Unraveling the roles of genotype and environment in the expression of plant defense phenotypes

Phenotypic variability, resulting from the combination of genetic information and environmental conditions, is a major driver of ecological interactions. However, determining what aspects of phenotypic variation are due to genetics, the environment, and their interaction is critical to understanding ecological interactions and their evolutionary consequences. To assess how genetic variation and local environmental variation act as drivers of phenotypic variation in the context of plant-herbivore interactions, we combined a series of common-garden experiments with data collected from a naturally growing population of Asclepias syriaca, common milkweed, from which the common garden seeds originated. Specifically, we compared phenotypes of 14 maternal lines in two common gardens with their 14 maternal genets and carried out a reciprocal transplant experiment with three of the maternal lines. These trials allowed us to identify genetic variation in traits of common milkweed and then compare the traits with those of the natural population of milkweed to assess how local growing environment dictates trait expression. We measured plant growth, defoliation, and arthropod abundances and sampled leaves for their cardenolide concentrations, latex exudation, and carbon and nitrogen concentrations. Our results suggest that milkweed foliar quality and milkweed-herbivore interactions are mediated by a combination of genetic variation and response to the local environment. We observed that milkweed resistance to chewing herbivores was genetically variable but appeared mediated by the local growing environment. We recorded that one type of milkweed defense, foliar cardenolides, was environmentally controlled and lacked genetic variation within our population of maternal lines. In contrast, we observed significant genetic variation among maternal lines in foliar latex exudation. Some variation in milkweed-aphid interactions was best explained by local environment while some variation remained unexplained. Strikingly, any genetic variation detected in a common garden was uncorrelated with the other common garden and this variation was unrelated to the naturally growing maternal genets. This indicates that even when genetic variation is present, environmental variation is the dominant driver of trait expression in this population of common milkweed. Our data also suggest that milkweeds are phenotypically plastic, with unknown consequences for their fitness under environmental change. This research aids in understanding which milkweed traits may be acted upon by natural selection and may therefore be most likely to adapt to the rapid environmental change seen in the modern world.Master of ScienceEcology and Evolutionary BiologyUniversity of Michiganhttps://deepblue.lib.umich.edu/bitstream/2027.42/155004/1/POTTS_THESIS_2020_final.pdfDescription of POTTS_THESIS_2020_final.pdf : Thesi