Hybrids and Herbivory: Genetic Patterns of Tolerance in Hybrids
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Hybrid offspring produced from genetically divergent lineages may show variable fitness with respect to the parental species. This variable fitness may result in ecological and evolutionary consequences, such as the strengthening of prezygotic isolating barriers or introgression of the divergent genomes. The outcomes of such consequences can be tested by determining the genetic modes of inheritance of fitness traits in both parental and hybrid crosses. Herbivores are known to affect plant fitness in nature, and plants have evolved defense mechanisms to reduce the chance and effects of herbivory. These defense mechanisms may be categorized as resistance or tolerance mechanisms. Resistance mechanisms act to reduce the incidence or level of herbivore damage, while tolerance is defined as the degree to which plant fitness is reduced by herbivore damage relative to the fitness of the plant in the undamaged state. In this study, I simulated herbivory by clipping parental and hybrid classes (cross types) of Louisiana Iris to determine the genetic modes of inheritance of tolerance with respect to four different fitness measurements: plant height, root weight, leaf weight, and rhizome weight. The difference in fitness effects of cross type due to herbivory treatment was significant for plant height and root weight variables. Heterosis was indicated for these two measures of asexual fitness, because F1 fitness was significantly greater than the midparent, and back crosses were significantly lower than the expectation of additive dominance. Because evidence of heterosis was observed in this study, and backcross hybrids are fit relative to their parents, the potential for gene flow is likely in this system.