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Summary Plastic responses of plants to their environment vary as a result of genetic differentiation within and among species. To accurately predict rangewide responses to climate change, it is necessary to characterize genotype‐specific reaction norms across the continuum of historic and future climate conditions comprising a species' range.The North American hybrid zone ofPopulus trichocarpaandPopulus balsamiferarepresents a natural system that has been shaped by climate, geography, and introgression. We leverage a dataset containing 44 clonal genotypes from this natural hybrid zone, planted across 17 replicated common garden experiments spanning a broad climatic range. Growth and mortality were measured over 2 yr, enabling us to model reaction norms for each genotype across these tested environments.Species ancestry and intraspecific genomic variation significantly influenced growth across environments, with genotypic variation in reaction norms reflecting a trade‐off between cold tolerance and growth. Using modeled reaction norms for each genotype, we predicted that genotypes with moreP. trichocarpaancestry may gain an advantage under warmer climates.Spatial shifts of the hybrid zone could facilitate the spread of beneficial alleles into novel climates. These results highlight that genotypic variation in responses to temperature will have landscape‐level effects. 

ABSTRACT Chromosomal inversion polymorphisms are ubiquitous across the diversity of diploid organisms and play a significant role in the evolution of adaptations in those species. Inversions are thought to operate as supergenes by trapping adaptive alleles at multiple linked loci through the suppression of recombination. While there is now considerable support for the supergene mechanism of inversion evolution, the extent to which inversions trap pre‐existing adaptive genetic variation versus accumulate new adaptive variants over time remains unclear. In this study, we report new insights into the evolution of a locally adaptive chromosomal inversion polymorphism (inv_chr8A), which contributes to the adaptive divergence between coastal perennial and inland annual ecotypes of the yellow monkeyflower,Mimulus guttatus. This research was enabled by the sequencing, assembly and annotation of new annual and perennial genomes ofM. guttatususing Oxford Nanopore long‐read sequencing technology. In addition to the adaptive inv_chr8A inversion, we identified three other large inversion polymorphisms, including a previously unknown large inversion (inv_chr8B) nested within inv_chr8A. Through population genomic analyses, we determined that the nested inv_chr8B inversion is significantly older than the larger chromosomal inversion in which it resides. We also evaluated the potential role of key candidate genes underlying the phenotypic effects of inv_chr8A. These genes are involved in gibberellin biosynthesis and anthocyanin regulation. Although little evidence was found to suggest that inversion breakpoint mutations drive adaptive phenotypic effects, our findings do support the supergene mechanism of adaptation and suggest it may sometimes involve nested inversions that evolve at different times. 

Summary In coevolving species, parasites locally adapt to host populations as hosts locally adapt to resist parasites. Parasites often outpace host local adaptation since they have rapid life cycles, but host diversity, the strength of selection, and external environmental influence can result in complex outcomes.To better understand local adaptation in host–parasite systems, we examined locally adapted switchgrass (Panicum virgatum), and its leaf rust pathogen (Puccinia novopanici) across a latitudinal range in North America. We grew switchgrass genotypes in 10 replicated multiyear common gardens, measuring rust severity from natural infection in a ‘host reciprocal transplant’ framework for testing local adaptation. We conducted genome‐wide association mapping to identify genetic loci associated with rust severity.Genetically differentiated rust populations were locally adapted to northern and southern switchgrass, despite host local adaptation to environmental conditions in the same regions. Rust resistance was polygenic, and distinct loci were associated with rust severity in the north and south. We narrowed a previously identified large‐effect quantitative trait locus for rust severity to a candidate YELLOW STRIPE‐LIKE gene and linked numerous other loci to defense‐related genes.Overall, our results suggest that both hosts and parasites can be simultaneously locally adapted, especially when parasites impose less selection than other environmental factors. 

PremiseIdentifying the environmental factors responsible for natural selection across different habitats is crucial for understanding the process of local adaptation in plants. Despite its importance, few studies have successfully isolated the environmental factors driving local adaptation in nature. In this study, we evaluated the agents of selection responsible for local adaptation of the monkeyflowerMimulus guttatusto California's coastal and inland habitats. MethodsWe implemented a manipulative reciprocal transplant experiment at coastal and inland sites, where we excluded aboveground stressors in an effort to elucidate their role in the evolution of local adaptation. ResultsExcluding aboveground stressors, most likely a combination of salt spray and herbivory, completely rescued inland annual plant fitness when transplanted to coastal habitat. The exclosures in inland habitat provided a benefit to the performance of coastal perennial plants. However, the exclosures are unlikely to provide much fitness benefit to the coastal plants at the inland site because of their general inability to flower in time to escape from the summer drought. ConclusionsOur study demonstrates that a distinct set of selective agents (aboveground vs. belowground) are responsible for local adaptation at opposite ends of an environmental gradient.