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

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 <i>Crataegus</i>, which includes both diploid and polyploid species reproducing sexually or via apomixis. In Central Europe, <i>C. monogyna</i> and <i>C. laevigata</i> are predominantly diploid sexuals, while <i>C. rhipidophylla</i> is mainly a polyploid apomict. These species hybridize to form <i>C.</i> <i>×</i> <i>media</i>, <i>C.</i> <i>×</i> <i>macrocarpa</i>, and <i>C.</i> <i>×</i> <i>subsphaerica</i>. Our aim was to assess how hybridization, apomixis, and polyploidy shape <i>Crataegus</i> diversity by integrating genetic, morphological, and cytological data. Leaves and fruits were collected from ten natural populations where all three species coexist and hybridize. Species identification was performed with novel nuclear microsatellites, marking the first genetic-based <i>Crataegus</i> 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. Genotyping identified distinct genetic clusters for parental species and their hybrids, with geographic structuring within <i>C. laevigata</i> and <i>C. rhipidophylla</i>. Morphological data clearly separated genetically defined parental species, although hybrids can be difficult to distinguish from parents due to a big overlap in morphology. FCM indicated that <i>C.</i> <i>×</i> <i>media</i> is predominantly a diploid sexual hybrid like its diploid parents, while other tri- or tetraploid hybrids with polyploid <i>C. rhipidophylla</i> as a parent are apomictic. Ploidy rather than hybridization dictates the mode of reproduction.