Systematic revision ofPlatantherain the Azorean archipelago: not one but three species, including arguably Europe’s rarest orchid

Abstract

Background and Aims.The Macaronesian islands represent an excellent crucible for exploring speciation. This dominantly phenotypic study complements a separate genotypic study, together designed to identify and circumscribePlatantheraspecies (butterfly-orchids) on the Azores, and to determine their geographic origin(s) and underlying speciation mechanism(s).Methods.216 individuals ofPlatantherafrom 30 Azorean localities spanning all nine Azorean islands were measured for 38 morphological characters, supported by light and scanning electron microscopy of selected flowers. They are compared through detailed multivariate and univariate analyses with four widespread continental European relatives in theP. bifolia-chloranthaaggregate, represented by 154 plants from 25 populations, and with the highly misleading original taxonomic descriptions. Physiographic and ecological data were also recorded for each study population.Key Results.Despite limited genetic divergence, detailed phenotypic survey reveals not one or two but three discrete endemic species ofPlatantherathat are readily distinguished using several characters, most floral:P. pollostantha(newly named, formerlyP. micrantha) occupies the widest range of habitats and altitudes and occurs on all nine islands;P. micrantha(formerlyP. azorica) occurs on eight islands but is restricted to small, scattered populations in laurisilva scrub; the trueP. azoricaappears confined to a single volcanigenic ridge on the central island of São Jorge.Conclusions.Although hybridity seems low, the excess of phenotypic over genotypic divergence suggests comparatively recent speciation. The most probable of several credible scenarios is that AzoreanPlatantheras represent a single migration to the archipelago of airborne seed from ancestral population(s) located in southwest Europe rather than North America, originating from within theP. bifolia-chloranthaaggregate. We hypothesise that an initial anagenetic speciation event, aided by the founder effect, was followed by the independent origins of at least one of the two rarer endemic species from within the first-formed endemic species, via a cladogenetic speciation process that involved radical shifts in floral development, considerable phenotypic convergence, and increased mycorrhizal specificity. The recent amalgamation by IUCN of AzoreanPlatantherasinto a single putative species on their Red List urgently requires overruling, as (a)P. azoricais arguably Europe’s rarestbona fideorchid species and (b) the almost equally rareP. micranthais one of the best indicators of semi-natural laurisilva habitats remaining on the Azores. Both species are threatened by habitat destruction and invasive alien plants. These orchids constitute a model system that illustrates the general advantages of circumscribing species by prioritising field-based over herbarium-based morphological approaches.