The Balkan Peninsula represents an important center of plant diversity, exhibiting remarkable ecological heterogeneity that renders it an optimal region for studying the diversification patterns of complex taxa such as Teucrium montanum. In the Balkan Peninsula, T. montanum is a highly plastic and morphologically variable species with unresolved taxonomic status. To ascertain the patterns of genetic and morphological diversification, a comparative genetic and morphological analysis was conducted. In total, 57 populations were subjected to analysis using AFLP and a multivariate morphometric approach. A Bayesian analysis of population structure distinguished two main genetic clusters, labelled A and B. Cluster B was found to be geographically restricted to the northwestern Dinarides, while cluster A occurred in the rest of the Balkans. Genetic cluster A was further subdivided into four subclusters that were spatially separated from each other. The contact populations between the subclusters exhibited a mixed genetic structure. There was a partial correlation between genetic and morphological diversification. The peripheral populations of the genetic clusters displayed morphological differences, while both genetic and morphological differences decreased in the contact zones. The observed genetic structure can be attributed to the reproductive biology of this species and the complex geological history of the Balkan Peninsula.

In this work, the overall morpho-anatomical variability of Teucrium montanum sensu lato on the Balkan Peninsula was described. A total of 60 populations were collected all over the Balkans. For each analysed individual, the morphological characters of the stem, leaves, and flowers, as well as the anatomical characters of leaves were measured. The habitats of each population were characterized by altitude zone, type of geological substrate, vegetation, and climate type. Basic descriptive statistics were calculated for each character state, and multivariate analyses were used to compare all assessed characters and groups. The results showed that the analysed populations exhibit high morphological variability and broad ecological preferences. Five of the seven morphologically defined groups fit the already described taxa (T. montanum subsp. montanum, T. montanum subsp. helianthemoides, T. pannonicum, T. skorpili, T. montanum var. parnassicum), whereas two groups are morphologically specific but did not fit the existing taxonomic concepts. The patterns of observed morphological variability only partially match the patterns of ecological differentiation of the habitats of the analysed populations. The occurrence of different morphological groups inhabiting similar habitats in the same biogeographic region suggests that the morpho-anatomical differentiation of T. montanum s.l. may be due to genetic or evolutionary factors.

This paper represents the fourth part of the inventory of the flora of Serbia (Niketić et al. 2018, 2020, 2021), which contains nomenclatural, taxonomic and floristic notes related to taxa from the Magnoliopsida group. At the same time, this contribution is the basis for publication of subsequent volumes of the An annotated checklist of vascular flora of Serbia in order to supplement the data on vascular plants in our country.

Teucrium montanum is a species widely distributed in Europe, occurring also in North Africa and Asia Minor. Because of its wide distribution and high morphological variability, many taxa related to T. montanum have been described. In this paper we discuss and lectotypify the following six names of taxa described or reported for the Balkan Peninsula: T. helianthemoides, T. montanum var. hirsutum, T. montanum var. modestum, T. montanum var. parnassicum, T. pannonicum, and T. skorpilii.

Teucrium montanum L. and T. capitatum L. are two plant species with overlapping distribution in the Balkan Peninsula, especially in Croatia, where several occurrences of their putative hybrid species T. × rohlenae have been recorded. Next-generation sequencing of both species and de novo assembly was carried out resulting in 120 contigs for T. montanum and 1685 contigs for T. capitatum assembled. The Dig-up primers pipeline was used for SSR mining of both assemblies, applying different criteria that resulted in 112 SSR candidates for testing. A subset of 41 SSRs was selected and after two rounds of testing, twelve SSRs were developed and characterized. A total of 232 alleles were detected with 5 to 29 alleles per locus, based on the test sample. The genetic diversity analysis of three Teucrium taxa from a single location revealed a higher level of diversity in T. montanum than in T. capitatum with intermediate values for the hybrid species. The NeighborNet diagram and genetic structure analysis grouped the taxa into two separate clusters, the first of which consisted exclusively of T. montanum, while the other was composed of intermixed T. capitatum and T. × rohlenae individuals. The availability of SSR markers for two Teucrium taxa will allow in-depth analysis of genetic diversity and structure, as well as molecular identification of their putative hybrids in the future.

In this work, we analyzed the morphology and genetic structure of Teucrium montanum, T. capitatum and their hybrid T. × rohlenae from three syntopic populations. A morphometric study showed that the parents and their hybrids exhibited continuous morphological variation, with the hybrid positioned exactly between the parents. Genetic analysis revealed that plants morphologically identified as T. × rohlenae are fertile hybrids that produce hybrid swarms dominated by later-generation hybrids. This suggests that introgression, rather than speciation, is the more likely outcome of hybridization between these plant species. The extent and direction of gene flow between the two species differed markedly between the three syntopic localities. At the Trilj locality, it was clearly unidirectional, with T. capitatum playing the dominant role. At the Sićevo locality, gene flow was slightly asymmetric, favoring the genetic background of T. capitatum, while at the Sliven site, it was completely asymmetric in the opposite direction. The extreme case of unidirectional gene flow was observed at the Trilj locality where plants morphologically identified as T. montanum could not be genetically distinguished from T. capitatum. This suggests that interspecific hybridization occurred long ago, leading to introgression and cryptic hybrids, blurring of species boundaries and generating evolutionary noise.