Patterns of herbivory and fungal infection in experimental Louisiana Iris hybrids

Background: The consequences of hybridization vary depending on the fitness of hybrids and pure species plants. Herbivory and fungal infection may affect fitness and often differ in their effect on hybrid versus pure-species genotypes. Questions: Do herbivory and fungal infection rates differ between pure species and hybrids? What is the underlying genetic basis of resistance to herbivory and fungal infection? Study system: Two ecologically similar species of Louisiana Iris: I. brevicaulis and I. fulva, and their F sub(1) and first-generation backcross hybrids. Methods: Mammalian herbivory (macroherbivory), insect herbivory (microherbivory), and fungal infection were assayed in an experimental plot of 930 irises, including pure species and three hybrid classes planted in native Louisiana Iris habitat in the Atchafalaya swamp of southern Louisiana. Results: Macroherbivory rates were low in the field plot and did not differ significantly between crosstypes. Microherbivory and fungal infection rates differed significantly between crosstypes. Microhervibory did not differ significantly between the pure species plants and was lower than expected in one backcross class. The fungus Puccinia iridis infected one F sub(1) hybrid class at significantly greater rates than all other experimental crosstypes. Backcross hybrids towards I. brevicaulis were also infected by P. iridis at rates intermediate between the highly infected F sub(1) class and its recurrent parent, suggesting a pattern of underdominance for resistance traits in the F sub(1).