic incompatibility and ancillary features may or may not occur under inbreeding depression.Both models emphasize monomorphic ancestor,different selective forces and the sequence of morphological and physiological traits.Distyly has been hypothesized to improve compatible pollination between anthers and stigmas located at the same level(i.e.,inter-morph)or avoid sexual interference within the same flower or reduce the wastage of male gametes by decreasing the number of incompatible pollen grains deposited on stigmas.Given that both models involve style-length polymorphism,to examine the evolution of distyly,the distylous common buckwheat(Fagopyrum esculentum i.e.approach and reverse herkogamy)and homostylous tatary buckwheat(F.tataricum)were used here to simulate different style-length dimorphisms by three combinations.All flowers of one morph in each combination were emasculated and stigmas were collected to analyze stigmatic pollen loads.We concluded that pollen was not effectively transferred in the stage of style-length polymorphism.When tatary buckwheat was used as pollen donor,the percentage of pollen and pollen number on stigmas of L-morph were significantly higher than those on stigmas of S-morph.Tatary buckwheat as pollen recipient,the percentage of pollen and pollen from L-morph on stigmas were significantly higher than those from S-morph.Most distylous flowers are tube-shaped or similar to tube-shaped.Flowers of Fagopyrum exhibit dish-shaped.Tube-shaped flowers have been recognized to constrain pollinator taxa and pollinator behavior.When pollinators visit flowers,two types of pollen are carried on different parts of bodies to promote compatible pollination of different morphs.To test the adaptive significance of distyly,we observed visitors and determined pollen removal and disposed pollen number of a single visit.Subsequently,we test the part of pollinator's body which contact stigmas by smearing fluorescent powder on stigmas.We captured visitors in the field,and counted pollen grains on parts of the insect bodies.We found syrphid flies were the principal pollinator preferring for S-morph,other insects may make indirect as well as direct contributions to pollination in common buckwheat.We observed that proximal part of syrphid flies contacted the stigmas of L-morph and distal part contacted the stigmas of S-morph,while legs of halictid bees contacted the stigmas of L-morph or S-morph.We concluded main pollen-carrying parts of syrphid flies were limb and rostrum,halictid bees were limb.Percentage of pollen of two types on each part was not significantly different.Two types of pollen could not been separately placed on the pollinator body,resulting in ineffective pollination.