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The breakdown of distyly in Mussaenda esquirolii(Rubiaceae)

【摘要】:正Distyly is a genetically controlled flower dimorphism.In distyly,the flowers of one morph have a long style and short stamens(the pin morph),while the flowers of the other morph have a short style and long stamens(the thrum morph). Rubiaceae has more heterostylous species than any other families.Most species in the genus Mussaenda are distylous or functional dioecous,a sexual system that has presumably been evolved from distyly.The Chinese endemic species Mussaenda esquirolii is monomorphic and the populations consist of approach herkogamous flowers only.Different pathways leading to monomorphism from distyly has been proposed:1).One of the two morphs of the self-compatible distylous or stigma height dimorphic species lost.2) Recombination within supergene generated a new genotype with long-style and long-stigma,which is complete self-compatible,and the ancestral types eliminated gradually.The aims of the present study are to illuminate the characteristics of floral monomorphism in Mesquirolii,and to elucidate the mechanism behind the evolution of monomorphism from distyly.Field experiments have been carried out in three wild populations.The floral biology,pollinator and breeding system of M.esquirolii were studied.The results indicated that M.esquirolii is a self-compatible,partially autogamous species,pollinators are needed to maximize its seed set.Partial autogamy is caused by the nearly homostyly(stigma and anthers are almost at the same level).Anthers dehisced and pollen grains exhibited the highest viability 5-12h before flower open,stigmas were receptive in bud,and self pollen grains were deposited on stigmas,resulting in cleistogamy.Pollen viability decreased rapidly after flower open, while stigma receptivity lasted to the third day.Pollinators visited the flowers of M.esquirolii, frequently at the study sites,and the primary pollinators were solitary bees,butterflies and sphinx moths.The most effective durations of male and female functions were much shorter than the nominal periods of pollen and stigma presentation.Artificial pollination showed that M.esquirolii was self-compatible,without apomixis.There is no significant difference of pollen tube growth rates between self- and cross-pollination,with pollen tubes reached ovules within 16h in both treatments,indicating the lack of cryptic SI and supporting the hypothesis that recombination within supergene causes the breakdown of self-incompatible system in the species.

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