The effects of hybridization on the physiology and ecology of two louisiana iris species, I. brevicaulis and I. fulva

Abstract

Mounting evidence suggests that hybridization between species is more common and more important to plant evolution than was previously believed by evolutionary biologists. This body of research addresses the physiological changes that occur in hybrids between Iris brevicaulis and I. fulva, and the ecological and evolutionary implications of that change. In a series of experiments in the field and greenhouse, I have documented that hybrids live in a different range of habitat than their parent species, and are capable of outperforming parent species in some habitats. The ability to survive in different habitat conditions is attributed to new trait combinations in recombinant hybrid individuals. Explicit tests of flood, drought, and shade tolerance indicate that some hybrid individuals are more tolerant of flooded conditions, but I. brevicaulis is superior in dry conditions, and I. fulva is superior in the shade. Physiological traits measured on parent species and hybrid individuals were quite similar, and not adequate to explain the observed patterns of relative hybrid fitness. By transplanting both species and several hybrid types into natural habitat in the field, I was that both vegetative and sexual fitness of hybrid individuals was higher than that of both parent species in flooded, I. fulva-like habitat, and equivalent to parent species in other habitats tested. Comparisons of seed and adult stage fitness in both field and greenhouse conditions indicate that hybrids and parent species have fairly equivalent fitness at the seed and seedling stage, while adult fitness of hybrid groups is more variable, and more responsive to the environment. Overall, this work suggests that hybrids in natural populations of I. brevicaulis and I. fulva are important sources of new genetic and phenotypic variation. Hybrids have likely played a large role in shaping the ecology and physiology of the species, and will continue to do so through introgression and ecological divergence of hybrids.