Different species or altitudinal morphotypes? Testing the taxonomic value of Dianthus brachycalyx (Caryophyllaceae)

Defining species boundaries within plant groups distributed along wide elevational and geographic gradients may lead to unstable taxonomic delimitations using morphological data only. Within the taxonomically challenging genus Dianthus, through an integrative approach we tested the taxonomic value of D. brachycalyx, a putative species endemic to mountain peaks of the central-southern Apennines, with respect to the widespread central-Mediterranean D. virgineus. We measured 30 morphological characters in 452 individuals from 25 populations and obtained 3202 single nucleotide polymorphisms using ddRAD-seq techniques in 394 individuals from 36 populations. For climatic niche comparison, we built a dataset of 348 occurrence points. By comparing morphometric, genetic, and climatic niche data, we showed that D. brachycalyx cannot be considered a distinct species. Morphometric separation between the two species is detectable, but high-elevation populations of D. virgineus are similar to D. brachycalyx. Genetic analyses revealed that the genetic structure of populations of D. brachycalyx and D. virgineus is mainly shaped by isolation-by-distance, irrespective of their taxonomic attribution. The climatic niches of the two species are overlapping, and the niche differences are just due to different availability of climatic conditions in their ranges. Accordingly, multiple lines of evidence do not support a separation of D. brachycalyx from D. virgineus, and the former should be considered a heterotypic synonym of the latter.