| Summary: | <p>Abstract</p> <p>Background</p> <p>Few studies address the issue of hybridization in a more than two-species context. The species-rich <it>Quercus </it>complex is one of the systems which can offer such an opportunity. To investigate the contemporary pattern of hybridization we sampled and genotyped 320 offspring from a natural mixed forest comprising four species of the European white oak complex: <it>Quercus robur</it>, <it>Q. petraea</it>, <it>Q. pubescens</it>, and <it>Q. frainetto</it>.</p> <p>Results</p> <p>A total of 165 offspring were assigned unambiguously to one of the pollen donors within the study plot. The minimum amount of effective pollen originating from outside the plot varied markedly among the seed parents, ranging from 0.18 to 0.87. The majority of the successful matings (64.1%) occurred between conspecific individuals indicating the existence of reproductive barriers between oak species. However, the isolation was not complete since we found strong evidence for both first-generation (8.4%) and later-generation hybrids (27.5%). Only two out of eight seed parents, belonging to <it>Q. petraea </it>and <it>Q. robur</it>, showed a high propensity to hybridize with <it>Q. pubescens </it>and <it>Q. petraea</it>, respectively. Significant structure of the effective pollen pools (<it>Φ</it><sub><it>pt </it></sub>= 0.069, P = 0.01) was detected in our sample. However, no support was found for the isolation by distance hypothesis. The proportion of hybrids was much higher (79%) in the seed generation when compared to the adult tree generation.</p> <p>Conclusion</p> <p>First-generation hybrids were observed only between three out of six possible species combinations. Hybrids between one pair of species preferred to mate with one of their parental species. The observation of first and later-generation hybrids in higher frequency in acorns than in adults might be explained by selection against hybrid genotypes, the history of this uneven-aged forest or past introgression between species.</p>
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