Journal of Phylogenetics & Evolutionary Biology

Development and Maintenance of a Cross-mixed Mating System in the Orchid Bulbophyllum orientale

Abstract

Li-Jun Chen, Guo-Qiang Zhang, Li-Qiang Li, Yu-Ting Zhang, Wen-Hui Rao and Zhong-Jian Liu

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.