The Maintenance Of Interactions Between Silene Species And Pollinating Seed-predator Moths

Understanding of the origin and maintenance of interspecific antagonisms and mutualisms remains a big challenge in plant-animal interactions,one of crucial questions in evolutionary biology for a long time.Previous studies on obligate plant-pollinator interactions such as fig-fig wasp,yucca-yucca moth and leafflower-leafflower moth indicated that obligate mutualisms originate from antagonistic associations,and could also reverse to parasitism,i.e.,pollinators act as seed predators.Mechanisms such as host sanction and selective flower abortion have been hypothesized to maintain the evolutionary stability in these systems.However,these highly specialized systems are difficult for us to understand how interactions build and maintain in the early stage of evolutionary history of interspecific mutualisms.In facultative nursery pollination systems,floral visitors may act as only pollinators or both pollinators and seed predators.The Silene-moth interactions can be mutualistic or parasitic in different species,providing a model system to study the the origin and maintenance of mutualisms under the variable factors.Here,we studied five Silene species in Shangri-La,southwest China for six years,estimated the interactions between different species and nocturnal moths,and compared the strategies to limit overexploitation from each other at different aspects(oviposition selection,predator satiation,population dynamic and interspecific association)between nursery pollination systems and parasitic systems.To estimate the interactions between Silene species and nocturnal moths,we observed diurnal and nocturnal visitors,bagged flowers to diurnal or nocturnal exposure,and surveyed oviposition rate in fresh flowers and fruit predation.The results indicated that moths oviposited in three Silene species(S.chungtienensis,S.gracilicaulis,S.yunnanensis)while only pollination of S.gracilicaulis relied on moths,suggesting the nursery pollination between moths and S.gracilicaulis but parasitism with other two species.To estimate the selection of oviposition on single-flower egg number,flower phase and floral traits,we surveyed and compared the number and proportion of eggs between male and female phase in three Silene species under different flowering stages.Then we compared observed single-flower egg distribution and predicted distribution under random oviposition.Meanwhile,we measured and compared floral traits between flowers with and without moth eggs.The results indicated that single-flower egg number was selected to be one,and the moths preferred to lay eggs in previously unparasitized flowers with wider petal and in female phase in the nursery system(S.gracilicaulis),whereas oviposition was random or even with a trend to lay more eggs in a single flower in the parasitic systems(S.chungtienensis and S.yunnanensis).To investigate the effects of predator satiation at the individual and population scales and population dynamic between plants and moths on fruit predation,we investigated population size(plant number)and predation rate among years,individual fruit number and predation rate,and then translocated eggs among individuals to remove predator satiation.Meanwhile,we counted flowers of tagged individuals(10-20)every four or seven days,and used the oviposition rate at different flowering stages multiplied by the corresponding flower number to estimate the moth population size.The results indicated that predator satiation limited fruit predation at both individual and population scale in the nursery pollination system(S.gracilicaulis),but only at population scale in the parasitic system(S.yunnanensis).Moreover,plants in nursery pollination system tend to be peak flowering with short flowering period,and the first and peak flowering date were more stable among years than parasitic systems,which could match moth population better.To compare the effects of interspecific pollen transfer on plant fitness between the nursery pollination and parasitic systems,we investigated conspecific pollen,heterospecific pollen(HP),HP diversity,HP proportion on stigmas and seed set in three Silene species,then conducted hand-pollination treatments with different HP number and proportion.Our results indicated that HP loads were significantly lower in S.gracilicaulis although Silene species had evolved tolerance mechanisms to HP effects.In other words,nursery pollination system(S.gracilicaulis)was likely to involve avoidance of HP loads from interspecific pollen interference.In summary,despite of the serious conflicting interests between plants and insects,strategies about oviposition selection,population dynamics and interspecific pollen interference together may have limited the overexploitation from each other in the nursery pollination systems,which maintain the stability of systems and might play important roles in the transition from antagonism to mutualism.