The Adaptive Advantages Of Pistillate And Perfect Flowers In Gynomonoecious Nepeta Stewartiana(Lamiaceae)

Angiosperms have diversed sexual systems,such as hermaphroditism,that the female and male coexist on the same individual,or dioecy,that the female and male separate on different individuals,Among which,gynomonoecy is the mode of sex expression that plants have both pistillate and perfect flowers,occurs in at least 15 families.Currently,there still remained several questions in the adaptive significance of gynomonoecy,such as why do plants have functional pistillate flowers(loss of male function)?Why does not the gynomonoecy evolve into monoecy?What is the significance of keeping perfect flowers in gynomonoecious plants?To answer these questions,this study used a gynomonoecious perennial herb Nepeta stewartiana(Lamiaceae)as the model system,to explore the adaptation advantages of gynomonoecious plants,through a series of experiments and analyses,including the comparison of sex allocation between two flower morphs,observation of foraging behavior,and evaluating the influence of the number of pistillate flowers on reproductive success under the condition of controlling the proportion of pistillate flowers.We intend to test the following four hypotheses:(1)Resource allocation hypothesis:the presence of the two flower types in gynomonoecious plants may permit flexible allocation of resources to female and male reproductive function.To compare the resource allocation and reproductive success between pistillate and perfect flowers,we measured floral traits and pollinator preferences in the field.The results showed that pistillate flowers were smaller than perfect flowers,and produced lesser nectar,but with similar sugar concentration.Pistillate flowers allocated more resources to female functions than perfect flowers,manifested by a higher proportion of dry weight of pistils and a longer stigma.Thus,the existence of pistillate and perfect flowers in Nepeta stewartiana permits flexible allocation of resources to female and male functions to some extent.(2)Reproductive assurance hypothesis:the reason of gynomonoecious plants keeping perfect flowers is that perfect flowers were capable of self-fertilization under a scarcity of pollinators.In this study,bumblebee and solitary bee were the main floral visitors and preferred perfect flowers.Hand pollination experiment showed that although pistillate flowers experienced more pollen limitation,but the dry weight of seed under open-pollinated was not significantly different between two flower morphs.Perfect flowers received more visits between multiple flowers on the same plant,the high seed set of perfect flowers was related to its ability of autonomous self-fertilization,supporting the retention of perfect flowers was a reproductive assurance strategy.(3)Avoiding pollen stealing hypothesis:if perfect flowers are susceptible to biased predation by pollen theft,the presence of pistillate flowers in gynomonoecious plants can reduce the damage of male function.We noticed the pollen stealing of solitary bee on perfect flowers in Nepeta stewartiana.Through emasculation of perfect flowers,we compared pollen deposition under the condition of natural pollen stealing and emasculation simulated pollen stealing.We found that both bumblebee and solitary bee preferred perfect flowers,the emasculated perfect flowers did not affect the visit of bumblebee,but the visiting rate of solitary bee was significantly decreased.Solitary bees actively collect pollen and its pollination contribution was small,although its pollen stealing behavior resulted in the waste of pollen source.The population with high abundance of solitary bee has less pollen on the stigma of both pistillate and perfect flowers.These results showed that the emergence of pistillate flowers may be caused by the selection of avoiding pollen theft,providing a new evolutionary impetus for the generation of pistillate flowers in gynomonoecious plants.(4)Promoting outcrossing hypothesis:pistillate flowers do not produce pollen and receive less selfing pollen.We predicted that plant with a high proportion of pistillate flowers will be favorable for outcrossing.For pistillate and perfect flowers,we investigated the flowering dynamics,recorded the number of flowers and dynamic changes of the proportion of pistillate and perfect flowers at both individual and population levels in six populations.The results showed that flowering phenology of both pistillate and perfect flowers presented synchronization at the individual level,but among individuals in a population showed asynchronism,and at the same time,the number of flowers varied greatly with time,thus sexual dimorphism appeared temporally in the population,this temporary separation of sex facilitating outcrossing pollination in pistillate flowers.We examined the effect of the different proportion of pistillate flowers(0,16.7%,25%,50%,75%&83.3%)on pollen receipt(geitonogamy and outcrossing)by pollen dyeing experiment.Pollen dyeing experiment showed that when the proportion of pistillate flowers was about 25%,for the whole population,individual received more pollen grains(geitonogamy and outcrossing).In Nepeta stewartiana,the proportion of pistillate flowers maintained between 20%and 30%that may reduce geitonogamy and promote outcrossing to a large degree.Generally,we found that the presence of pistillate flowers in gynomonoecious Nepeta stewartiana avoided pollen theft and promoted outcrossing,but pistillate flowers suffered serious pollen limitation,the retention of female function in perfect flowers may be a reproductive assurance mechanism.This study supports resource allocation hypothesis,reproductive assurance hypothesis,avoiding pollen stealing hypothesis and promoting outcrossing hypothesis.In this study,we found that the appearance of pistillate flowers could reduce the pollen stealing of solitary bees,which provided a further basis for the study on the evolution of pistillate flowers.This study mainly revealed the adaptive signicance of pistillate flowers in gynomonoecious Nepeta stewartiana,providing insights into the evolution of gynomonoecy.