Niche breadth and overlap of pseudogamous apomictic Crataegus hybrids and their progenitors in north-western Romania

Hybridization followed by introgression is well documented within the genus Crataegus and although many hybrids are known and widespread, factors influencing gene flow are still poorly understood. In this study, the niche breadth and overlap between hybrids and their progenitors are compared assuming that a relatively wide niche is advantageous in the spread of hybrids outside of the hybrid zone and the ecological factors potentially influencing hybrid and progenitor distribution and frequency are investigated. For this, specimens of Crataegus were sampled in four subregions in north-western Romania that differed in terms of elevation, forest cover, fragmentation and the extent of anthropogenic effect on the landscape. We used dynamic range boxes to estimate niche breadth and overlap based on climatic, landscape and microenvironmental variables. CCA models were used to explore the effects of environmental factors on the distribution of taxa of Crataegus. Cluster analysis was used to explore the phytosociological affiliation of the taxa studied. In this field investigation, 10 Crataegus taxa were identified: five species and five hybrids. Most Crataegus hybrids were relatively rare compared to their progenitors, except Crataegus ×subsphaerica (C. monogyna × C. rhipidophylla), which was the most common taxon in the area studied, being present even outside of the hybrid zone, suggesting range expansion. Based on the results, it seems that the distribution of the studied Crataegus taxa is determined primarily by climatic factors and light availability, and they appear to be affiliated with major habitat categories, such as low altitude broad-leaved forest or grasslands and scrublands. The results also revealed that most hybrids had parent-like niche breadths, and rare hybrids had surprisingly wide niches, suggesting that hybrid frequency is influenced more by phenological overlap between progenitors and effective long-distance dispersal of apomictic seeds, rather than environmental selection against hybrids. In addition, the presumed intermediate position in ecological preferences of the hybrid Crataegus relative to its progenitors was not as evident as expected, probably due to introgression towards one of the progenitors. Considering the frequent occurrence of hybrids outside the hybrid zone and the mosaic distribution pattern of the habitat of sympatric species, a combination of the mosaic and evolutionary novelty model best describe hybrid zone of the studied species.