Non-Nitrogen-Fixing Can Escape Sanctions in Indeterminate Alfalfa Nodules, Exhibiting Parasitic Growth.

The soil bacterium can proliferate by leveraging its nitrogen-fixing symbiosis with legumes that form indeterminate root nodules, such as (alfalfa) and . In contrast to determinate-nodulating legumes, e.g.

How might bacteriophages shape biological invasions?

Invasions by eukaryotes dependent on environmentally acquired bacterial mutualists are often limited by the ability of bacterial partners to survive and establish free-living populations. Focusing on the model legume-rhizobium mutualism, we apply invasion biology hypotheses to explain how bacteriophages can impact the competitiveness of introduced bacterial mutualists. Predicting how phage-bacteria interactions affect invading eukaryotic hosts requires knowing the eco-evolutionary constraints of introduced and native microbial communities, as well as their differences in abundance and diversity.

Invasive legumes can associate with many mutualists of native legumes, but usually do not.

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.

Selection for cheating across disparate environments in the legume-rhizobium mutualism.

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.

Specialization-generalization trade-off in a Bradyrhizobium symbiosis with wild legume hosts.

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.

Transcriptomic insights into mechanisms of symbiotic cooperation.

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).

Precision of host sanctions in the fig tree-fig wasp mutualism: consequences for uncooperative symbionts.

Host sanctions that reduce the relative fitness of uncooperative symbionts provide a mechanism that can limit cheating and thus stabilise mutualisms over evolutionary timescales. Sanctions have been demonstrated empirically in several mutualisms. However, if multiple individual symbionts interact with each host, the precision with which individual cheating symbionts are targeted by host sanctions is critical to their short- and long-term effectiveness.

Permanent genetic resources added to Molecular Ecology Resources Database 1 October 2011-30 November 2011.

This article documents the addition of 139 microsatellite marker loci and 90 pairs of single-nucleotide polymorphism sequencing primers to the Molecular Ecology Resources Database. Loci were developed for the following species: Aglaoctenus lagotis, Costus pulverulentus, Costus scaber, Culex pipiens, Dascyllus marginatus, Lupinus nanus Benth, Phloeomyzus passerini, Podarcis muralis, Rhododendron rubropilosum Hayata var. taiwanalpinum and Zoarces viviparus.

Negotiation, sanctions, and context dependency in the legume-Rhizobium mutualism.

Two important questions about mutualisms are how the fitness costs and benefits to the mutualist partners are determined and how these mechanisms affect the evolutionary dynamics of the mutualism. We tackle these questions with a model of the legume-rhizobium symbiosis that regards the mutualism outcome as a result of biochemical negotiations between the plant and its nodules. We explore the fitness consequences of this mechanism to the plant and rhizobia and obtain four main results.

Selection on herbivory resistance and growth rate in an invasive plant.

The evolution of increased competitive ability (EICA) hypothesis proposes that invasive species evolve decreased defense and increased competitive ability following natural enemy release. Previous tests of EICA examined the result of evolution by comparing individuals from home and introduced ranges, but no previous study of this hypothesis has examined the process of evolution by analyzing patterns of selection. On the basis of EICA, there should be selection for competitive ability without herbivores and selection for defense with herbivores.

Pathways to mutualism breakdown.

Mutualisms are ubiquitous in nature despite the widely held view that they are unstable interactions. Models predict that mutualists might often evolve into parasites, can abandon their partners to live autonomously and are also vulnerable to extinction. Yet a basic empirical question, whether mutualisms commonly break down, has been mostly overlooked.

An empirical test of partner choice mechanisms in a wild legume-rhizobium interaction.

Mutualisms can be viewed as biological markets in which partners of different species exchange goods and services to their mutual benefit. Trade between partners with conflicting interests requires mechanisms to prevent exploitation. Partner choice theory proposes that individuals might foil exploiters by preferentially directing benefits to cooperative partners.

Epistasis and genotype-environment interaction for quantitative trait loci affecting flowering time in Arabidopsis thaliana.

A major goal of evolutionary biology is to understand the genetic architecture of the complex quantitative traits that may lead to adaptations in natural populations. Of particular relevance is the evaluation of the frequency and magnitude of epistasis (gene-gene and gene-environment interaction) as it plays a controversial role in models of adaptation within and among populations. Here, we explore the genetic basis of flowering time in Arabidopsis thaliana using a series of quantitative trait loci (QTL) mapping experiments with two recombinant inbred line (RIL) mapping populations [Columbia (Col) x Landsberg erecta (Ler), Ler x Cape Verde Islands (Cvi)].

Quantitative trait loci affecting delta13C and response to differential water availibility in Arabidopsis thaliana.

