Effect of temperature and genetic structure on adaptive evolution at a dynamic range edge in the North American gypsy moth (Lymantria dispar L.)

The study of biological invasions is not only essential to regulate their vast potential for ecological and economical harm, they offer a unique opportunity to study adaptive evolution in the context of recent range expansions into novel environments. The North American invasion of the gypsy moth, Lymantria dispar L., since its introduction in 1869 to Massachusetts, has expanded westward to Minnesota, northward to Canada, and southward to North Carolina. Fluctuating range dynamics at the southern invasive edge are heavily influenced by heat exposure over their optimal (supraoptimal) during the larval stage of development. We coupled genomic sequencing with reciprocal transplant and laboratory-rearing experiments to examine the interactions of phenotypic, genetic, and environmental variation under selective supraoptimal regimes. We demonstrate that while there is no evidence to support local adaptation in the fitness-related physiological traits we measured, there are clear genomic patterns of adaptation due to differential survival in higher temperatures. Mapping of loci identified as contributing to local adaptation in a selective environment and those associated with phenotypic variation highlighted that variation in larval development time is partly driven by pleiotropic loci also affecting survival. Overall, I highlight the necessity and inferential power gained through replicating environmental conditions using both phenotypic and genome-wide analyses

Geographic Variation in Larval Metabolic Rate Between Northern and Southern Populations of the Invasive Gypsy Moth

Thermal regimes can diverge considerably across the geographic range of a species, and accordingly, populations can vary in their response to changing environmental conditions. Both local adaptation and acclimatization are important mechanisms for ectotherms to maintain homeostasis as environments become thermally stressful, which organisms often experience at their geographic range limits. The spatial spread of the gypsy moth (Lymantria dispar L.) (Lepidoptera: Erebidae) after introduction to North America provides an exemplary system for studying population variation in physiological traits given the gradient of climates encompassed by its current invasive range. This study quantifies differences in resting metabolic rate (RMR) across temperature for four populations of gypsy moth, two from the northern and two from southern regions of their introduced range in North America. Gypsy moth larvae were reared at high and low thermal regimes, and then metabolic activity was monitored at four temperatures using stop-flow respirometry to test for an acclimation response. For all populations, there was a significant increase in RMR as respirometry test temperature increased. Contrary to our expectations, we did not find evidence for metabolic adaptation to colder environments based on our comparisons between northern and southern populations. We also found no evidence for an acclimation response of RMR to rearing temperature for three of the four pairwise comparisons examined. Understanding the thermal sensitivity of metabolic rate in gypsy moth, and understanding the potential for changes in physiology at range extremes, is critical for estimating continued spatial spread of this invasive species both under current and potential future climatic constraints

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss (3,69). Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops (13,36,51,80)

History and climate adaptation drive the spatial genetic structure of foundational shrub species in western North America

Widely distributed plant taxa of western North America often display such variation due to divergent selection across heterogeneous landscapes that support environmental gradients as well as historical periods of isolation owing to biogeographical barriers and distributional shifts associated with Quaternary climate oscillations. Temporal and spatial variation in these processes often leads to a continuum of phenotypic and genetic divergence with variable reproductive isolation among populations and species. Polyploidization has additionally been a prominent driving factor in diversification with extended ecological consequences. My dissertation research is applying landscape genetic analyses of high throughput sequencing data to address factors underlying the formation and maintenance of diversification and reproductive barriers across two foundational shrub species of western North America, rubber rabbitbrush (Ericameria nauseosa) and big sagebrush (Artemisia tridentata). Overall, the two shrubs illustrate convergent evolution of subspecific genetic differentiation with limited admixture across the range that is strongly predicted by environmental variation (i.e., mosaic hybrid zone). Within subspecific designations of both species, there is strong population structure that coincides with both geography and environment elucidating that they are likely independently evolving lineages. Furthermore, A. tridentata differentiation and admixture is not only determined by the environment, but also by ploidal variation. Only individuals of the same ploidal level (e.g., diploid [2n] or tetraploid [4n]) can interbreed and those of different levels are reproductively isolated (i.e., triploid block). These results provide a range-wide perspective on how genetic variation and reproductive isolation is organized among and within subspecific lineages, ploidal levels, and environments, and have relevance for understanding the genomic and environmental factors shaping reproductive isolation across heterogeneous environments. Given its ecological significance of these two foundational shrub species, further understanding of these patterns and processes should inform ecological restoration and expectations for evolution across changing environments in western North America

History and climate adaptation drive the spatial genetic structure of foundational shrub species in western North America

Widely distributed plant taxa of western North America often display such variation due to divergent selection across heterogeneous landscapes that support environmental gradients as well as historical periods of isolation owing to biogeographical barriers and distributional shifts associated with Quaternary climate oscillations. Temporal and spatial variation in these processes often leads to a continuum of phenotypic and genetic divergence with variable reproductive isolation among populations and species. Polyploidization has additionally been a prominent driving factor in diversification with extended ecological consequences. My dissertation research is applying landscape genetic analyses of high throughput sequencing data to address factors underlying the formation and maintenance of diversification and reproductive barriers across two foundational shrub species of western North America, rubber rabbitbrush (Ericameria nauseosa) and big sagebrush (Artemisia tridentata). Overall, the two shrubs illustrate convergent evolution of subspecific genetic differentiation with limited admixture across the range that is strongly predicted by environmental variation (i.e., mosaic hybrid zone). Within subspecific designations of both species, there is strong population structure that coincides with both geography and environment elucidating that they are likely independently evolving lineages. Furthermore, A. tridentata differentiation and admixture is not only determined by the environment, but also by ploidal variation. Only individuals of the same ploidal level (e.g., diploid [2n] or tetraploid [4n]) can interbreed and those of different levels are reproductively isolated (i.e., triploid block). These results provide a range-wide perspective on how genetic variation and reproductive isolation is organized among and within subspecific lineages, ploidal levels, and environments, and have relevance for understanding the genomic and environmental factors shaping reproductive isolation across heterogeneous environments. Given its ecological significance of these two foundational shrub species, further understanding of these patterns and processes should inform ecological restoration and expectations for evolution across changing environments in western North America

Data from: Geographic variation in larval metabolic rate between northern and southern populations of the invasive gypsy moth

Thermal regimes can diverge considerably across the geographic range of a species, and accordingly, populations can vary in their response to changing environmental conditions. Both local adaptation and acclimatization are important mechanisms for ectotherms to maintain homeostasis as environments become thermally stressful, which organisms often experience at their geographic range limits. The spatial spread of the gypsy moth (Lymantria dispar L.) after introduction to North America provides an exemplary system for studying population variation in physiological traits given the gradient of climates encompassed by its current invasive range. This study quantifies differences in resting metabolic rate (RMR) across temperature for four populations of gypsy moth, two from the northern and two from southern regions of their introduced range in North America. Gypsy moth larvae were reared at high and low thermal regimes, then metabolic activity was monitored at four temperatures using stop-flow respirometry to test for an acclimation response. For all populations, there was a significant increase in RMR as respirometry test temperature increased. Contrary to our expectations, we did not find evidence for metabolic adaptation to colder environments based on our comparisons between northern and southern populations. We also found no evidence for an acclimation response of RMR to rearing temperature for three of the four pairwise comparisons examined. Understanding the thermal sensitivity of metabolic rate in gypsy moth, and understanding the potential for changes in physiology at range extremes, is critical for estimating continued spatial spread of this invasive species both under current and potential future climatic constraints

Data

Please see the accompanying README file for details

Southern Range Analysis

R script for analysis of Southern Range Data and Southern Range Survival Dat