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Sonia E. Sultan






Environments experienced by parent individuals can profoundly affect offspring phenotypes. These inherited environmental effects can include specific developmental adjustments that improve offspring growth under the conditions that induced them. Such adaptive transgenerational plasticity has garnered much research interest, yet it remains unclear over how many generations these effects persist, if they influence fitness, and how they are inherited. Similarly, the molecular basis of adaptive within-generation plasticity is unclear in most cases. Recent epigenetics research suggests that DNA methylation can mediate adaptive plasticity, both within and across generations. This dissertation combines phenotypic plasticity experiments and DNA methylation analysis in order to address these outstanding questions.

Chapter One reviews examples of adaptive transgenerational plasticity in plants, the potential mechanistic bases of these inherited effects, and their ecological and evolutionary implications. Chapter Two demonstrates that adaptive transgenerational effects of drought stress persist over two generations in the annual plant Polygonum persicaria. These inherited effects enhanced the growth and survival of grandoffspring grown under severe drought stress. Chapter Three shows, through experimental demethylation, that DNA methylation mediates the inherited effects of drought stress in P. persicaria. Furthermore, these methylation-mediated effects of parental drought were genotype-specific. A central conclusion of this study is that genotype, epigenotype, and parental soil-moisture environment interact to adaptively influence functional traits in P. persicaria. Chapter Four examines the relationship between DNA methylation and adaptive within-generation plasticity. Drought stress, low-nutrient stress, and shade each induced DNA methylation changes, as measured by methylation-sensitive AFLP. However, stress-induced methylation changes were not detected in response to each stress in each genetic line. Because genetic lines expressed similar degrees of adaptive plasticity, there was not a consistent association between stress-induced changes in phenotypes and methylation patterns. While this subject requires further study, these results suggest that genotypespecific DNA methylation changes may contribute to the expression of adaptive plasticity. Such genotypic differences underscore the importance of incorporating genetic variation into ecological epigenetics studies.

Together, these studies indicate that interactions between genotype, epigenotype, and environmental signals – including those in previous generations – are a meaningful source of phenotypic variation. Further investigating these interactions represents a promising new direction in evolutionary biology.



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