Rachel Vannette

Pollinators, including honey bees, routinely encounter potentially harmful microorganisms and phytochemicals during foraging. However, the mechanisms by which honey bees manage these potential threats are poorly understood. In this study, we examine the expression of antimicrobial, immune and detoxification genes in Apis mellifera and compare between forager and nurse bees using tissue-specific RNA-seq and qPCR. Our analysis revealed extensive tissue-specific expression of antimicrobial, immune signaling, and detoxification genes. Variation in gene expression between worker stages was pronounced in the mandibular and hypopharyngeal gland (HPG), where foragers were enriched in transcripts that encode antimicrobial peptides (AMPs) and immune response. Additionally, forager HPGs and mandibular glands were enriched in transcripts encoding detoxification enzymes, including some associated with xenobiotic metabolism. Using qPCR on an independent dataset, we verified differential expressio...

Background/Question/Methods Mutualistic interactions between two species can be facilitated or undermined by third-party species, whose impact not only depends on their intrinsic traits, but also on the population ratio of the species involved in the interactions. Furthermore, the role of third-party species in mediating the mutualistic interactions may change when other species coexist and compete for the niche. For example, species of yeasts (e.g., Metschnikowia reukaufii) and bacteria (e.g., Gluconobacter sp.) are commonly found in the nectar of flowers. Recent empirical work indicates that the microbes differentially affect nectar chemistry, pollinator preference for nectar, and resulting seed set by plants, and as a result, yeasts and bacteria may benefit or inhibit the plant-pollinator mutualism, respectively. In addition, empirical results also demonstrate competitive interactions between yeasts and bacteria in nectar. Here we take a mathematical approach integrating a biolog...

Background/Question/Methods Predicting the strength of priority effects among species remains difficult, presenting a major challenge in explaining variation in the structure and function of ecological communities. Here we argue that mechanistic predictions of the strength of priority effects within and among environments are possible by considering three related but distinct components of the species’ niche, namely, impact niche, requirement niche, and niche overlap. Specifically, we develop four general hypotheses: 1) Species that impact the environment to a larger extent will exert stronger priority effects, 2) species whose growth rate is more sensitive to changes in the environment will experience stronger priority effects, 3) priority effects are stronger among species with higher niche overlap, and 4) environments that support more rapid growth will be characterized by stronger priority effects. We test these predictions using nectar yeast communities grown in four synthetic ...

Background/Question/Methods An increasing number of studies suggest that microorganisms found in floral nectar can affect plant fitness by altering nectar chemistry and, consequently, the attractiveness of flowers to pollinators. It has also been shown that nectar microbes rely on pollinators for flower-to-flower dispersal. These studies point to the potential for significant feedback among microbes, animals, and plants. However, it is not well understood how microbial species may vary in their effects on plants and animals or how they may develop and function as multi-species communities, despite ample evidence for the functional diversity of microbial species and the importance of species interactions in structuring microbial communities. We address these questions by synthesizing four lines of evidence from several field and laboratory experiments we recently conducted. Results/Conclusions First, experimental inoculation of hummingbird-pollinated Mimulus aurantiacus flowers with ...

Background/Question/Methods Symbiotic microbes can substantially influence how their hosts interact with the environment. Recent work suggests that plant-associated microbes can affect plant interactions with their biotic environment, pollinators, by changing nectar chemistry. Because microbes can vary greatly in their role as symbionts, determining the processes that shape species composition of symbiotic microbes is important to understanding their influence on host-environment interactions. Here, we examine how one process, dispersal, shapes microbial communities in floral nectar and their effects on nectar chemistry. Multiple modes of dispersal potentially exist, including via different pollinators and other flower-visiting animals and via wind and water. These characteristics of nectar microbial communities make them uniquely conducive to addressing the effects of dispersal mode. At the Jasper Ridge Biological Preserve in California, we manipulated microbial access to flowers o...

Aims: It is unclear how changing atmospheric conditions, including rising carbon dioxide concentration, influence interactions between above and below-ground systems and if intraspecific variation exists in this response.
Methods: We assessed interactive effects of atmospheric CO2 concentration, above-ground herbivory, and plant genotype on root traits and mycorrhizal associations. Plants from five families of Asclepias syriaca, a perennial forb, were grown under ambient and elevated atmospheric CO2 concentrations for three months. Foliar herbivory by either lepidopteran caterpillars or phloem-feeding aphids was imposed. Mycorrhizal colonization, below-ground biomass, root biomass, and secondary defensive chemistry in roots were quantified.
Results: We observed substantial genetic variation among A. syriaca families in their mycorrhizal colonization levels in response to elevated CO2 and herbivory treatments. Elevated CO2 treatment increased root biomass in all genetic families, whereas foliar herbivory tended to decrease root biomass. Root cardenolide concentration and composition varied greatly among plant families, and elevated CO2 treatment increased root cardenolides in two of the five plant families. Moreover, herbivores differentially affected the composition of cardenolides expressed below ground.
Conclusions: Increased atmospheric CO2 has the potential to influence interactions among plants, herbivores and mycorrhizal fungi and intraspecific variation suggests that such interactions can evolve.

