Does reproductive isolation reflect the segregation of color forms in <i>Spiranthes sinensis</i> (Pers.) Ames complex (Orchidaceae) in the Chinese Himalayas?

Abstract

Isolation between species, or taxa sharing a common lineage, depends primarily on the relative strengths of various reproductive barriers. Previous studies on reproductive isolation between orchids emphasized mechanical and ethological barriers in flowers of species showing food and/or sexual mimicry. In this study, we investigated and quantified a series of prepollination and postpollination barriers between pink and white forms of <i>Spiranthes sinensis</i> sl, a nectar-secreting complex. We generated ML trees based on <i>trn</i>S-G and <i>mat</i>K to explore phylogenetic relationships in this species complex. <i>Spiranthes sinensis</i> sl segregated from some other congeners, but the white form constituted a distinct clade in relation to the pink form. The white form secreted 2-Phenylethanol as it is a single-scent compound and was pollinated almost exclusively by native, large-bodied <i>Apis cerana</i> and <i>Bombus</i> species (Apidae). <i>Apis cerana</i> showed a high floral constancy to this form. The scentless, pink form was pollinated primarily by smaller bees in the genera <i>Ceratina</i> (Apidae), and members of the family Halictidae, with infrequent visits by <i>A. cerana</i> and <i>Bombus</i> species. Fruit set and the production of large embryos following interform pollination treatments were significantly lower compared to intraform pollination results for the white form. Our results suggested that pollinator isolation, based on color and scent cues, may result in greater floral constancy in white populations when both forms are sympatric as two different, guilds of pollinators forage selectively preventing or reducing prospective gene flow. Postpollination barriers appear weaker than prepollination barriers but they also play a role in interform isolation, especially in the white form. Our findings suggest that floral color forms in <i>S. sinensis</i> do not represent an unbalanced polymorphism. Interpretations of the evolutionary status of these forms are discussed.