Integrating morphology, niche modelling, and molecular data to disentangle taxonomic challenges in a species complex of Calibrachoa (Solanaceae)

Background: Plant species delimitation can be difficult when clearly visible diagnostic characteristics are lacking. Historically, phenotypic similarity has been used as a criterion to group individuals into species. However, these traits can fail to discriminate between morphologically similar species or improperly subdivide species through inaccurate interpretation of natural phenotypic diversity.Aims: We aimed to clarify the taxonomic status of two Calibrachoa taxa for which some polymorphism sharing is evident.Methods: We evaluated the diversity between the taxa in terms of molecular diversity, niche divergence, and floral and foliar differences.Results: Overall, a high level of polymorphism sharing between the taxa was indicated, possibly reflecting their rapid divergence. However, leaf morphology was a good predictor of taxa associated with phylogenetically informative sequences. Diversity among taxa was due to differentiation more than local adaptation. Ecological niche modelling indicated no niche divergence between the two taxa.Conclusions: Our results suggest that the recent synonymisation of Calibrachoa thymifolia and C. linearis should be reviewed, with both taxa possibly considered as valid species. More molecular markers should be used to test this possibility, ideally examining divergence across their genomes.