Plant Traits as Predictors of Population Differentiation and Climate Sensitivity: Investigating Gene Flow and Regeneration Niche of Asclepias in the Midwest U.S.

Finch, Jessamine Hope Schmoyer

doi:https://doi.org/10.21985/n2-vma3-y760

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Plant Traits as Predictors of Population Differentiation and Climate Sensitivity: Investigating Gene Flow and Regeneration Niche of Asclepias in the Midwest U.S.

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Plant life history traits influence the extent and degree of gene flow across a species range. The resulting genetic neighborhood size can influence population differentiation and the scale of local adaptation, with implications for species response to environmental change and sourcing of plant materials for ecological restoration. To assess the impact of trait variation on gene flow of species with shared evolutionary history, this dissertation examined members of the genus Asclepias L. (milkweeds; Apocynaceae). Asclepias incarnata (swamp milkweed) is primarily self-compatible, and is largely restricted to wetlands, wet prairies, and shorelines. Conversely, Asclepias syriaca is primarily self-incompatible and is habitat generalist, commonly found along roadsides, forest edges, and in prairies and old fields. This group is an important nectar source for pollinators, and supports many specialist herbivores, including the imperiled monarch butterfly (Danaus plexippus), for which it serves as the obligate larval host. A neutral genetic study of wild populations (Chapter 1) from three collection regions (Minnesota, Illinois, and Missouri), revealed extremely high gene flow for A. syriaca, with no evidence of isolation by distance. While not universal, results suggest that greater habitat-specificity A. incarnata can result in colonization of isolated habitat patches, ultimately reducing gene flow and neighborhood size for some populations. Despite generally high gene flow and low genetic variation in these species, a lab germination study (Chapter 2) identified significant differences in germination among species and populations. This study included a third congener, A. verticillata, which is self-incompatible and partial to dry prairies. A. syriaca demonstrated the broadest germination niche, and all species displayed latitudinal trends in germination, although patterns were not consistent across species. A field recruitment study at two field sites (Illinois and Missouri; Chapter 3), found low to no variation among seed sources within a field site for A. syriaca, while A. incarnata displayed significant effects of seed source, with fluctuating latitudinal patterns across early life stages. Overall analysis of fitness derived from aster models using biomass as the best proxy of fitness in these data found a positive relationship with latitude of seed origin for A. incarnata and less variation among local sources for A. syriaca. A comparison of germination and neutral genetic diversity and structure of wild populations and commercially available seed lots in the region (Minnesota, Wisconsin, Illinois, Iowa & Missouri; Chapter 4), found evidence for neutral and adaptive changes associated with commercial milkweed production, with implications for restoration success and natural populations. Collectively, these studies support the model that variation in plant traits within genera has implications for gene flow, genetic neighborhood size, and potentially variation in adaptive traits at early life stages. Generalist species with ubiquitous gene flow may show less variation among populations and thus, be less sensitive to climate change and amendable to broader seed transfer zones, while specialist species may show greater sensitivity and mandate more restricted seed transfer. Importantly, commercial seed differs from wild material, potentially as a result of genetic bottlenecks, selection, plasticity, and/or hybridization. Understanding and minimizing genetic changes in production, and avoiding commercial seed when the risk is too great for remnant populations, will be important for the success of monarch conservation efforts and the fitness of milkweed populations. Chapter 2 was published in Plant Biology in 2018 and a copy of the final, formatted publication is included in supplementary materials.

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