Megan. E. Frederickson

Summary: Microbiomes that enhance the performance of host plants are likely to be co‐introduced with their host during colonization because of their intimate association. Yet, it is unclear how co‐introduced microbes will impact host colonization, as both the microbiome and its effects could vary upon introduction into a new habitat. Using the duckweed Lemna japonica – a cosmopolitan, freshwater angiosperm – and its microbiome, we tracked the colonization of both plants and microbes during an experimental co‐introduction in the wild. We tested how plant performance varied during colonization when plants were co‐introduced with microbes from their home habitat or with microbes local to the introduced habitat. We found that plant performance was substantially reduced when plants were co‐introduced with microbes from their home habitat (i.e. with microbes that are non‐local to the introduced habitat), relative to hosts with a local microbiome. Moreover, negative impacts from the initial, non‐local microbiome persisted for multiple host generations despite a rapid turnover in microbiome composition. Our results suggest that the initial microbiome plants are co‐introduced and can leave lasting impacts on plant performance during colonization. Considering the identity of the co‐introduced microbiome will therefore be critical to predicting plant colonization dynamics in an era of global change.

Most microbes that live in or on hosts are not obligate symbionts. Instead, they can cycle between host-associated and free-living phases and experience selection in both environments. Consequently, while the benefits that microbes confer to hosts are often assumed to be a product of host selection, they may also evolve independent of the host-microbe interaction, while the microbe is free-living. We investigated this hypothesis by experimentally evolving a beneficial Allorhizobium bacterium we had previously isolated from duckweed ( Lemna japonica ), evolving the strain in the absence of any host at high and low salinity and at high and low nitrogen. Inoculating evolved strains back onto the plants, we found microbial adaptation to high-salinity benefited host plants in high-salinity environments, but only when microbes evolved under low-nitrogen conditions. Microbial adaptation to high nitrogen reduced plant growth, in some cases below that of the uninoculated control. In low-nitrogen environments, microbial growth rate predicted plant benefit in matching salinity environments, suggesting the traits underlying microbial salinity adaptation may be associated with the microbial benefits. Whole-genome sequencing of the evolved strains revealed significant genomic shifts between selective treatments, including plasmid variation and point mutations associated with osmotic regulation, although many of the affected genes have unclear functions. The emergence of microbial benefits to hosts as a byproduct of microbial adaptation highlights that these benefits do not depend on host selection. Rather, predicting the evolution of these symbioses may require understanding both abiotic and biotic selective agents acting on key microbial traits mediating the host-microbe interaction.

Although mutualism is widely expected to broaden the realized niche of participating species, there have been no global syntheses examining how mutualism affects species' niches. Here, we combine mutualism trait data on 2,668 legume species with geographic occurrence, climate, and soil nitrogen data. We use these data to investigate the effects of generalized defensive ant-plant mutualisms and more specialized nutritional legume-rhizobium mutualisms on the range of temperature, precipitation, and soil nitrogen values where legumes occur. Legumes engaged in mutualisms with ants or rhizobia occupied significantly more biomes and generally larger niches than non-mutualist species. Legumes with extrafloral nectaries, a trait that attracts a generalized guild of defensive ant partners, had wider niches across all three niche axes, regardless of latitude. Effects of interacting with rhizobia-- a more specialized mutualism-- on niche breadth were also mainly positive, but depended on latitude, with this mutualism increasing precipitation and soil nitrogen niche breadth only at high latitude, and increasing temperature niche breadth at low latitude, but decreasing temperature niche breadth at high latitude. Mutualism therefore affects niche breadth, but in ways that depend on interaction type and biogeographic trends that vary latitudinally.

Synopsis: Course-based undergraduate research experiences (CUREs) provide a scalable model for engaging students in authentic scientific inquiry, bridging core biological concepts with real-world environmental applications. We introduce a new CURE lab tailored for introductory biology students at the undergraduate level, utilizing duckweed as a model organism to investigate ecological interactions and environmental management. Our paper presents a curriculum that engages students in hands-on research with a focus on duckweed’s role in ecosystem dynamics, pollutant remediation, and its potential as a bioresource, along with scientific results from student projects that serve as tangible examples of the curriculum’s outcomes. Through experimentation, students explore how duckweed can be applied to address real-world environmental challenges, utilizing advanced laboratory techniques and data analysis tools. Successfully implemented with 192 students across three semesters at our institutions, this CURE lab has produced reliable duckweed growth data with high reproducibility. This curriculum addresses the gap between traditional laboratory exercises and authentic research experiences through introducing opportunities to conduct reproducible experiments, analyze real data, and communicate scientific findings in meaningful contexts.

