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Microbiota comprise the bulk of life's diversity, yet we know little about how populations of microbes accumulate adaptive diversity across natural landscapes. Adaptation to stressful soil conditions in plants provides seminal examples of adaptation in response to natural selection via allelic substitution. For microbes symbiotic with plants however, horizontal gene transfer allows for adaptation via gene gain and loss, which could generate fundamentally different evolutionary dynamics. We use comparative genomics and genetics to elucidate the evolutionary mechanisms of adaptation to physiologically stressful serpentine soils in rhizobial bacteria in western North American grasslands. In vitro experiments demonstrate that the presence of a locus of major effect, the nre operon, is necessary and sufficient to confer adaptation to nickel, a heavy metal enriched to toxic levels in serpentine soil, and a major axis of environmental soil chemistry vari-ation. We find discordance between inferred evolutionary histories of the core genome and nreAXY genes, which often reside in putative genomic islands. This suggests that the evolutionary history of this adaptive variant is marked by frequent losses, and/or gains via horizontal acquisition across divergent rhizobium clades. However, different nre alleles confer distinct levels of nickel resistance, suggesting allelic substitution could also play a role in rhizobium adaptation to serpentine soil. These results illustrate that the interplay between evolution via gene gain and loss and evolution via allelic substitution may underlie adaptation in wild soil microbiota. Both processes are important to consider for understanding adaptive diversity in microbes and improving stress-adapted microbial inocula for human use. [ABSTRACT FROM AUTHOR]

Aims: Domestication and crop breeding has improved plants for human use, but may have unintended consequences for traits that are not selected upon. Disruption in the mycorrhizal mutualism has been observed in crops; possibly due to breeding in fertilised soil. Little is known about whether the legume-rhizobia mutualism can be disrupted by domestication and crop evolution, despite the importance of symbiotic nitrogen fixation. We aimed to identify differences in mutualistic outcomes between five legume cultivars and their wild progenitors in terms of their reactions across varying nitrogen levels. Methods: With five greenhouse experiments, we characterised symbiosis traits in chickpea, lentil, pea, peanut, and soybean across a gradient of N fertilisation to characterise whether symbiosis traits differ context specifically between wild and domesticated lines. Results: At lower levels of N addition, wild soybean benefited more from rhizobia than domesticated soybean. Chickpea cultivars abandoned symbiosis at lower N than wild chickpea accessions. Chickpea cultivars reduced per-nodule colony forming units (CFU) in response to N addition more than wild chickpeas, but lentil cultivars reduced CFU less than lentil accessions. The lentil, pea, and peanut cultivars did not differ from their wild relatives in rhizobial benefit, nor in nodulation response to N addition. Conclusions: Differences in the regulation of the root nodule symbiosis are evident between domesticated and wild chickpea and soybean, but not lentil, pea, or peanut. This indicates that mutualism disruption—a decrease in the magnitude of the mutually symbiotic interaction—is a possible, but not necessary, consequence of domestication. [ABSTRACT FROM AUTHOR]

While significant progress has been made in understanding global carbon (C) cycling, the mechanisms regulating belowground C fluxes and storage are still uncertain. New molecular technologies have the power to elucidate these processes, yet we have no widespread standardized implementation of molecular techniques. To address this gap, we introduce the Molecular Observation Network (MONet), a decadal vision from the Environmental Molecular Sciences Laboratory (EMSL), to develop a national network for understanding the molecular composition, physical structure, and hydraulic and biological properties of soil and water. These data are essential for advancing the next generation of multiscale Earth systems models. In this paper, we discuss the 1000 Soils Pilot for MONet, including a description of standardized sampling materials and protocols and a use case to highlight the utility of molecular-level and microstructural measurements for assessing the impacts of wildfire on soil. While the 1000 Soils Pilot generated a plethora of data, we focus on assessments of soil organic matter (SOM) chemistry via Fouriertransform ion cyclotron resonance-mass spectrometry and microstructural properties via X-ray computed tomography to highlight the effects of recent fire history in forested ecosystems on belowground C cycling. We observed decreases in soil respiration, microbial biomass, and potential enzyme activity in soils with high frequency burns. Additionally, the nominal oxidation state of carbon in SOM increased with burn frequency in surface soils. This results in a quantifiable shift in the molecular signature of SOM and shows that wildfire may result in oxidation of SOM and structural changes to soil pore networks that persist into deeper soils. [ABSTRACT FROM AUTHOR]