The way species affect one another in ecological communities often depends on the order of species arrival. The magnitude of such historical contingency, known as priority effects, varies across species and environments, but this variation has proven difficult to predict, presenting a major challenge in understanding species interactions and consequences for community structure and function. Here, we argue that improved predictions can be achieved by decomposing species' niches into three components: overlap, impact and requirement. Based on classic theories of community assembly, three hypotheses that emphasise related, but distinct influences of the niche components are proposed: priority effects are stronger among species with higher resource use overlap; species that impact the environment to a greater extent exert stronger priority effects; and species whose growth rate is more sensitive to changes in the environment experience stronger priority effects. Using nectar-inhabiting microorganisms as a model system, we present evidence that these hypotheses complement the conventional hypothesis that focuses on the role of environmental harshness, and show that niches can be twice as predictive when separated into components. Taken together, our hypotheses provide a basis for developing a general framework within which the magnitude of historical contingency in species interactions can be predicted.

The effects of mutualistic interactions on partner phenotype and fitness can vary with many factors, including the abundance of interacting partners. Partner abundance may determine the relative costs and benefits associated with the interaction. Although arbuscular mycorrhizal fungi (AMF) can strongly influence plant phenotype and community interactions, the effects of AMF abundance on plant resistance traits and multitrophic interactions are not well understood. We tested the hypothesis that increasing AMF abundance in soil will increase mycorrhizal colonization and affect plant biomass, foliar phosphorus concentration, the expression of plant resistance and herbivore performance.
We inoculated Asclepias syriaca seedlings with Glomus etunicatum, Scutellospora fulgida and a mix of the two species in 11 AMF abundance treatments. We quantified plant phosphorus (P), growth and resistance phenotype and the performance of a specialist herbivore, Danaus plexippus on plants associated with varying amounts of fungi.
Increasing abundance of S. fulgida or G. etunicatum in soil increased the proportion of plant root colonized by AMF, but root colonization by a mix of the fungi was not related to inoculum density. The abundance of S. fulgida, but not G. etunicatum, increased per cent foliar P and trichome density, but decreased latex exudation. Abundance of all AMF treatments tended to decrease specific leaf mass (SLM), and the two single-species treatments unimodally affected the expression of total foliar cardenolides. Increasing abundance of the mix of AMF species also increased above-ground biomass, foliar P and trichome density, but had little effect on other traits. The presence of AMF, species identity and the AMF abundance all explained significant variation in the expression of plant traits, although their relative contribution varied depending on the trait examined. Mycorrhizal abundance strongly increased caterpillar growth rate, which was associated with a decline in SLM.
Synthesis. Variation in mycorrhizal abundance can profoundly influence the expression of plant resistance and subsequent herbivore performance. AMF abundance may be a key, but overlooked factor in determining the outcome of mycorrhizal mutualisms.

Mutualistic interactions are often subject to exploitation by species that are not directly involved in the mutualism. Understanding which organisms act as such ‘third-party’ species and how they do so is a major challenge in the current study of mutualistic interactions. Here, we show that even species that
appear ecologically similar can have contrasting effects as third-party species. We experimentally compared the effects of nectar-inhabiting bacteria and yeasts on the strength of a mutualism between a hummingbird-pollinated shrub, Mimulus aurantiacus, and its pollinators. We found that the common bacterium Gluconobacter sp., but not the common yeast Metschnikowia reukaufii, reduced pollination success, seed set and nectar consumption by pollinators, thereby weakening the plant–pollinator mutualism. We also found that the bacteria reduced nectar pH and total sugar concentration more greatly than
the yeasts did and that the bacteria decreased glucose concentration and increased fructose concentration whereas the yeasts affected neither. These distinct changes to nectar chemistry may underlie the microbes’ contrasting effects on the mutualism. Our results suggest that it is necessary to understand
the determinants of microbial species composition in nectar and their differential modification of floral rewards to explain the mutual benefits that plants and pollinators gain from each other.

