Browsing by Author "Mitchell-Olds, Thomas"

How genetic variation is maintained in the face of persistent natural selection is a central question in evolutionary biology. Here, I leverage a focal polymorphism, leaf chemical profile in a perennial wildflower (Boechera stricta, Brassicaceae) to investigate the ecological and genetic mechanisms that may influence the maintenance of variation in this trait.

In the first chapter, I present data from a suite of common garden and greenhouse experiments showing that the alleles underlying variation in chemical profile have contrasting fitness effects across environments. I identify two putative selective drivers on chemical profiles, and utilize phenotype-environment associations and molecular genetic analyses to test for additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, likely caused by pleiotropic effects of genes that influence secondary chemical biosynthesis on herbivore defense and drought response.

In the second chapter, I utilize a multi-year, manipulative field experiment to test for the effects of variation in selective drivers on genotypes conferring contrasting chemical profiles. I integrate variable effects of the environment on fitness components across life history and across environments to assess patterns of lifetime fitness. These data suggest that environmental conditions in which contrasting genotypes can both persist may be widespread.

Together, these chapters provide complementary perspectives on the question of persistent natural variation, suggesting that variation in secondary metabolic profiles in B. stricta may persist at present due to balancing selection, and may continue to persist in the future under variable environmental scenarios.

Grazing herbivores can significantly reduce plant fitness by causing apical meristem damage and completely removing reproductive structures. Compensation is a type of tolerance to grazing herbivores by which plants replace the removed reproductive structures. However compensation is possibly costly because of resource allocation to new reproductive structures, which can cause a tradeoff in survival probability overwinter in perennial plants. We show that variation in compensation is heritable and correlated with the environment from which the accessions originated in B. stricta. Specifically, length of lateral branches produced upon apical meristem damage when flowering was associated with climatic variables that change along an elevation gradient. Genotypes originating from low elevations— where the temperature is warmer and growing seasons are longer — produce longer lateral branches. Genotypes from high elevation sites — where the temperature is cooler and growing seasons are shorter — produce shorter lateral branches. This study suggests that compensation in the form of lateral branch length is affected by natural selection, which is potentially the result of tradeoffs between survival and reproduction.