Transgressive character expression in a hybrid sunflower species

Diploid hybrid lineages often are ecologically distinct from their parental species. However. it is unclear whether this niche divergence is typically achieved via hybrid intermediacy, a mixture of parental traits, and/or the evolution of extreme (transgressive) morphological and ecophysiological fe atures. Here we compare an extensively studied hybrid sunflower species. He lianthus anomalus. with its putative parents. H. annuus and H. petiolaris, for 41 morphological and 12 ecophysiological traits. Helianthus anomalus wa s morphologically intermediate for one trait (2.4%), parental-like for 23 t raits (56.1%), and transgressive for 17 traits (41.5%). For ecophysiologica l traits, H. anomalus was not significantly different from one or both pare nts for nine traits (75%), and was transgressive for the remaining three (2 5%). Thus, H. anomalus appears to be a mosaic of parental-like and transgre ssive phenotypes. Although the fitness effects of the transgressive charact ers are not yet known, many of these characters are consistent with adaptat ions reported for other sand dune plants. Genetic studies are currently und erway to ascertain whether these extreme characters arose as a direct bypro duct of hybridization or whether they evolved via mutational divergence.