In mutualism, hosts select symbionts via partner choice and preferentially direct more resources to symbionts that provide greater benefits via sanctions. At the initiation of symbiosis, prior to resource exchange, it is not known how the presence of multiple symbiont options (i.e. the symbiont social environment) impacts partner choice outcomes. Furthermore, little research addresses whether hosts primarily discriminate among symbionts via sanctions, partner choice or a combination. We inoculated the legume, Acmispon wrangelianus, with 28 pairs of fluorescently labelled Mesorhizobium strains that vary continuously in quality as nitrogen-fixing symbionts. We find that hosts exert robust partner choice, which enhances their fitness. This partner choice is conditional such that a strain's success in initiating nodules is impacted by other strains in the social environment. This social genetic effect is as important as a strain's own genotype in determining nodulation and has both transitive (consistent) and intransitive (idiosyncratic) effects on the probability that a symbiont will form a nodule. Furthermore, both absolute and conditional partner choice act in concert with sanctions, among and within nodules. Thus, multiple forms of host discrimination act as a series of sieves that optimize host benefits and select for costly symbiont cooperation in mixed symbiont populations. [ABSTRACT FROM AUTHOR]

The rhizosphere has been called "one of the most complex ecosystems on earth" because it is a hotspot for interactions among millions of microbial cells. Many of these are microbes are also participating in a dynamic interplay with host plant tissues, signaling pathways, and metabolites. Historically, breeders have employed a plant‐centric perspective when trying to harness the potential of microbiome‐derived benefits to improve productivity and resilience of economically important plants. This is potentially problematic because: (i) the evolution of the microbes themselves is often ignored, and (ii) it assumes that the fitness of interacting plants and microbes is strictly aligned. In contrast, a microbe‐centric perspective recognizes that putatively beneficial microbes are still under selection to increase their own fitness, even if there are costs to the host. This can lead to the evolution of sophisticated, potentially subtle, ways for microbes to manipulate the phenotype of their hosts, as well as other microbes in the rhizosphere. We illustrate this idea with a review of cases where rhizosphere microbes have been demonstrated to directly manipulate host root growth, architecture and exudation, host nutrient uptake systems, and host immunity and defense. We also discuss indirect effects, whereby fitness outcomes for the plant are a consequence of ecological interactions between rhizosphere microbes. If these consequences are positive for the plant, they can potentially be misconstrued as traits that have evolved to promote host growth, even if they are a result of selection for unrelated functions. The ubiquity of both direct microbial manipulation of hosts and context‐dependent, variable indirect effects leads us to argue that an evolutionary perspective on rhizosphere microbial ecology will become increasingly important as we continue to engineer microbial communities for crop production. [ABSTRACT FROM AUTHOR]

Oomycete and fungal pathogens cause billions of dollars of damage to crops worldwide annually. Therefore, there remains a need for broad-spectrum resistance genes, especially ones that target pathogens but do not interfere with colonization by beneficial microbes. Motivated by evidence suggesting that phosphatidylinositol-3-phosphate (PI3P) may be involved in the delivery of some oomycete and fungal virulence effector proteins, we created stable transgenic soybean plants that express and secrete two different PI3P-binding proteins, GmPH1 and VAM7, in an effort to interfere with effector delivery and confer resistance. Soybean plants expressing the two PI3P-binding proteins exhibited reduced infection by the oomycete pathogen Phytophthora sojae compared to control lines. Measurements of nodulation by nitrogen-fixing mutualistic bacterium Bradyrhizobium japonicum , which does not produce PI3P, revealed that the two lines with the highest levels of GmPH1 transcripts exhibited reductions in nodulation and in benefits from nodulation. Transcriptome and plant hormone measurements were made of soybean lines with the highest transcript levels of GmPH1 and VAM7 , as well as controls, following P. sojae - or mock-inoculation. The results revealed increased levels of infection-associated transcripts in the transgenic lines, compared to controls, even prior to P. sojae infection, suggesting that the plants were primed for increased defense. The lines with reduced nodulation exhibited elevated levels of jasmonate-isoleucine and of transcripts of a JAR1 ortholog encoding jasmonate-isoleucine synthetase. However, lines expressing VAM7 transgenes exhibited normal nodulation and no increases in jasmonate-isoleucine. Overall, together with previously published data from cacao and from P. sojae transformants, the data suggest that secretion of PI3P-binding proteins may confer disease resistance through a variety of mechanisms. [ABSTRACT FROM AUTHOR]

