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Does the Platanthera dilatata (Orchidaceae) complex contain cryptic species or continuously variable populations?

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Abstract

Floral phenotypic traits are expected to reflect evolutionary changes and are used as a reliable basis for species delimitation. However, when traits overlap among populations of newly emerging species, this confounds identification of evolutionarily distinct lineages and reduces taxonomic stability. In this study, we quantified variation in ten floral traits and plastid DNA sequences across 26 populations of Platanthera dilatata (Orchidaceae) in North America to determine geographic structure among populations and to evaluate support for three varieties recognized in the current taxonomy. k-means clustering analysis, in the absence of a priori designation of groups, indicated two morphologically distinct groups. Spur length was the most distinctive character between groups. The group containing larger flowers with longer spurs corresponds to the var. leucostachys and most samples in this group are from western North America. The vars. albiflora and dilatata could not be distinguished within the second group, which exhibited flowers with short to intermediate spurs and include samples from eastern and western North America. Morphological variation in P. dilatata may reflect pollinator-mediated selection, particularly in spur length, which is known to vary in association with pollinators across Platanthera. Significant genetic divergence was observed between the two groups (F ST = 0.15; P ≤ 0.001), but we did not find corresponding phylogenetic structure, which may reflect recent divergence and retention of ancestral polymorphisms. Based on these results, we suggest preserving the current intraspecific taxonomy until further studies determine the origin of floral variation and the extent of gene flow between morphologically divergent populations.

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Acknowledgments

This work is based on research conducted for a M.S. by BA. This research was funded by the Mississippi State University Department of Biological Sciences and Office of Research and Economic Development Research Initiation Program. We thank personnel at the U.S. National Forest Service for permission to collect samples, S. Datwyler for providing leaf samples, C. Sheviak for helpful discussions, and M. Welch, B. Counterman and two anonymous reviewers for providing comments on previous versions of the manuscript.

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  1. Binaya Adhikari

    Present address: Department of Biological Sciences, Kent State University, Kent, OH, 44242, USA

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  1. Department of Biological Sciences, Mississippi State University, Mississippi State, MS, 39762, USA

    Binaya Adhikari & Lisa E. Wallace

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  1. Binaya Adhikari
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Correspondence to Lisa E. Wallace.

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Adhikari, B., Wallace, L.E. Does the Platanthera dilatata (Orchidaceae) complex contain cryptic species or continuously variable populations?. Plant Syst Evol 300, 1465–1476 (2014). https://doi.org/10.1007/s00606-013-0974-8

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