A study on biogeography and floral trait evolution in Platanthera subgenus Limnorchis (Orchidaceae) using phylogenomic approaches

Abstract

Platanthera is one of the largest genera of temperate orchids and exemplifies a lineage that has adaptively radiated into diverse habitats within North America, Asia, Europe, North Africa, Borneo, and Sarawak. Major centers of diversity in this genus are western North America and eastern Asia. The diversity in floral morphological traits such as floral color, shape and the length of nectar spur is associated with numerous pollination syndromes, making Platanthera an ideal system to study the evolution of floral traits and pollination biology. Despite its diversity, a thorough phylogenetic hypothesis for the genus is lacking because no studies have yet sampled taxa exhaustively or developed a robust molecular toolkit. Nevertheless, in past phylogenetic studies some intrageneric groups of species appear to be monophyletic. One of these groups is subgenus Limnorchis, but the majority of taxa in this group have not been included in a phylogenetic analysis. In this study, I developed a new toolkit for Platanthera consisting of genomic information from 617 low-copy nuclear loci. Using a targeted enrichment approach, I collected high-throughput sequence data from these loci in 23 accessions, including nine of the 12 diploids of subgenus Limnorchis and nine outgroup Platanthera species. A maximum likelihood search was performed on a 570,818 nucleotide supermatrix to generate a phylogeny. This analysis resolved a strongly supported monophyletic clade for subgenus Limnorchis. This phylogeny was then used to test hypotheses of biogeographic diversification and floral trait evolution of subgenus Limnorchis. Ancestral biogeographic reconstruction indicated that subgenus Limnorchis originated in western North America ca. 3 – 4.5 Mya from an ancestor that was widespread in western North America and eastern Asia and subsequently diversified in western North America, followed by dispersal of some species to eastern North America. Floral macro and micro-morphological traits were characterized across the subgenus. Ancestral character reconstruction suggests convergent evolution of spur length, spur shape and viscidium shape, possibly in response to selection by similar pollinators. Understanding the biogeographic history and morphological diversification of subgenus Limnorchis within a phylogenetic context will contribute to an updated taxonomy for the subgenus.