Environment-dependent performance and fitness of Iris brevicaulis, I-fulva(Iridaceae), and hybrids

We tested the relative fitness of two Louisiana Iris species (Iris brevicau lis and I. fulva) and their first-generation backcross hybrids in three exp erimental watering treatments: dry, field capacity, and hooded. Leaf area e xpansion rate, gas exchange (A(max), g(s), c(i)), and biomass at final harv est were measured for each species and hybrid class in ail three environmen tal treatments. Fitness (based on total biomass) of the four genotypic clas ses differed significantly with environment. All genotypic classes performe d most poorly in the dry treatment. The fitness ranking of genotypic class also changed across environments (significant genotypic class by treatment interaction) with hybrid genotype fitness shifting relative to parental gen otypes. Integrating over all treatments. backcrosses to I. fulva showed the lowest fitness, whereas backcrosses to I. brevicaulis outperformed I. fulv a. The differences in fitness were apparently achieved by a combination of differences in photosynthesis and allocation. In this system, hybrids are n ot necessarily less fit than their parents, and the relationship between hy brid and parental fitness is influenced by environmental conditions, lendin g support to the Hybrid Novelty model of hybrid zone evolution.