In mutualisms, variation at genes determining partner fitness provides the raw material upon which coevolutionary selection acts, setting the dynamics and pace of coevolution. However, we know little about variation in the effects of genes that underlie symbiotic fitness in natural mutualist populations. In some species of legumes that form root nodule symbioses with nitrogen‐fixing rhizobial bacteria, hosts secrete nodule‐specific cysteine‐rich (NCR) peptides that cause rhizobia to differentiate in the nodule environment. However, rhizobia can cleave NCR peptides through the expression of genes like the plasmid‐borne Host range restriction peptidase (hrrP), whose product degrades specific NCR peptides. Although hrrP activity can confer host exploitation by depressing host fitness and enhancing symbiont fitness, the effects of hrrP on symbiosis phenotypes depend strongly on the genotypes of the interacting partners. However, the effects of hrrP have yet to be characterised in a natural population context, so its contribution to variation in wild mutualist populations is unknown. To understand the distribution of effects of hrrP in wild rhizobia, we measured mutualism phenotypes conferred by hrrP in 12 wild Ensifer medicae strains. To evaluate context dependency of hrrP effects, we compared hrrP effects across two Medicago polymorpha host genotypes and across two experimental years for five E. medicae strains. We show for the first time in a natural population context that hrrP has a wide distribution of effect sizes for many mutualism traits, ranging from strongly positive to strongly negative. Furthermore, we show that hrrP effect size varies across host genotypes and experiment years, suggesting that researchers should be cautious about extrapolating the role of genes in natural populations from controlled laboratory studies of single genetic variants. [ABSTRACT FROM AUTHOR]

Specialization in mutualisms is thought to be a major driver of diversification, but few studies have explored how novel specialization evolves, or its relation to the evolution of other niche axes. A fundamental question is whether generalist interactions evolve to become more specialized (i.e., oscillation hypothesis) or if partner switches evolve without any change in niche breadth (i.e., musical chairs hypothesis). We examined alternative models for the evolution of specialization by estimating the mutualistic, climatic, and edaphic niche breadths of sister plant species, combining phylogenetic, environmental, and experimental data on Acmispon strigosus and Acmispon wrangelianus genotypes across their overlapping ranges in California. We found that specialization along all three niche axes was asymmetric across species, such that the species with broader climatic and edaphic niches, Acmispon strigosus, was also able to gain benefit from and invest in associating with a broader set of microbial mutualists. Our data are consistent with the oscillation model of specialization, and a parallel narrowing of the edaphic, climatic, and mutualistic dimensions of the host species niche. Our findings provide novel evidence that the evolution of specialization in mutualism is accompanied by specialization in other niche dimensions. [ABSTRACT FROM AUTHOR]

Although most invasive species engage in mutualism, we know little about how mutualism evolves as partners colonize novel environments. Selection on cooperation and standing genetic variation for mutualism traits may differ between a mutualism's invaded and native ranges, which could alter cooperation and coevolutionary dynamics. To test for such differences, we compare mutualism traits between invaded‐ and native‐range host‐symbiont genotype combinations of the weedy legume, Medicago polymorpha, and its nitrogen‐fixing rhizobium symbiont, Ensifer medicae, which have coinvaded North America. We find that mutualism benefits for plants are indistinguishable between invaded‐ and native‐range symbioses. However, rhizobia gain greater fitness from invaded‐range mutualisms than from native‐range mutualisms, and this enhancement of symbiont fecundity could increase the mutualism's spread by increasing symbiont availability during plant colonization. Furthermore, mutualism traits in invaded‐range symbioses show lower genetic variance and a simpler partitioning of genetic variance between host and symbiont sources, compared to native‐range symbioses. This suggests that biological invasion has reduced mutualists' potential to respond to coevolutionary selection. Additionally, rhizobia bearing a locus (hrrP) that can enhance symbiotic fitness have more exploitative phenotypes in invaded‐range than in native‐range symbioses. These findings highlight the impacts of biological invasion on the evolution of mutualistic interactions. [ABSTRACT FROM AUTHOR]

