Abstract
SummaryThe digitate-tubered clade (Dactylorhiza s.l. plus Gymnadenia s.l.) within subtribe Orchidinae is an important element of the North-temperate orchid flora and has become a model system for studying the genetic and epigenetic consequences of organism-wide ploidy change. Here, we integrate morphological phylogenetics with Sanger sequencing of nrITS and the plastid region trnL-F in order to explore phylogenetic relationships and phenotypic character evolution within the clade. The resulting morphological phylogenies are strongly incongruent with the molecular phylogenies, instead reconstructing through parsimony the genus-level boundaries recognised by traditional 20th Century taxonomy. They raise fresh doubts concerning whether Pseudorchis is sister to Platanthera or to Dactylorhiza plus Gymnadenia. Constraining the morphological matrix to the topology derived from ITS sequences increased tree length by 20%, adding considerably to the already exceptional level of phenotypic homoplasy. Both molecular and morphological trees agree that D. viridis and D. iberica are the earliest-diverging species within Dactylorhiza (emphasising the redundancy of the former genus Coeloglossum). Morphology and ITS both suggest that the former genus Nigritella is nested within (and thus part of) Gymnadenia, the Pyrenean endemic 'N.' gabasiana apparently forming a molecular bridge between the two radically contrasting core phenotypes. Comparatively short subtending molecular branches plus widespread (though sporadic) hybridisation indicate that Dactylorhiza and Gymnadenia approximate the minimum level of molecular divergence acceptable in sister genera. They share similar tuber morphologies and base chromosome numbers, and both genera are unusually prone to polyploid speciation. Another prominent feature of multiple speciation events within Gymnadenia is floral paedomorphosis. The 'traditional' morphological and candidate-gene approaches to phylogeny reconstruction are critically appraised.
Highlights
Subtribe Orchidinae s.s. dominates the Eurasian orchid flora, encompassing a considerable range of phenotypes (Fig. 1) and evolutionary mechanisms
Two nodes persist across all five trees, but they are the two nodes of greatest relevance to the present study: that linking Dactylorhiza and Gymnadenia as sister genera, and that linking this sister pairing to the remainder of the taper-tubered clade (i.e. Platanthera plus Galearis plus Neolindleya plus Pseudorchis)
1) Several molecular phylogenetic studies based on Sanger sequencing have provided an optimal circumscription of genera within the taper-tubered clade of subtribe Orchidinae
Summary
Subtribe Orchidinae s.s. (i.e. excluding the still poorly resolved subtribe Habenariinae) dominates the Eurasian orchid flora, encompassing a considerable range of phenotypes (Fig. 1) and evolutionary mechanisms (reviewed by Bateman 2009, 2012a). (i.e. excluding the still poorly resolved subtribe Habenariinae) dominates the Eurasian orchid flora, encompassing a considerable range of phenotypes (Fig. 1) and evolutionary mechanisms (reviewed by Bateman 2009, 2012a) This clade has, through the last two decades, been subjected to several molecular phylogenetic studies utilising many samples that together spanned the subtribe (e.g. Bateman et al 2003; Inda et al 2012; Jin et al 2014; Tang et al 2015; Jin et al 2017). These two genera have been consistently found to be sisters; they have been shown to share digitate tubers (i.e., those emitting more than one large root from the tuber apex) and a presumed chromosomal fusion event that converted n = 21 to n = 20 (Pridgeon et al 1997; Bateman et al 2003). These genera were segregated as subtribe Dactylorhizinae by Vermeulen (1977)
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