Extensive gene flow blurs species boundaries among Veronica barrelieri, V. orchidea and V. spicata (Plantaginaceae) in southeastern Europe

Little is known about the contribution of interspecific hybridization, a frequent phenomenon in plants, to the high plant diversity in southeastern Europe, one of the continent's diversity hot spots. A good system to study the relevance of hybridization for biodiversity in this region is Veronica subg. Pseudolysimachium sect. Pseudolysimachion (Plantaginaceae). Depending on the presumed frequency of hybridization, existing taxonomic concepts in this group range from distinguishing only morphological races without explicit taxonomic status to recognizing several species each with a series of intraspecific taxa. Using genetic (plastid sequences and AFLP fingerprints), ploidy-level and morphometric data, three core groups, pertaining to the currently recognized species V barrelieri, V orchidea, and V spicata, were identified. All three species are, however, connected by numerous and gradual genotypic transitions and show rampant discrepancies between genetic and morphometric-taxonomic assignments. Complete homogenization of the three core groups is probably prevented by geographic isolation, ecological divergence and ploidy differences. Misinterpretation of hybrid swarms as separate taxa and the mosaic distribution of different indumentum types have led to a gross overestimation of taxonomic diversity of V. sect. Pseucloly.simachion in southeastern Europe. Taxonomically, this might be accommodated by reducing V barrelieri, V orchidea and V spicata to subspeci fie rank and abandoning recognition at least of those subspecies and varieties included in our study.