Global climate change and shifting land‐use are increasing plant stress due to abiotic factors such as drought, heat, salinity and cold, as well as via the intensification of biotic stressors such as herbivores and pathogens. The ability of plants to tolerate such stresses is modulated by the bacteria and fungi that live on or inside of plant tissues and comprise the plant microbiome. However, the impacts of diverse classes of beneficial members of the microbiome and the contrasting stresses that impact plants are most commonly studied independently of each other.Our meta‐analysis of 288 experiments across 89 studies moves beyond previous studies in that we simultaneously compare the roles of bacterial versus fungal microbiome members that live within plant tissues and colonize plant surfaces in ameliorating biotic versus abiotic sources of plant stress.The magnitude of microbial stress amelioration can be measured as the greater proportional impact of beneficial microbes on plant performance in more stressful environments. In the plant experiments we examine, the magnitude of microbial stress amelioration is substantial: it is 23% of the effect size of the typical impact of stress and 56% of the effect size of beneficial microbiome members in the absence of stress.The amount of benefit microbiome members confer to plants differs among classes of microbes, depending on whether plants are grown in stressful or non‐stressful environments. In the absence of stress, beneficial bacteria tend to confer greater plant benefits than do fungi. However, symbiotic fungi, especially arbuscular mycorrhizal fungi, more strongly ameliorate plant stress than do bacteria. In particular, beneficial microbes ameliorate salinity, foliar herbivory and fungal pathogen stress.These results highlight the fact that the impacts of beneficial and antagonistic components of the microbiome on plant performance depend on biotic and abiotic environmental contexts. Furthermore, beneficial microbes are especially critical for plant health in stressful environments and thus present opportunities to mitigate negative consequences of global change. A free plain language summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Students often struggle to connect concepts with evidence, sometimes because development of research skills has not been emphasized in their science courses. We developed a strategy and protocol to train high school students in research and experimental investigation of questions related to course material on legume biology. The richness of this subject matter allows for adaptations of our framework to address diverse areas of science, including principles in ecology, environmental science, agriculture, microbiology, and evolution. Our framework includes a hands-on classroom inquiry that investigates the symbiotic relationship between nitrogen-fixing rhizobial bacteria and legumes. This student-led, inquiry-based project employs an intellectually demanding, hands-on method of education to build critical research skills using an adaptable model and inexpensive materials. We also report positive student feedback from a post hoc survey to gauge student attitudes toward the activity and the effectiveness of this framework. [ABSTRACT FROM AUTHOR]

To establish and spread in a new location, an invasive species must be able to carry out its life cycle in novel environmental conditions. A key trait underlying fitness is the shift from vegetative to reproductive growth through floral development. In this study, we used a common garden experiment and genotyping‐by‐sequencing to test whether the latitudinal flowering cline of the North American invasive plant Medicago polymorpha was translocated from its European native range through multiple introductions, or whether the cline rapidly established due to evolution following a genetic bottleneck. Analysis of flowering time in 736 common garden plants showed a latitudinal flowering time cline in both the native and invaded ranges where genotypes from lower latitudes flowered earlier. Genotyping‐by‐sequencing of 9,658 SNPs in 446 individuals revealed two major subpopulations of M. polymorpha in the native range, only one of which is present in the invaded range. Additionally, native range populations have higher genetic diversity than invaded range populations, suggesting that a genetic bottleneck occurred during invasion. All invaded range individuals are closely related to plants collected from native range populations in Portugal and southern Spain, and population assignment tests assigned invaded range individuals to this same narrow source region. Taken together, our results suggest that latitudinal clinal variation in flowering time has rapidly evolved across the invaded range despite a genetic bottleneck following introduction. [ABSTRACT FROM AUTHOR]

