Abstract
Outbreeding is usually advantageous because inbreeding suffers from depression. Nevertheless, mixed mating is very common in nature. We found two co-existing plant types, self-compatible and self-incompatible, in populations of the orchid Bulbophyllum orientale. The floral parts of this plant form a device to promote cross-pollination. Rancid substances are excreted to lure pollinators to the labellum, and pollens are attached to pollinators through a delicate mechanism. Given that many inflorescences and flowers are present on a clone and each inflorescence, respectively, pollinating insects may continuously visit inflorescences of the same clone and flowers of the same inflorescence but rarely continuously visit different populations separated by large distances. Consequently, self-compatible plants produce seeds from both crossing and selfing, and self-incompatible plants only bear crossing seeds. Thus, a crossmixed mating system is created in the population. Individuals capable of producing both crossed and selfed seeds have better chances in natural selection. The strict crossing system is broken down, and a cross-mixed mating system consisting of both mixed mating and strict crossing is formed. The cross-mixed mating system fluctuates with varying behavior of pollinating insects. The mixed mating system is favored because a population has many clone individuals and because each individual has many multi-flower inflorescences in B. orientale. The partial strict crossing is retained, and it can counteract the latent harm caused by selfing and assist in the maintenance of this cross-mixed mating system. The successful evolution of flowering plants is demonstrated by the mode of attraction for pollinating insects, the smart use of the cross-pollinating facility, and the tradeoff between crossing and mixed mating.
Highlights
Plant breeding systems comprise inbreeding, outbreeding, and apomixes [1]
The inbreeding depression caused by self-pollination is deemed to be a selective pressure in the evolution of plant breeding systems [4,7]
Given that pollination pattern and pollinator feeding behavior affect development of the breeding system, this study aims to examine the pollination mechanism, including flowering phenology, flower structure, stigma receptivity, pollen activity and floral odor and breeding system of Bulbophyllum orientale, to investigate the evolutionary dynamics of the mating system of B. orientale, and to reveal the development and maintenance of its breeding system diversity
Summary
Plant breeding systems comprise inbreeding, outbreeding, and apomixes [1]. Among these systems, outbreeding is considered the primary driver of pollination diversity. Selfing has many benefits, such as opening up new habitats [5,6], avoiding unresponsive pollinators [7,8,9,10], and automatically transferring genes to offspring [11]. These benefits are severely abated because of the high mating costs incurred from the pollen and ovule discounting. The inbreeding depression caused by self-pollination is deemed to be a selective pressure in the evolution of plant breeding systems [4,7]. These relationships can influence population dynamics [15,16]
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