Using an integrated approach to identify cryptic species, divergence patterns and hybrid species in Asian ladies’ tresses orchids (Spiranthes, Orchidaceae)

Abstract

Spiranthes (∼36 species, Orchidaceae) is a small genus with a global distribution. It has a center of diversity in North America with only a few species occurring in Asia. This study focuses on the Asian Spiranthes with an emphasis on understanding their biogeographic relationships and species delimitations using molecular markers. Our phylogenetic trees based on nuclear (ITS) and chloroplast (trnL-trnLF, matK and trnS-G) sequences from samples across their range in Asia revealed the Asian Spiranthes are monophyletic. Ancestral area optimization suggested that North America forms the ancestral region for the Asian Spiranthes rather than Europe suggesting that they originated from a single long-distance dispersal event. Our study also revealed the presence of a cryptic species S. himalayensis, which was discovered based on molecular data thus emphasizing the importance of wide geographical sampling in phylogenetic studies. Sequences of cloned ITS provided support for the hypothesis that natural hybridization between S. sinensis and the newly described S. himalayensis resulted in the allotetraploid S. hongkongensis, with S. himalayensis as the paternal parent. One of the species complexes known in Asia is the S. sinensis complex, which shows a wide occurrence and is known for local geographical variants. Some of these variants have been described as new species in Australia and New Zealand. Our studies show that all the sampled variants including the Australian and New Zealand species show monophyly despite having long branches. This suggests that there may be high rates of gene flow between the geographically distinct forms resulting in lack of species resolution within the S. sinensis complex.