Disentangling subspecies delimitation in Heldreichia bupleurifolia Boiss. (Brassicaceae) through genomic and quantitative morphological approaches

Abstract

Heldreichia bupleurifolia Boiss., the only species in its genus, is distributed across the Taurus and Anatolian Diagonal Mountains in Türkiye and presents considerable taxonomic challenges. The species is currently divided into five subspecies and one variety and has long posed challenges for infraspecific classification due to overlapping morphological traits and unresolved phylogenetic relationships. To address these issues and clarify the boundaries of the described subspecies and their evolutionary history, this study integrates comprehensive morphometric analyses of 502 individuals from 36 populations, as well as SNP- and locus-based genomic analyses from RAD-seq data of 136 individuals from 20 populations. While morphometric assessments showed significant differentiation in foliar and fruit traits, especially between western and eastern populations, genomic analyses identified distinct genetic clusters corresponding to geographic regions. High levels of genetic admixture and morphological plasticity were seen in the Central Taurus populations, indicating either historical hybridization or the preservation of ancestral polymorphisms. In accordance with morphological and admixture analyses, coalescent-based subspecies delimitation supported a three-subspecies model, resulting in revised taxonomic classifications that included new combinations. To further contextualize these results within an evolutionary perspective, we adopted the speciation continuum framework, interpreting morphological, genetic, and reproductive differentiation as components of a continuous process of divergence. These results emphasize the drawbacks of using morphology alone for taxonomy and the value of integrative methods for determining subspecies boundaries in areas with complex topography.