Exotic, invasive plants and animals can wreak havoc on ecosystems by displacing natives and altering environmental conditions. However, much less is known about the identities or evolutionary dynamics of the symbiotic microbes that accompany invasive species. Most leguminous plants rely upon symbiotic rhizobium bacteria to fix nitrogen and are incapable of colonizing areas devoid of compatible rhizobia. We compare the genomes of symbiotic rhizobia in a portion of the legume's invaded range with those of the rhizobium symbionts from across the legume's native range. We show that in an area of California the legume Medicago polymorpha has invaded, its Ensifer medicae symbionts: (i) exhibit genome-wide patterns of relatedness that together with historical evidence support host-symbiont co-invasion from Europe into California, (ii) exhibit population genomic patterns consistent with the introduction of the majority of deep diversity from the native range, rather than a genetic bottleneck during colonization of California and (iii) harbor a large set of accessory genes uniquely enriched in binding functions, which could play a role in habitat invasion. Examining microbial symbiont genome dynamics during biological invasions is critical for assessing host-symbiont co-invasions whereby microbial symbiont range expansion underlies plant and animal invasions. [ABSTRACT FROM AUTHOR]

Mutualistic interactions can strongly influence species invasions, as the inability to form successful mutualisms in an exotic range could hamper a host's invasion success. This barrier to invasion may be overcome if an invader either forms novel mutualistic associations or finds and associates with familiar mutualists in the exotic range. Here, we ask (1) does the community of rhizobial mutualists associated with invasive legumes in their exotic range overlap with that of local native legumes and (2) can any differences be explained by fundamental incompatibilities with particular rhizobial genotypes? To address these questions, we first characterized the rhizobial communities naturally associating with three invasive and six native legumes growing in the San Francisco Bay Area. We then conducted a greenhouse experiment to test whether the invasive legume could nodulate with any of a broad array of rhizobia found in their exotic range. There was little overlap between the Bradyrhizobium communities associated with wild-grown invasive and native legumes, yet the invasive legumes could nodulate with a broad range of rhizobial strains under greenhouse conditions. These observations suggest that under field conditions in their exotic range, these invasive legumes are not currently associating with the mutualists of local native legumes, despite their potential to form such associations. However, the promiscuity with which these invading legumes can form mutualistic associations could be an important factor early in the invasion process if mutualist scarcity limits range expansion. Overall, the observation that invasive legumes have a community of rhizobia distinct from that of native legumes, despite their ability to associate with many rhizobial strains, challenges existing assumptions about how invading species obtain their mutualists. These results can therefore inform current and future efforts to prevent and remove invasive species. [ABSTRACT FROM AUTHOR]

Cheating is a focal concept in the study of mutualism, with the majority of researchers considering cheating to be both prevalent and highly damaging. However, current definitions of cheating do not reliably capture the evolutionary threat that has been a central motivation for the study of cheating. We describe the development of the cheating concept and distill a relative-fitness-based definition of cheating that encapsulates the evolutionary threat posed by cheating, i.e. that cheaters will spread and erode the benefits of mutualism. We then describe experiments required to conclude that cheating is occurring and to quantify fitness conflict more generally. Next, we discuss how our definition and methods can generate comparability and integration of theory and experiments, which are currently divided by their respective prioritisations of fitness consequences and traits. To evaluate the current empirical evidence for cheating, we review the literature on several of the best-studied mutualisms. We find that although there are numerous observations of low-quality partners, there is currently very little support from fitness data that any of these meet our criteria to be considered cheaters. Finally, we highlight future directions for research on conflict in mutualisms, including novel research avenues opened by a relative-fitness-based definition of cheating. [ABSTRACT FROM AUTHOR]

