Challenges of applying monophyly in the phylogenetic shallows: taxonomic reappraisal of the Dactylorhiza maculata group

Richard M Bateman

doi:10.1007/s12225-021-09971-2

Abstract

SummaryThe first study of the taxonomically critical European orchid genus Dactylorhiza to use next-generation DNA sequencing generated the statistically best-supported reconstruction of its phylogeny to date. However, the two competing topologies obtained within the monophyletic Section Maculatae differed radically in the placement of the D. maculata s.s.–D. foliosa clade. Both topologies showed D. foliosa to be nested deeply within D. maculata s.s., and suggested that D. saccifera s.l. is paraphyletic, D. saccifera s.s. from south-eastern Europe and Asia Minor diverging before D. gervasiana from south-central Europe (a taxon typically viewed as a subspecies of D. saccifera or, more often, ignored completely). The poorly-sampled but character-rich morphometric comparison presented here suggests that D. saccifera s.s. and D. gervasiana cannot be distinguished with confidence using morphological characters and that, if D. gervasiana is accepted as a species, it is effectively cryptic. The diploid D. foliosa is accepted as an island endemic species, despite rendering paraphyletic the autotetraploid D. maculata s.s.; all other named taxa within the D. maculata s.s. clade are considered infraspecific. Dactylorhiza fuchsii is indisputably a bona fide species rather than a subspecies but it does not merit becoming the basis of a taxonomic section separate from D. maculata s.s. The strongly contrasting degrees of molecular vs morphological disparity reported here are representative of a more general trend evident in groups that have experienced comparatively recent speciation, as is the need to recognise occasional paraphyletic species when circumscribing species by integrating genotypic and phenotypic data categories. Explicit taxonomic criteria combined with a comparative, monographic approach are needed to achieve consistency of ranking. Disproving hypotheses of species status is judged a criminally underrated activity, actually being as important to outcomes as much-vaunted species "discovery".

Highlights

Brandrud et al (2020) published a well-received and thoroughly sampled phylogenetic study of the genus Dactylorhiza (Necker ex Nevski) Soó that relied upon a comparatively recent 'next-generation' genetic analytical technique termed RAD-seq
As RAD-seq had already been applied to the genus Ophrys (Bateman et al 2018a), Gymnadenia s.l. (Brandrud et al 2019) and the Epipactis helleborine aggregate (Sramkó et al 2019), Dactylorhiza was the obvious European orchid genus to benefit from this welcome technical advance
In the case of the derived clade within Dactylorhiza that includes the D. maculata and D. incarnata groups, allopolyploids have long been known to originate through repeated hybridisation events between similar parental lineages within these two species groups, each event being combined with genome doubling to generate a novel lineage whose ability to reproduce with either parent is considerably reduced

Summary

Introduction

Brandrud et al (2020) published a well-received and thoroughly sampled phylogenetic study of the genus Dactylorhiza (Necker ex Nevski) Soó that relied upon a comparatively recent 'next-generation' genetic analytical technique termed RAD-seq (restriction siteassociated sequencing). Dactylorhiza sambucina has itself long been a source of instability in trees of the genus, and in the RAD-seq likelihood trees it was placed as sister to the combined incarnata-umbrosa-euxina and fuchsiimaculata-aristata clades with only 50% support. This result pushed the incarnata group into a more derived position within the tree than was found in any previous molecular study (reviewed by Bateman et al 2018b). The position of the Siberian-Alaskan D. aristata as sister to the D. maculata group has molecular precedence only in the nrITS + rpl tree of Pillon et al (2006) and is still in need of explanation, given its vast geographic disjunction from all Dactylorhiza species other than the circumpolar D. viridis

Methods

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Discussion

Conclusion