Phenotypic plasticity is an important response mechanism of plants to environmental heterogeneity. Here, we explored the genetic basis of plastic responses of Arabidopsis thaliana to water deficit by experimentally mapping quantitative trait loci (QTL) in two recombinant inbred populations (Cvi x Ler and Ler x Col). We detected genetic variation and significant genotype-by-environment interactions for many traits related to water use.

Genetic variation and relationships of constitutive and herbivore-induced glucosinolates, trypsin inhibitors, and herbivore resistance in Brassica rapa.

We examined genetic variation in inducibility and in constitutive and herbivore-induced levels of glucosinolates, trypsin inhibitors, and resistance to herbivory in families of Brassica rapa originating from a wild population. We also examined phenotypic and genetic correlations among absolute levels of these traits in control and induced plants. We grew seedlings of 10 half-sib families in pairs in pots, and exposed one plant per pair to folivory by Trichoplusia ni larvae.

Sanctions and mutualism stability: why do rhizobia fix nitrogen?

Why do rhizobia expend resources on fixing N(2) for the benefit of their host plant, when they could use those resources for their own reproduction? We present a series of theoretical models which counter the hypotheses that N(2) fixation is favoured because it (i) increases the exudation of useful resources to related rhizobia in the nearby soil, or (ii) increases plant growth and therefore the resources available for rhizobia growth. Instead, we suggest that appreciable levels of N(2) fixation are only favoured when plants preferentially supply more resources to (or are less likely to senesce) nodules that are fixing more N(2) (termed plant sanctions). The implications for different agricultural practices and mutualism stability in general are discussed.

Partner choice in nitrogen-fixation mutualisms of legumes and rhizobia.

Mutualistic interactions are widespread and obligatory for many organisms, yet their evolutionary persistence in the face of cheating is theoretically puzzling. Nutrient-acquisition symbioses between plants and soil microbes are critically important to plant evolution and ecosystem function, yet we know almost nothing about the evolutionary dynamics and mechanisms of persistence of these ancient mutualisms. Partner-choice and partner-fidelity are mechanisms for dealing with cheaters, and can theoretically allow mutualisms to persist despite cheaters.

Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus).

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance.

PATERNAL EFFECTS IN INHERITANCE OF A PATHOGEN RESISTANCE TRAIT IN IPOMOEA PURPUREA.

For continuously variable, polygenic characters, the response to selection depends upon the proportion of phenotypic variance that is caused by additive genetic variance, or narrow-sense heritability. Thus, a major goal of quantitative genetics is to partition phenotypic variance for a trait in a way that isolates additive genetic variance from other causes. The variance among paternal half-sib families, which is frequently used to estimate additive variance, is commonly recognized to include additive epistatic effects.

Pathogen-induced systemic resistance in Ipomoea purpurea.

Infection of Ipomoea purpurea by anthracnose, the disease caused by the fungal pathogen Colletotrichum dematium, increases resistance to subsequent infections on previously uninfected leaves. Fungal isolates varied in their levels of virulence but not in the extent to which they induced resistance. Induced resistance was equally effective against all isolates tested.

COSTS AND BENEFITS OF PLANT RESPONSES TO DISEASE: RESISTANCE AND TOLERANCE.

A major assumption of models of the evolution of plant resistance to disease is that plant resistance involves fitness costs. To test this assumption, a field experiment was performed so that a quantitative-genetic analysis could be used to detect fitness costs to Ipomoea purpurea of resistance to different fungal isolates of Colletotrichum dematium, a pathogenic fungus causing the disease anthracnose. This experiment yielded no evidence that resistance to anthracnose involves direct fitness costs.

EXAMINING SELECTION ON THE MULTIVARIATE PHENOTYPE: PLANT RESISTANCE TO HERBIVORES.

Cost-benefit theory posits that stabilizing selection, produced by a trade-off between associated costs and benefits, often maintains phenotypic traits at intermediate equilibrium values. Measurement of selection on one type of trait, resistance to herbivory, should provide evidence to test this prediction. However, most plants host more than one species of herbivore, and resistance to various herbivores may be phenotypically correlated.

THE EVOLUTION OF RESISTANCE TO HERBIVORY IN IPOMOEA PURPUREA. I. ATTEMPTS TO DETECT SELECTION.

In this study, we looked for evidence of directional or stabilizing/disruptive selection on plant size and on the level of damage (resistance) caused by four types of herbivores in the annual morning glory Ipomoea purpurea. Selection was estimated by standard phenotypic regression analysis and by regression on breeding values. The phenotypic regression analysis revealed directional selection for all five characters (i.