Abstract
BackgroundOutcrossing is known to carry genetic advantages in comparison with inbreeding. In many cases, flowering plants develop a self-incompatibility mechanism, along with a floral component adaptation mechanism, to avoid self-pollination and to promote outbreeding. Orchids commonly have a lip in their flower that functions as the a visiting plate for insect pollinators. Aside from the lip, however, many species (including Coelogyne rigida) have sheaths around the axis of inflorescence. The function of these sheaths remains unknown, and has long been a puzzle to researchers.Methodology/Principal FindingsWe investigated the function of these sheaths in relation to the lip and the pollinators, as well as their role in the modes of pollination and reproduction of Coelogyne rigida in 30 flowering populations of orchids in the limestone area of Southeast Yunnan, China. We found that self-incompatible C. rigida developed specialized bird perches around the basal axis of inflorescence to attract sunbirds and to complement their behavioral tendency to change foraging locations frequently. This self-incompatibility mechanism operates separately from the floral component adaptation mechanism. This mechanism thus prevents bees from repeatedly visiting the floral lip of the same plant which, in turn, results in autogamy. In this way, instead of preventing autogamy, C. rigida responds to these negative effects through a highly efficient cross-pollination method that successfully transfers pollen to different plants.ConclusionsThe proposed method ensures reproductive success, while offsetting the infertile self-pollination by insects, thereby reducing mating costs and addressing the lack of cross-pollination. The adaptation provides a novel and striking example of structural adaptation that promotes cross-pollination in angiosperms.
Highlights
The transition from outcrossing to self-fertilization is one of the most common evolutionary trends in plants [1], [2]
The aims of this study are as follows: (1) to determine the role of the sheaths surrounding the inflorescence basal axis; (2) to determine how these sheaths facilitate the transfer of pollen by sunbirds; (3) to quantify the level of fruit production affected by sheaths; and (4) to determine the relationship between the sheaths and the lips so that the roles of birds and insects as pollinators in the evolution of the C. rigida breeding system can be assessed and compared
We discovered that unlike the floral lip utilized for insect visitation, the sheaths around the inflorescence axis of the C. rigida provide perches for the sunbird, A. gouldiae, its effective pollinator
Summary
The transition from outcrossing to self-fertilization is one of the most common evolutionary trends in plants [1], [2]. Self-incompatibility effectively mitigates the harmful effects of self-mating and inbreeding depression. This mechanism does not prevent self-pollination, which results in ovule and pollen discounting at a high mating cost and does not directly promote crossing. Self-incompatible plants commonly evolve through floral component adaptation mechanisms to prevent self-pollination and/or to promote crosspollination. In many cases, flowering plants develop a self-incompatibility mechanism, along with a floral component adaptation mechanism, to avoid selfpollination and to promote outbreeding. Aside from the lip, many species (including Coelogyne rigida) have sheaths around the axis of inflorescence The function of these sheaths remains unknown, and has long been a puzzle to researchers
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