Background As our world becomes warmer, agriculture is increasingly impacted by rising soil salinity and understanding plant adaptation to salt stress can help enable effective crop breeding. Salt tolerance is a complex plant phenotype and we know little about the pathways utilized by naturally tolerant plants. Legumes are important species in agricultural and natural ecosystems, since they engage in symbiotic nitrogen-fixation, but are especially vulnerable to salinity stress. Results Our studies of the model legume Medicago truncatula in field and greenhouse settings demonstrate that Tunisian populations are locally adapted to saline soils at the metapopulation level and that saline origin genotypes are less impacted by salt than nonsaline origin genotypes; these populations thus likely contain adaptively diverged alleles. Whole genome resequencing of 39 wild accessions reveals ongoing migration and candidate genomic regions that assort non-randomly with soil salinity. Consistent with natural selection acting at these sites, saline alleles are typically rare in the range-wide species' gene pool and are also typically derived relative to the sister species M. littoralis. Candidate regions for adaptation contain genes that regulate physiological acclimation to salt stress, such as abscisic acid and jasmonic acid signaling, including a novel salt-tolerance candidate orthologous to the uncharacterized gene AtCIPK21. Unexpectedly, these regions also contain biotic stress genes and flowering time pathway genes. We show that flowering time is differentiated between saline and non-saline populations and may allow salt stress escape. Conclusions This work nominates multiple potential pathways of adaptation to naturally stressful environments in a model legume. These candidates point to the importance of both tolerance and avoidance in natural legume populations. We have uncovered several promising targets that could be used to breed for enhanced salt tolerance in crop legumes to enhance food security in an era of increasing soil salinization. [ABSTRACT FROM AUTHOR]

The primary dilemma in evolutionarily stable mutualisms is that natural selection for cheating could overwhelm selection for cooperation. Cheating need not entail parasitism; selection favours cheating as a quantitative trait whenever less-cooperative partners are more fit than more-cooperative partners. Mutualisms might be stabilised by mechanisms that direct benefits to more-cooperative individuals, which counter selection for cheating; however, empirical evidence that natural selection favours cheating in mutualisms is sparse. We measured selection on cheating in single-partner pairings of wild legume and rhizobium lineages, which prevented legume choice. Across contrasting environments, selection consistently favoured cheating by rhizobia, but did not favour legumes that provided less benefit to rhizobium partners. This is the first simultaneous measurement of selection on cheating across both host and symbiont lineages from a natural population. We empirically confirm selection for cheating as a source of antagonistic coevolutionary pressure in mutualism and a biological dilemma for models of cooperation. [ABSTRACT FROM AUTHOR]

Background: Specialized interactions help structure communities, but persistence of specialized organisms is puzzling because a generalist can occupy more environments and partake in more beneficial interactions. The “Jack-of-all-trades is a master of none” hypothesis asserts that specialists persist because the fitness of a generalist utilizing a particular habitat is lower than that of a specialist adapted to that habitat. Yet, there are many reasons to expect that mutualists will generalize on partners. Plant-soil feedbacks help to structure plant and microbial communities, but how frequently are soil-based symbiotic mutualistic interactions sufficiently specialized to influence species distributions and community composition? To address this question, we quantified realized partner richness and phylogenetic breadth of four wild-grown native legumes (Lupinus bicolor, L. arboreus, Acmispon strigosus and A. heermannii) and performed inoculation trials to test the ability of two hosts (L. bicolor and A. strigosus) to nodulate (fundamental partner richness), benefit from (response specificity), and provide benefit to (effect specificity) 31 Bradyrhizobium genotypes. Results: In the wild, each Lupinus species hosted a broader genetic range of Bradyrhizobium than did either Acmispon species, suggesting that Acmispon species are more specialized. In the greenhouse, however, L. bicolor and A. strigosus did not differ in fundamental association specificity: all inoculated genotypes nodulated both hosts. Nevertheless, A. strigosus exhibited more specificity, i.e., greater variation in its response to, and effect on, Bradyrhizobium genotypes. Lupinus bicolor benefited from a broader range of genotypes but averaged less benefit from each. Both hosts obtained more fitness benefit from symbionts isolated from conspecific hosts; those symbionts in turn gained greater fitness benefit from hosts of the same species from which they were isolated. Conclusions: This study affirmed two important tenets of evolutionary theory. First, as predicted by the Jack-of-alltrades is a master of none hypothesis, specialist A. strigosus obtained greater benefit from its beneficial symbionts than did generalist L. bicolor. Second, as predicted by coevolutionary theory, each test species performed better with partner genotypes isolated from conspecifics. Finally, positive fitness feedback between the tested hosts and symbionts suggests that positive plant-soil feedback could contribute to their patchy distributions in this system. [ABSTRACT FROM AUTHOR]

