Asymmetric Introgression and Cryptic Natural Hybridization between Two Species of Section (Lamiaceae) on the Balkan Peninsula.

In this work, we analyzed the morphology and genetic structure of , and their hybrid × 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 × 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 playing the dominant role. At the Sićevo locality, gene flow was slightly asymmetric, favoring the genetic background of , 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 could not be genetically distinguished from . This suggests that interspecific hybridization occurred long ago, leading to introgression and cryptic hybrids, blurring of species boundaries and generating evolutionary noise.