Novel phenotypes among early generation hybrids of two Louisiana iris species: flooding experiments


*Present address and correspondence: Jill A. Johnston, Department of Plant Biology, University of Minnesota, St Paul, MN 55108, USA (e-mail


  • 1Recent studies of hybridization have shown that extreme (e.g. transgressive) phenotypes can be generated during massive genetic recombination events such as interspecific hybridization. Extreme hybrid phenotypes appear to have the potential to create new lineages and lead to evolutionary divergence.
  • 2Hybrid genotypes of some salt marsh species have proven capable of reengineering their ecosystems, while in other cases, hybridization has transformed plant communities by facilitating species invasion.
  • 3Using the Louisiana irises, a naturally hybridizing group of species, we assessed the potential for hybrids of Iris brevicaulis and I. fulva to possess extreme physiological and growth traits. Our glasshouse experiment used two soil water environments (wet and flood) to measure hybrid trait expression at both the genotypic class and individual genotype level.
  • 4At the genotypic class level, two of the three hybrid classes (F1s, and backcrosses towards each parental species) were transgressive for at least one physiological trait (F1s for root mass proportion, specific leaf area and final leaf area; backcrosses to I. brevicaulis for leaf area ratio). Three fitness components (total biomass, ramet production and flower production) were measured on each plant in the experiment. All three hybrid classes had greater clonal fitness (total biomass) but not greater sexual fitness (flower number) than parent species.
  • 5At the individual genotype level, two F1 hybrids showed extreme physiological trait expression: one for specific leaf area, and the other for shallow root allocation. Flower production was the only fitness component that showed some degree of environment-dependence at the individual genotype level.
  • 6Although early generation hybrid classes can contain rare genotypes with extreme phenotypic trait expression, and hybrid fitness components were equal or superior to both parental species in most cases, there was not a strong association between transgressive traits and elevated hybrid fitness.