【Societal Impact Statement】Understanding plant reproductive strategies is vital for conserving endangered species. This study reveals a novel self-pollination mechanism in the diminutive orchid Stigmatodactylus sikokianus, facilitated by the movement of a finger-like appendage beneath the stigma. While this less-than-1-mm-long structure inspired the name Stigmatodactylus, its ecological function remained unknown for over 130 years—until now. The findings suggest that this appendage facilitates delayed self-pollination, likely ensuring reproductive success under conditions of pollinator scarcity. Furthermore, the research highlights the value of integrating taxonomy with functional morphology to advance our understanding of pollination ecology. 【Summary】・Orchidaceae is one of the largest and most morphologically diverse plant families, largely due to intricate relationships with pollinators. However, autonomous self-pollination is relatively common, likely as an adaptation to limited pollinator availability. This study examines the reproductive mechanism of the diminutive orchid Stigmatodactylus sikokianus, focusing on the finger-like appendage beneath the stigma. / ・The reproductive biology of S. sikokianus was investigated through pollination observations, artificial pollination experiments, and detailed studies of column morphology. / ・Pollination experiments demonstrated that S. sikokianus primarily relies on autonomous self-pollination, as indicated by high fruit set in bagged flowers and no fruit set in emasculated ones. Microscopic analysis revealed that the dactyliform appendage arcs toward the stigma, maintaining its straight structure, and eventually adheres to the stigma surface, allowing contact between the appendage apex and pollinia. UV microscopy showed thick bundles of pollen tubes penetrating the appendage adhered to the stigma and reaching the base of the column. The absence of pollen tube formation before contact between the appendage apex and pollinia suggests that autogamy occurs exclusively through the appendage at a later flowering stage. / ・This study uncovers a novel self-pollination mechanism in orchids, where the movement of the stigma appendage likely facilitates delayed selfing. This mechanism ensures seed production and likely reproductive success in S. sikokianus, enabling it to thrive in shaded environments with limited pollinator availability. The discovery highlights the value of integrating taxonomic and ecological approaches, as morphological traits provide valuable insights into species life histories.

autogamy

理学研究科

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