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Orchid's unique structure reveals new self-pollination mechanism

130 years after a fungus-eating plant received its name, a Kobe University researcher has uncovered the purpose of the structure that inspired its name—revealing a novel mechanism by which plants ensure reproduction.

Makino Tomitaro, a towering figure in Japanese botany, named around 1,000 species and discovered about 600 new plants between 1887 and 1957. Among his notable discoveries was the diminutive orchid Stigmatodactylus sikokianus, first identified in 1889.

After Makino's discovery, the plant was named for the unique, tiny finger-like appendage (the "dactylus" part) on its stigma, the flower's female organ that receives pollen. Despite its fame as one of Japan's iconic orchids, the function of this structure has remained a mystery—until now.

The Kobe University botanist Suetsugu Kenji specializes in orchids that feed on soil fungi rather than sunlight, and Stigmatodactylus is one such plant.

He describes himself saying, "I'm particularly interested in their pollination mechanisms, employing an interdisciplinary approach that integrates taxonomy, ecology, and evolutionary biology."

This curiosity led him to ask, "What is the significance of the finger-like appendage that inspired the genus name, and what is its ecological role?"

To address these questions, Suetsugu carefully observed whether insects visited the plants and the conditions under which the plants could produce seeds. He also analyzed the flowers' morphology at various stages throughout their development to understand how pollination and fertilization occur.

His findings, published in Plants, People, Planet, reveal that the plants predominantly self-pollinate, that is, they don't rely on insects to transfer pollen from other plants, and that they do so about three days after the flowers open.

This delay in self-pollination carries important ecological implications. Growing in the dark understories of forests, often among leaf litter, and offering no nectar rewards, these plants are rarely visited by potential pollinators.

Suetsugu explains, "While self-pollination likely guarantees reproductive success, relying solely on this method risks inbreeding. This may drive the evolution of mechanisms that combine the benefits of self-pollination and outcrossing. Delayed self-pollination, postponed until opportunities for outcrossing are exhausted, is likely one such adaptation—a fail-safe mechanism."

Microscopy analysis provided insights into the role of the finger-like appendage in this self-pollination. By the third day after the flower opens, its stigma collapses and, with the finger-like appendage, comes into contact with the pollen-carrying anther. This enables the pollen to extend their tubes through the appendage into the stigma and subsequently into the ovary, thereby fertilizing the plant.

Suetsugu writes, "The movement of the stigma appendage represents, to the best of our knowledge, a novel self-pollination mechanism in orchids."

He adds, "The most exciting aspect of this study was revealing this previously unknown mechanism, highlighting the intricate evolutionary paths plants can take to ensure survival."

With Stigmatodactylus comprising 28 species, many of which share this structure, this mechanism may be present in other species as well.

The Kobe University botanist concludes, "The significance of this discovery lies in its ability to bridge historical botanical research and contemporary scientific inquiry. It underscores the value of integrating meticulous taxonomic analysis with ecological and evolutionary studies to gain novel insights.

"In an era where research is increasingly specialized, taxonomy and ecology are often conducted separately. However, this study demonstrates that traditional natural history research, unifying taxonomy, evolution and ecology, still has the power to uncover new phenomena today."