Experimental evidence for selection against hybrids between two interfertile red oak species

Reproductive isolation between related oak species within one taxonomic section is incomplete. Even though pre- and post­zygotic isolation mechanisms have been described for interfer­tile oak species, natural hybridization is common in contact zones between related oaks. The apparent restriction of inter­specific hybrids between ecologically divergent species to intermediate environments in contact zones suggests postzy­gotic isolation via environmental selection against hybrids in parental environments. Overrepresentation of hybrids in seeds as compared to adult trees provides additional indirect evi­dence for selection against hybrids. Here, we used genetic assignment analyses in progeny obtained from a sympatric stand of Quercus rubra and Quercus ellipsoidalis, two interfertile species with different adaptations to drought, to characterize the number of hybrids and “pure” species in the non-germinated acorns and in seedlings. The occurrence of 43.6 % F1 hybrids and introgressive forms among the non-germinated acorns and their very low frequency in the seedlings (9.3 %) is to our knowledge the first direct evidence for early selection against hybrids in oaks possibly as result of genetic incompatibilities. Additionally, we found a signature of positive selection on EST-SSR PIE200 in Q. rubra which needs further confirmation. These results contribute to our under­standing of reproductive isolation and divergence between interfertile oak species with different ecological adaptations.