Understanding what determines colonization success is a key challenge of global change ecology. Microbiomes that enhance host performance are likely to be co-introduced with hosts during colonization due to their intimate association. Yet, it is unclear how co-introduced microbes will impact host colonization, as both the microbiome and its effects could vary upon introduction into a new habitat. Here, we used duckweeds —a cosmopolitan, freshwater angiosperm— and their microbiome to track colonization dynamics during an experimental co-introduction in the wild. Notably, we found that host performance was substantially reduced when plants were co-introduced with microbes non-local to the introduced habitat, relative to hosts with a local microbiome. Moreover, negative impacts from the non-local microbiome persisted for multiple host generations despite a rapid turnover in microbiome composition during colonization. Overall, we suggest that considering the co-introduced microbiome can help predict host colonization success under global change.

Aim: Mutualism can shape biogeographic patterns at macroecological scales. Recently, Delavaux et al. (2024) found that plant mutualisms with mycorrhizae, nitrogen-fixing bacteria, and animal pollinators limit island colonization, weakening the latitudinal diversity gradient (LDG) on oceanic islands. Here, we revisit their analysis and examine whether ants and plants engaged in a common defensive mutualism mediated by extrafloral nectaries (EFN) are similarly under-represented on oceanic islands, and whether this defensive ant-plant mutualism also weakens the LDG on islands. Location: Global. Time period: Present-day species distributions and traits. Major taxa studied: Vascular plants and ants. Methods: We used global trait and occurrence databases to compare ant and plant species richness between oceanic islands and their likely source mainlands for taxa that do and do not interact mutualistically via EFNs. When analyzing the factors that determine whether a plant species occurs on oceanic islands, we also included the mutualism types studied by Delavaux et al. (2024), namely biotic pollination, mycorrhizal fungi, and nitrogen-fixing bacteria, and added plant habit, life history, and phylogeny as covariates. Results: Both plants with EFNs and EFN-visiting ants are significantly over-represented on islands. The species richness of EFN-visiting ants and EFN-bearing plants also positively covary across islands. In ants, engaging in mutualisms mediated by EFNs significantly strengthens, rather than weakens, the LDG on islands, while for plants, EFNs have no effect or strengthen the LDG on islands, depending on data filtering. After accounting for plant habit, life history, and phylogeny, only biotic pollination significantly limits plant colonization of islands, whereas mycorrhizae and nitrogen-fixing bacteria now have positive or non-significant effects, respectively, on island colonization. Main conclusions: EFNs facilitate rather than limit plant and ant colonization on islands. Mutualism does not ubiquitously limit island colonization, and some mutualism types strengthen rather than dampen the LDG on islands.

The Biotic Interactions Hypothesis (BIH) predicts that species interactions will intensify from the poles to the tropics. We studied the ant-plant defensive interaction mediated by extrafloral nectaries (EFNs) in the castor bean plant, Ricinus communis, across a ~ 21° latitudinal gradient in its naturalized range in India. Among our study sites, we found that the ant-EFN mutualism in castor bean gets stronger from sub-tropical to tropical latitudes. Investment in mutualism by plants and visitation by ants increased from higher to lower latitudes. Further, latitude significantly explained ant community composition. However, contrary to the predictions of the BIH, standing herbivory increased with latitude, perhaps because plants invest less in biotic defence at higher latitudes. To our knowledge, our study is the first to test for the patterns predicted by the BIH for an ant-plant interaction in the Paleotropics. Results from our study can also help inform sustainable pest control practices for R. communis since India is the world’s largest producer of castor oil.

The Biotic Interactions Hypothesis (BIH) predicts that species interactions will intensify from the poles to the tropics. We studied the ant-plant defensive interaction mediated by extrafloral nectaries (EFNs) in the castor bean plant, Ricinus communis, across a ∼21° latitudinal gradient in its naturalized range in India. Among our study sites, we found that the ant-EFN mutualism in castor bean gets stronger from sub-tropical to tropical latitudes. Investment in mutualism by plants and visitation by ants increased from higher to lower latitudes. Further, latitude significantly explained ant community composition. However, contrary to the predictions of the BIH, standing herbivory increased with latitude, perhaps because plants invest less in biotic defence at higher latitudes. To our knowledge, our study is the first to test for the patterns predicted by the BIH for an ant-plant interaction in the Paleotropics. Results from our study can also help inform sustainable pest control practices for R. communis since India is the world’s largest producer of castor oil.

Soil microbiomes play key roles in plant productivity and nutrient cycling, and we need to understand whether and how they will withstand the effects of global climate change. We exposed in situ soil microbial communities to multiple rounds of heat, drought, or heat and drought treatments, and profiled microbial communities with 16S rRNA and ITS amplicon sequencing during and after these climatic changes. We then tested how domain and symbiotic lifestyle affected responses. Fungal community composition strongly shifted due to drought and its legacy. In contrast, bacterial community composition resisted change during the experiment, but still was affected by the legacy of drought. We identified fungal and bacterial taxa with differential abundance due to heat and drought and found that taxa affected during climate events are not necessarily the taxa affected in recovery periods, showing the complexity and importance of legacy effects. Additionally, we found evidence that symbiotic groups of microbes important to plant performance respond in diverse ways to climate treatments and their legacy, suggesting plants may be impacted by past climatic events like drought and warming, even if they do not experience the event themselves.