Environmental gradients can drive adaptive evolutionary shifts in plant resource allocation among growth, reproduction, and herbivore resistance. However, few studies have attempted to connect these adaptations to underlying physiological and genetic mechanisms. Here, we evaluate potential mechanisms responsible for a coordinated locally adaptive shift between growth, reproduction, and herbivore defense in the yellow monkeyflower, Mimulus guttatus. Through manipulative laboratory experiments, we found that gibberellin (GA) growth hormones may play a role in the developmental divergence between perennial and annual ecotypes of M. guttatus. Further, we detected an interaction between a locally adaptive chromosomal inversion, DIV1, and GA addition. This finding is consistent with the inversion contributing to the evolutionary divergence between inland annual and coastal perennial ecotypes by reducing GA biosynthesis/activity in perennials. Finally, we found evidence that the DIV1 inversion is partially responsible for a coordinated shift in the divergence of growth, reproduction, and herbivore resistance traits between coastal perennial and inland annual M. guttatus. The inversion has already been established to have a substantial impact on the life‐history shift between long‐term growth and rapid reproduction. Here, we demonstrate that the DIV1 inversion also has sizable impacts on both the total abundance and composition of phytochemical compounds involved in herbivore resistance.

Premise Identifying 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 monkeyflower Mimulus guttatus to California's coastal and inland habitats. Methods We 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. Results Excluding 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. Conclusions Our study demonstrates that a distinct set of selective agents (aboveground vs. belowground) are responsible for local adaptation at opposite ends of an environmental gradient.

vi. Author Contributions: DL and DP designed the experiments; DL, DP and DB conducted the experiments; LH conducted the analyses of herbivore resistance compounds; DL and LH wrote the manuscript. vii. Acknowledgements: We would like to thank Sol Chavez for assisting with the quantification of PPGs. Seed collections were originally made possible by permission from the state parks of Oregon and California. Funding for this research was provided by Michigan State University through a startup package to DBL. viii. Raw data from experiments will be deposited at Dryad upon publication Mechanisms of a locally adaptive shift in allocation among growth, reproduction, and 1 herbivore resistance in Mimulus guttatus 2 3 ABSTRACT 4Environmental gradients can drive adaptive evolutionary shifts in plant resource allocation 5 among growth, reproduction, and herbivore resistance. However, few studies have attempted to 6 connect these adaptations to underlying physiological and genetic mechanisms. Here, we 7 evaluate potential mechanisms responsible for a coordinated locally adaptive shift between 8 growth, reproduction, and herbivore defense in the yellow monkeyflower, Mimulus guttatus. 9Through manipulative laboratory experiments we found that gibberellin (GA) growth hormones 10 may play a role in the developmental divergence between perennial and annual ecotypes of M. 11 guttatus. Further, we detected an interaction between a locally adaptive chromosomal inversion, 12 DIV1, and GA addition. This finding is consistent with the inversion contributing to the 13 evolutionary divergence between inland annual and coastal perennial ecotypes by reducing GA 14 biosynthesis/activity in perennials. Finally, we found evidence that the DIV1 inversion is 15 partially responsible for a coordinated shift in the divergence of growth, reproduction, and 16 herbivore resistance traits between coastal perennial and inland annual M. guttatus. The 17 inversion has already been established to have a substantial impact on the life-history shift 18 between long-term growth and rapid reproduction. Here, we demonstrate that the DIV1 inversion 19 also has sizable impacts on both the total abundance and composition of phytochemical 20 compounds involved in herbivore resistance. 21

Plant root−microbe interactions influence plant productivity, health, and resistance to stress. Although there is evidence that plant species and even genotypes can alter soil microbial community structure, environmental conditions can potentially outweigh plant genetic effects. Here, we used a reciprocal transplant experiment to understand the contributions of the environment and the host plant to rhizosphere microbiome composition in locally adapted ecotypes of Mimulus guttatus (syn. Erythranthe guttata). Two genotypes of a coastal ecotype and two genotypes of an inland ecotype were planted at coastal and inland sites. After 3 months, we collected rhizosphere and bulk soil and assessed microbial communities by 16S rRNA gene sequencing. We found that local environment (coastal versus inland site) strongly influenced rhizosphere communities, at least in part due to distinct local microbial species pools. Host identity played a smaller role: at each site, the ecotypes exhibited remarkably similar composition of microbial communities at the class level, indicating that divergent M. guttatus ecotypes recruit phylogenetically similar rhizosphere communities, even in environments to which they are maladapted. Nevertheless, the two ecotypes significantly differed in community composition at both sites due, in part, to an exclusive set of taxa associated with each ecotype. They also differed in alpha diversity at the inland site. Although this indicates that locally adapted M. guttatus ecotypes are genetically diverged in factors shaping rhizosphere communities, our findings highlight the context-specific interactions between host identity and local environment that shape those communities. [Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

24Identifying the environmental factors responsible for natural selection across different habitats is 25 crucial for understanding the process of local adaptation. Despite its importance, only a few 26 studies have successfully isolated the environmental factors driving local adaptation in nature. In 27 this study, we evaluated the agents of selection responsible local adaptation of the monkeyflower 28Mimulus guttatus to coastal and inland habitats in California. We implemented a manipulative 29 field reciprocal transplant experiment at coastal and inland sites, where we excluded 30 aboveground stressors in an effort to elucidate their role in the evolution of local adaptation. We 31 found that excluding these stressors, most likely a combination of salt spray and herbivory, 32 completely rescued inland plant fitness when transplanted to coastal habitat. In contrast, the 33 exclosures in inland habitat provided limited fitness benefit for either coastal or inland plants. 34