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

Project Abstract: 
Phenotypic variability is the driver of many ecological and evolutionary processes. However, the phenotype of an organism is a product of both its genes and its environment. To understand an organism’s phenotype, and consequently its interactions with organisms around it, we must understand the independent and interactive effects of both genotype and environment. By leveraging a 10-year data set and employing a three-tier experiment, I will assess the contributions of genotype and environment to the expression of defense within a plant population under decline. Populations are able to adapt to environmental change only if heritable genetic variation exists within the population. Assessing whether genotypic variation is responsible for phenotypic variation will allow a better prediction of the population’s ability to adapt to a rapidly changing environment.
Status of Research Project: 
Years Active: 
My project will include three experimental plant groups: 1) Field common garden, 2) Greenhouse common garden, and 3) Reciprocal field transplant. Common garden experiments create a common environment for all plants and therefore remove environment from the genotype-environment interaction. Therefore, the phenotype exhibited is due only to the genotype. The seeds for all experiments will come from mapped genetic individuals (genets) of A. syriaca growing at the University of Michigan Biological Station (UMBS). Over the past 10 summers, M.D. Hunter has measured plant defense traits, nutrient levels, and arthropod abundances from these genets. The seeds collected from the genets are half-siblings (multiple seed pods from unknown fathers for each genetic mother). Seeds and seedlings will be classified by the maternal genotype. If phenotypes of the common garden plants correlate strongly with their historic maternal phenotypes from Hunter’s data, this suggests that genotype plays a major role in phenotypic expression. Conversely, weak parent-offspring correlations will suggest a dominant role for the environment in expression of defense phenotype. Field Common Garden: A randomized block design will be employed at the University of Michigan Biological Station in the summer of 2019. The randomized block design will be held in a fenced enclosure and will consist of 18 blocks (replicates). Each block will contain 15 plants (location randomized), each of a unique genotype. This results in 15 maternal genotypes, 18 plants per genotype for 270 plants total. Each plant will be in 18 cm x 16 cm pot that will be set into the ground, making the top soil of the pot level with the ground. Plants will be watered ad libitum. Each week, arthropod abundance and plant size surveys will be taken for each A. syriaca plant. Once in June, July, and August, plants will be sampled for foliar cardenolides and foliar carbon and nitrogen concentrations. Samples will be analyzed in the Fall of 2019 in Ann Arbor. Prediction: If genotype is the dominant driver of phenotypic variation, then plants in the field common garden will show similar defense and nutrient patterns to their maternal genotypes as recorded in M.D. Hunter’s 10-year data set. Greenhouse Common Garden: Methods for the Greenhouse Common Garden will be modeled after the Field Common Garden except that all A. syriaca plants will be held in blocks on benches within the UMBS Greenhouse. The same genotypes will be represented in the same sample size. Tissue sampling will occur within the same week for all experiments. Prediction: If exposure to herbivores is a dominant driver of phenotypic variation, then plants in the greenhouse common garden will exhibit less phenotypic variation than the field common garden plants due to the absence of arthropods in the greenhouse. Environmental Transplant: I will choose three maternal genotypes for reciprocal field transplants. Chosen genotypes will vary across the known range of defense expression and originate from spatially separated locations within the UMBS population. Replicate seeds per genet will be grown in groups of reciprocal transplants within all of the source genets. Therefore, each genet location will have individual plants from each of the three source genets (including their own). Plants will be therefore in the soils and conditions of the “host” genet location. Plants will grow in randomized blocks at each of the three locations of the maternal genotypes. Each block will consist of 5 replicates of each of 3 genotypes, totaling 15 plants at each maternal genet site (45 total plants). Plants in each block will be protected by screen netting to prevent deer and rabbit browsing. Prediction: If environment is the driver of phenotypic variation, then all plants in one location will show similar phenotypes regardless of the maternal genotype. Timetable: May 2019: First month of growth in greenhouse for all plants. Preparation of field plots and experimental set up. June – August 2019: Field plots and greenhouse experiments set up. Foliar chemistry samples taken at the end of each month for all plants. Arthropod abundance taken weekly for each plant. Fall 2019: Chemistry and nutrient samples analyzed on campus.