Wide diversity in narrow geographic space: genetic, morphological and ploidy variation in three Central European Crataegus species with emphasis on their reproductive modes

Background and Aims Hybridization, polyploidization, and apomixis are evolutionary forces that obscure genetic differentiation and boost morphological variability. These processes have shaped the family Rosaceae, particularly the genus Crataegus, which includes both diploid and polyploid species reproducing sexually or via apomixis. In Central Europe, C. monogyna and C. laevigata are predominantly diploid sexuals, while C. rhipidophylla is mainly a polyploid apomict. These species hybridize to form C. × media, C. × macrocarpa, and C. × subsphaerica. Our aim was to assess how hybridization, apomixis, and polyploidy shape Crataegus diversity by integrating genetic, morphological, and cytological data. Methods Leaves and fruits were collected from ten natural populations where all three species coexist and hybridize. Species identification was performed using novel nuclear microsatellites, marking the first genetic-based Crataegus taxonomy in Central Europe. Ploidy levels were estimated by flow cytometry (FCM), including seed screening to infer reproductive modes. A combined morphological analysis of leaves and fruits was used to distinguish parental species and evaluate hybrid variability. Key Results Genotyping identified distinct genetic clusters for parental species and their hybrids, with geographic structuring within C. laevigata and C. rhipidophylla. Morphological data clearly separated genetically defined parental species, though hybrids can be difficult to distinguish from parents due to a big overlap in morphology. FCM indicated that C. × media is predominantly a diploid sexual hybrid as its diploid parents, while other tri- or tetraploid hybrids with polyploid C. rhipidophylla as a parent are apomictic. Ploidy rather than hybridisation dictates the mode of reproduction.