Although adaptive plant population divergence across contrasting soil conditions is often driven by abiotic soil factors, natural enemies may also contribute. Cryptic matching to the native soil color is a form of defensive camouflage that seeds can use to avoid detection by seed predators., The legume Acmispon wrangelianus occurs across a variety of gray-green serpentine soils and brown nonserpentine soils. Quantitative digital image analysis of seed and soil colors was used to test whether genetically based seed color is a closer match to the color of the native soil than to the color of other nearby soils., Lineages bear seeds that more closely match the color of their native serpentine or nonserpentine soil type than the opposing soil type. Further, even within a soil type, lineages bear seeds with a closer color match to the soil at their native site than to other sites., The striking concordance between seed and native soil color suggests that natural selection for locally camouflaged seed color morphs, probably driven by seed predators, may maintain adaptive divergence in pigmentation, despite the opportunity for migration between soil environments. [ABSTRACT FROM AUTHOR]

Specialization and concomitant trade-offs are assumed to underlie the non-neutral coexistence of lineages. Trade-offs across heterogeneous environments can promote diversity by preventing competitive exclusion. However, the importance of trade-offs in maintaining diversity in natural microbial assemblages is unclear, as trade-offs are frequently not detected in artificial evolution experiments. Stressful conditions associated with patches of heavy-metal enriched serpentine soils provide excellent opportunities for examining how heterogeneity may foster genetic diversity. Using a spatially replicated design, we demonstrate that rhizobium bacteria symbiotic with legumes inhabiting contrasting serpentine and nonserpentine soils exhibit a trade-off between a genotype's nickel tolerance and its ability to replicate rapidly. Furthermore, we detected adaptive divergence in rhizobial assemblages across soil type heterogeneity at multiple sites, suggesting that this trade-off may promote the coexistence of phenotypically distinct bacterial lineages. Trade-offs and adaptive divergence may be important factors maintaining the tremendous diversity within natural assemblages of bacteria. [ABSTRACT FROM AUTHOR]

Intraspecific genetic variation can affect community structure and ecosystem processes (Bolnick et al. 2011). It can also influence phenotypic expression by genotypes within other species to produce genotype-by-genotype ( G × G) interaction (Falconer & Mackay 1996). Evolution of one species drives correlated evolution of others when it causes G × G for fitness (Thompson 2005). However, the mechanisms by which species interact also influence evolutionary outcomes (Kummel & Salant 2006; Golubski & Klausmeier 2010; Akçay & Simms 2011; Grman et al. 2012). To identify genes and putative functional mechanisms underlying G × G interactions, Heath et al. (2012) analysed natural variation in the symbiotic transcriptome of the mutualistic nutritional symbiosis between a legume host Medicago truncatula and the facultative endosymbiotic rhizobium Sinorhizobium meliloti. Using twelve microarrays, the authors simultaneously measured host and symbiont gene expression in root nodules from four factorial pairings of host and symbiont genotypes that produced G × G in host fitness (Fig. 1, upper panel). Rhizobium gene expression was influenced by rhizobium and plant genotype and the G × G interaction (Fig. 1, lower panel), whereas plant gene expression was influenced primarily by plant genotype. The authors identified rhizobium genes that might contribute to G × G in host plant fitness. Heath et al. (2012) have moved beyond the constraints of single organism analysis towards a more realistic understanding of plants and bacteria as organisms inextricably linked with symbioses that affect even basic patterns of gene expression. [ABSTRACT FROM AUTHOR]

Plants are rife with bacteria and fungi that colonize roots and shoots both externally and internally. By providing novel nutritional and defense pathways and influencing plant biochemical pathways, microbes can fundamentally alter plant phenotypes. Here we review the widespread nature of microbially mediated plant functional traits. We highlight that there is likely fitness conflict between hosts and symbionts and that fitness outcomes can depend on partner genotypes and ecological factors. Microbes may influence ecosystems through their effects on the functional trait values and population dynamics of their plant hosts. These effects may feed back on symbiont evolution by altering transmission rates of symbionts and scale up to ecosystem processes and services. We end by proposing new avenues of research in this emerging field. [ABSTRACT FROM AUTHOR]

