Conserving Milkweed: The Effects Of Environmental Change And Viral Prevalence On Asclepias Syriaca
Published 2017 · Biology
Monarch butterflies (Danaus plexippus L.) undergo a cross-continental migration each year, depending on the presence of common milkweed (Asclepias syriaca L.), both as a food source and for breeding purposes. The past two decades have seen a marked decline in monarch populations. These declines are partially attributed to a loss of milkweed plants in agricultural areas. To prevent monarch butterfly extinction, conservation efforts are working to increase milkweed prevalence. Common milkweed is able to grow in a variety of habitat types and environmental conditions; however, the plants response to environmental change has not been extensively studied. The first chapter in this thesis examined the response of northeastern milkweed populations to changes in temperature and precipitation. In addition, the response of plants to increasing levels of soil salinity was investigated, as milkweed plants in the northeast are particularly vulnerable to exposure to high salt levels. Growth patterns were captured by the following response variables: plant height, number of nodes and leaves, stem diameter, stem, root, and whole plant biomass, and leaf area. We found a mixed response of growth characteristics to elevated temperature; leaf area and root biomass were smaller, while node count was larger, and all other variables did not have consistent trends across trials. Across variables, drought conditions reduced plant growth compared to control and excess precipitation treatments, while excess was generally larger than control, although not consistently across response variables. As soil salinity increased, plant growth characteristics decreased, but low salt levels did not differ significantly from the control; therefore we concluded that common milkweed is moderately salt tolerance. These findings indicate that common milkweed populations may experience decreased growth rate with changing environmental conditions; however, the changes we observed will not likely result in an overall significant decrease. Conservation initiatives are currently focusing on planting milkweed near agricultural areas. While this is helpful for monarch butterflies, common milkweed can act as a host to many viral plant pathogens, which is problematic for growers. The second chapter of this thesis surveyed wild populations of northeastern common milkweed for the presence of cucumber mosaic virus (CMV). Tissue samples were tested for CMV using Immunostrip and ELISA methods. Seed samples were subsequently collected from plants testing positive for the virus and grown up to determine the rate of seed transmission. We found low levels (2.5%) of CMV positive plants throughout the northeast, with most field sites testing negative for the disease. Seed transmission studies were all negative, indicating that common milkweed seeds do not transmit CMV. These results demonstrate that common milkweed does not substantially contribute to the spread of CMV in the northeastern United States. The final chapter of this thesis presents methods for propagating common milkweed in tissue culture. Common milkweed has a high level of genetic variation, due to outcrossing, complex pollination methods, and incompatibility in selfpollination. Callus induction was obtained on Murashige and Skoog media containing 10 μM BAP + 5 μM NAA; shoot proliferation was obtained on media containing 1.25 μM BAP + 5 μM NAA; and root production occurred on media with 1.25 μM BAP and 2.5 μM NAA. The formulations in this thesis allow for the production of in vitro plantlets for future research efforts.