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

## TL;DR

This study explores how hybridization, polyploidy, and asexual reproduction shape genetic and morphological diversity in three hawthorn species and their hybrids in Central Europe.

## Contribution

The study introduces new genetic markers for Crataegus taxonomy and integrates genetic, morphological, and cytological data to clarify hybridization and reproductive modes.

## Key findings

- Genetic clusters of parental species and hybrids show geographic structuring.
- Hybrids with polyploid parents are apomictic, while diploid hybrids remain sexual.
- Morphology overlaps between hybrids and parents make identification difficult.

## Abstract

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. 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 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. 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, although 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 like its diploid parents, while other tri- or tetraploid hybrids with polyploid C. rhipidophylla as a parent are apomictic. Ploidy rather than hybridization dictates the mode of reproduction.

Hybridization, polyploidy, and asexual seed formation can blur species boundaries in plants and create complex patterns of variation. To understand how these processes interact in natural populations, we studied three Central European hawthorn species and their hybrids using newly developed genetic markers, leaf and fruit morphology, and flow cytometry of both plants and seeds. We show that species remain genetically distinct despite frequent hybridization, and that reproductive mode is largely driven by ploidy rather than hybrid origin: diploids reproduce sexually, while polyploids and their hybrids are mostly apomictic. These contrasting reproductive strategies help explain patterns of clonality, gene flow, and morphological overlap. Our work highlights the value of integrating genetic, cytological, and morphological data to resolve complex plant groups.