Fitness of reciprocal F hybrids between Rhinanthus minor and Rhinanthus major under controlled conditions and in the field.

The performance of first-generation hybrids determines to a large extent the long-term outcome of hybridization in natural populations. F hybrids can facilitate further gene flow between the two parental species, especially in animal-pollinated flowering plants. We studied the performance of reciprocal F hybrids between Rhinanthus minor and R. major, two hemiparasitic, annual, self-compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. Germination under laboratory conditions was much lower for the F hybrid formed on R. major compared with the reciprocal hybrid formed on R. minor, and this confirmed previous results from similar experiments. However, this difference was not found under field conditions, which seems to indicate that the experimental conditions used for germination in the laboratory are not representative for the germination behaviour of the hybrids under more natural conditions. The earlier interpretation that F hybrid seeds formed on R. major face intrinsic genetic incompatibilities therefore appears to be incorrect. Both F hybrids performed at least as well as and sometimes better than R. minor, which had a higher fitness than R. major in one of the two years in the greenhouse and in the field transplant experiment. The high fitness of the F hybrids confirms findings from naturally mixed populations, where F hybrids appear in the first year after the two species meet, which leads to extensive advanced-hybrid formation and introgression in subsequent generations.