Many organisms live in temporally or spatially heterogeneous environments. One adaptive response to environmental heterogeneity is phenotypic plasticity, or the ability of an organism to change its phenotype in response to environmental variation. Plasticity is particularly important for plants, which cannot easily escape environmental stresses (e.g., competition, herbivory, or drought), and therefore is common in many morphological and physiological traits that allow plants to cope with stresses. Despite numerous examples of the adaptive value of plasticity in plant traits, the reaction norms of many plant traits do not evolve to their predicted optima: there is often considerable genetic variation in trait plasticities, and some populations display sub-optimal responses to environmental stresses. This mismatch may reflect a balance between the fitness benefits (adaptive value) of plasticity and evolutionary constraints, such as strong genetic correlations with other traits, a lack of genetic variation in the reaction norm, and fitness costs of plasticity.
This dissertation focuses on the evolution of plasticity in plant defense responses to insect herbivores. Plant induced defenses are well-studied ecologically, but little is known about potential constraints on their evolution or the effects of these constraints on insect herbivores. Using the andromonecious herb Solanum carolinense, I examined the adaptive value of plasticity and tested for several evolutionary constraints on plant induced defenses and tolerance. I found damage-induced plasticity in several defense traits, and herbivores significantly reduced plant fitness in the field. However, neither tolerance nor plasticity in induced defenses was adaptive, and most traits were selected against (i.e., costly) in both environments with and without insect herbivores.
I uncovered several genetic constraints on plasticity in the plant defense response, including a) a lack of genetic variation in some resistance traits, b) strong genetic correlations between constitutive resistance and inducibility, and c) genetic correlations among the plasticities of different defense traits. Combined, these genetic constraints structured variation in the plant-mediated interaction between the specialist folivores Manduca sexta and Leptinotarsa juncta; this is the first study to demonstrate that genetic trade-offs within a plant species can affect interactions at higher trophic levels.
Although there were no fitness costs of plasticity in induced chemical/mechanical defenses, there were fitness costs of tolerance, and I was able to detect selection acting directly on trait plasticities. Selection on plasticities was generally positive or stabilizing, indicating fitness benefits of canalization of two defense traits I measured. Fitness costs of tolerance were present both in environments with and without herbivores, meaning that tolerance of herbivory was never adaptive. My dissertation is the first to simultaneously test for both the adaptive value and a breadth of evolutionary constraints (including costs of plasticity) on the plant defense response.
Combined, my results indicate that selection should reduce both the plasticity and mean expression of S. carolinense defenses; however, correlated selection on plastic traits or a lack of genetic variation in plasticity may prevent this from occurring. Most importantly, these results suggest that two basic predictions of evolutionary theory - that phenotypic plasticity should be both adaptive and costly - do not always hold true for plant defense traits. This underscores the importance of measuring selection on any plastic defense trait assumed to be adaptive, and investigating other potential evolutionary constraints on defensive plasticity besides fitness costs. My study shows that genetic correlations may also be important, but correlations among defense trait plasticities have been rarely investigated and most studies do not account for the presence of these correlations when examining selection on individual defense traits.
|Commitee:||Davis, Frederick, Inouye, Brian D., Travis, Joseph, Winn, Alice A.|
|School:||The Florida State University|
|School Location:||United States -- Florida|
|Source:||DAI-B 75/06(E), Dissertation Abstracts International|
|Subjects:||Plant biology, Ecology, Evolution and Development|
|Keywords:||Genetic trade-offs, Herbivory, Induced defenses, Phenotypic plasticity, Plant-mediated interactions, Solanum carolinense|
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