Transgressive segregation provides a simple and plausible explanation for the niche divergence and phenotypic novelty often associated with plant hybrid lineages. QTL studies in numerous plant species indicate that transgressive segregation is frequent in plants, and that it is an expected consequence of the genetic architecture of differentiated populations or species. However, existing QTL studies of transgressive traits in plants have not attempted to validate the fitness effects of transgressive traits in natural habitats, nor have they measured the effects of individual QTL alleles on hybrid fitness. Studies like this may bridge the gap between transgressive segregation and the origin of novel adaptation in nature. We are currently conducting field experiments using synthetic hybrid lineages between Helianthus annuus and H. petiolaris , the two sunflower species that gave rise to H. paradoxus , the Pecos puzzle sunflower. Our goal is to study the relationships between hybrid fitness and transgressive traits associated with niche divergence (salt tolerance in this model system). Ultimately, we plan to study the fitness effects of individual QTL alleles in natural "hybrid habitat". Preliminary results of these selection experiments will be presented and discussed.

Key words: Helianthus , adaptation, fitness, QTLs, transgressive segregation