Ecological interactions are complex networks, but have typically been studied in a pairwise fashion. Examining how third-party species can modify the outcome of pairwise interactions may allow us to better predict their outcomes in realistic systems. For instance, arbuscular mycorrhizal fungi (AMF) can affect plant interactions with other organisms, including below-ground herbivores, but the mechanisms underlying these effects remain unclear.
Here, we use a comparative, phylogenetically controlled approach to test the relative importance of mycorrhizal colonization and plant chemical defences (cardenolides) in predicting plant survival and the abundance of a generalist below-ground herbivore across 14 species of milkweeds (Asclepias spp.). Plants were inoculated with a mixture of four generalist AMF species or left uninoculated. After 1 month, larvae of Bradysia sp. (Diptera: Sciaridae), a generalist below-ground herbivore, colonized plant roots.
We performed phylogenetically controlled analyses to assess the influence of AMF colonization and toxic cardenolides on plant growth, mortality and infestation by fungus gnats. Overall, plants inoculated with AMF exhibited greater survival than did uninoculated plants. Additionally, surviving inoculated plants had lower numbers of larvae in their roots and fewer non-AM fungi than surviving uninoculated plants. In phylogenetic controlled regressions, gnat density in roots was better predicted by the extent of root colonized by AMF than by root cardenolide concentration. Taken as a whole, AMF modify the effect of below-ground herbivores on plants in a species-specific manner, independent of changes in chemical defence.
This study adds to the growing body of literature demonstrating that mycorrhizal fungi may improve plant fitness by conferring protection against antagonists, rather than growth benefits. In addition, we advocate using comparative analyses to disentangle the roles of shared history and ecology in shaping trait expression and to better predict the outcomes of complex multitrophic interactions.

How species interactions may modify the effects of environmental change on evolutionary adaptation is poorly understood. Elevated CO2 is known to alter plant–herbivore interactions, but the evolutionary consequences for plant populations have received little attention. We conducted an experiment to determine the effects of elevated CO2 and herbivory by a specialist insect herbivore (Danaus plexippus) on the expression of constitutive and induced plant defense traits in five genotypes of Asclepias syriaca, and assessed the heritability of these traits. We also examined changes in relative fitness among plant genotypes in response to altered CO2 and herbivory. The expression of plant defense traits varied significantly among genotypes. Elevated CO2 increased plant growth and physical defenses (toughness and latex), but decreased investment in chemical defenses (cardenolides). We found no effect of elevated CO2 on plant induction of cardenolides in response to caterpillar herbivory. Elevated CO2 decreased the expression of chemical defenses (cardenolides) to a different extent depending on plant genotype. Differential effects of CO2 on plant defense expression, rather than direct effects on relative fitness, may alter A. syriaca adaptation to changing climate.

"General theories of plant defence often fail to account for complex interactions between the resources required for defence expression. For example, the carbon that is used for carbon-based defence is acquired using nutrient-rich photosynthetic pigments, while nutrient gain itself requires
substantial carbon allocation belowground. We should therefore expect the expression of plant defence to reflect the tight linkage between carbon and nutrient gain, yet mechanistic studies linking resource gain with plant defence theory have been slow to emerge."

The presence and identity of mycorrhizal fungi can strongly affect plant growth, survival, and reproduction in natural and managed systems. Although mycorrhizal fungal densities vary within and among environments, the effect of mycorrhizal abundance on plant response and trophic consequences has rarely been examined. Using predictions from mutualism theory, we hypothesized that increasing density of arbuscular mycorrhizal fungi in the soil would increase both plant nutrition and carbon costs.

Background/Question/Methods Rising atmospheric CO2 causes distinct changes in organism physiology and phenotype, which may affect interactions among organisms. Recent work demonstrates that aboveground organisms can substantially affect belowground interactions. How does environmental change, specifically increasing atmospheric CO2, affect above-belowground interactions?

An organism's phenotype determines in large part how it interacts with the world around it. While many forces shape organism phenotype, among the most important are organism genotype, abiotic conditions, and interspecific interactions. In the work presented here, we describe the relative effects of each of these factors on the defensive phenotype of Asclepias syriaca (common milkweed).

Floral nectar is an important reward for pollinators, but is often colonized by pollinator-vectored yeast and bacteria that can attain high densities in nectar. These nectar inhabitants have been hypothesized to degrade floral resources, or conversely, to improve pollinator effectiveness. Changes in nectar chemistry have been implicated in both effects. Multiple species of yeast and bacteria can inhabit the nectaries of a hummingbird-pollinated shrub native to California, Mimulus aurantiacus. We conducted field and laboratory experiments ...