Flowering Phenology May Shape Hybridization Patterns of Hawthorn (Crataegus L.) Species

Abstract

Asynchronous flowering phenology is an important prezygotic barrier to hybridization, especially in the case of sympatric species, while the degree of overlap in flowering can influence hybridization odds and shift introgression. In our study, we investigated the flowering phenology of three sympatric Crataegus-species and their hybrids in Transylvania, Romania. C. laevigata flowered first at the end of April, followed by C. rhipidophylla and the hybrid taxa C. × subsphaerica and C.× media, C. × macrocarpa 8 days later, while C. monogyna flowered last, 3 days later. The parental species of the most frequent hybrid taxa C. × subsphaerica have been found to have the greatest overlap in their flowering, while hybrids of C. laevigata, which overlap narrowly in their flowering with the other two species, are rare. Interestingly, all three hybrid taxa overlapped almost perfectly in their flowering phenology with that of C. rhipidophylla, and except for C.× media, hybrid taxa are not intermediate in the timing of their flowering relative to their parents. Our results provide evidence that distribution patterns and frequency of Crataegus-hybrids in the landscape are at least partially shaped by the parental species’ overlap in flowering phenology, which influences hybridization odds and may shift introgression towards one of the parental species.