Plants are rife with bacteria and fungi that colonize roots and shoots both externally and internally. By providing novel nutritional and defense pathways and influencing plant biochemical pathways, microbes can fundamentally alter plant phenotypes. Here we review the widespread nature of microbially mediated plant functional traits. We highlight that there is likely fitness conflict between hosts and symbionts and that fitness outcomes can depend on partner genotypes and ecological factors. Microbes may influence ecosystems through their effects on the functional trait values and population dynamics of their plant hosts. These effects may feed back on symbiont evolution by altering transmission rates of symbionts and scale up to ecosystem processes and services. We end by proposing new avenues of research in this emerging field. [ABSTRACT FROM AUTHOR]

Species interactions play a critical role in biological invasions. For example, exotic plant and microbe mutualists can facilitate each other's spread as they co-invade novel ranges. Environmental context may influence the effect of mutualisms on invasions in heterogeneous environments, however these effects are poorly understood. We examined the mutualism between the legume, Medicago polymorpha, and the rhizobium, Ensifer medicae, which have both invaded California grasslands. Many of these invaded grasslands are composed of a patchwork of harsh serpentine and relatively benign nonserpentine soils. We grew legume genotypes collected from serpentine or non-serpentine soil in both types of soil in combination with rhizobium genotypes from serpentine or non-serpentine soils and in the absence of rhizobia. Legumes invested more strongly in the mutualism in the home soil type and trends in fitness suggested that this ecotypic divergence was adaptive. Serpentine legumes had greater allocation to symbiotic root nodules in serpentine soil than did nonserpentine legumes and non-serpentine legumes had greater allocation to nodules in non-serpentine soil than did serpentine legumes. Therefore, this invasive legume has undergone the rapid evolution of divergence for soil-specific investment in the mutualism. Contrary to theoretical expectations, the mutualism was less beneficial for legumes grown on the stressful serpentine soil than on the non-serpentine soil, possibly due to the inhibitory effects of serpentine on the benefits derived from the interaction. The soil-specific ability to allocate to a robust microbial mutualism may be a critical, and previously overlooked, adaptation for plants adapting to heterogeneous environments during invasion. [ABSTRACT FROM AUTHOR]

Thousands of minute scissurellid gastropods (Vetigastropoda: Scissurellidae) were observed and collected at floating light traps in Moorea, French Polynesia, where a mass spawning event occurred on November 11, 2002. More than 20,000 live snails were recovered from the traps, and epidemic spawning continued in the laboratory, where clouds of white sperm and red-orange eggs were broadcast into the water. No aggregations of this magnitude have been reported previously in vetigastropods, nor is the capacity for sustained swimming known from other vetigastropod taxa. Snails had emerged from a local benthic habitat of coral sand and swum through ≥2 m of water to reach the traps. Scanning electron micrographs of the ornate shells confirm the identity of the most abundant species as Scissurella spinosa, with lesser numbers of Sinezona plicata. Our observations indicate that scissurellids are not exclusively benthic and that they are not rare, even though they have seldom been collected and studied alive. Because little is known about the basic biology and anatomy of scissurellids and because their position within the basal Gastropoda is poorly resolved, we include information on the design and construction of inexpensive light traps to promote comparative investigation of other poorly known species. [ABSTRACT FROM AUTHOR]

Premise of the Study: Current methods for quantifying herbivore‐induced alterations in plant biochemistry are often unusable by researchers due to practical constraints. We present a cost‐effective, high‐throughput protocol to quantify multiple biochemical responses from small plant tissue samples using spectrophotometric techniques. Methods and Results: Using Solanum lycopersicum and Medicago polymorpha leaves pre‐ and post‐herbivory, we demonstrate that our protocol quantifies common plant defense responses: peroxidase production, polyphenol oxidase production, reactive oxygen species production, total protein production, and trypsin‐like protease inhibition activity. Conclusions: Current protocols can require 500 mg of tissue, but our assays detect activity in less than 10 mg. Our protocol takes two people 6 h to run any of the assays on 300 samples in triplicate, or all of the assays on 20 samples. Our protocol enables researchers to plan complex experiments that compare local versus systemic plant responses, quantify environmental and genetic variation, and measure population‐level variation. [ABSTRACT FROM AUTHOR]