Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.

Multicellularity was one of the most significant innovations in the history of life, but its initial evolution remains poorly understood. Using experimental evolution, we show that key steps in this transition could have occurred quickly. We subjected the unicellular yeast Saccharomyces cerevisiae to an environment in which we expected multicellularity to be adaptive. We observed the rapid evolution of clustering genotypes that display a novel multicellular life history characterized by reproduction via multicellular propagules, a juvenile phase, and determinate growth. The multicellular clusters are uniclonal, minimizing within-cluster genetic conflicts of interest. Simple among-cell division of labor rapidly evolved. Early multicellular strains were composed of physiologically similar cells, but these subsequently evolved higher rates of programmed cell death (apoptosis), an adaptation that increases propagule production. These results show that key aspects of multicellular complexity, a subject of central importance to biology, can readily evolve from unicellular eukaryotes.T he evolution of multicellularity was transformative for life on earth (1). In addition to larger size, multicellularity increased biological complexity through the formation of new biological structures. For example, multicellular organisms have evolved sophisticated, higher-level functionality via cooperation among component cells with complementary behaviors (2, 3). However, dissolution and death of multicellular individuals occurs when cooperation breaks down, cancer being a prime example (4). There are multiple mechanisms to help ensure cooperation of component cells in most extant multicellular species (5-8), but the origin and the maintenance of multicellularity are two distinct evolutionary problems. Component cells in a nascent multicellular organism would appear to have frequent opportunities to pursue noncooperative reproductive strategies at a cost to the reproduction of the multicellular individual. How, then, does the transition to multicellularity occur?Understanding the evolution of complex multicellular individuals from unicellular ancestors has been extremely challenging, largely because the first steps in this process occurred in the deep past (>200 million years ago) (9, 10). As a result, transitional forms have been lost to extinction, and little is known about the physiology, ecology, and evolutionary processes of incipient multicellularity (11). Nonetheless, several key steps have been identified for this transition. Because multicellular organisms are composed of multiple cells, the first step in this transition was likely the evolution of genotypes that form simple cellular clusters (1, 3, 12-16). It is not known whether this occurs more readily through aggregation of genetically distinct cells, as in biofilms, or by mother-daughter cell adhesion after division. Once simple clusters have evolved, selection among multicelled clusters must predominate over selection among single cells within clusters (1,15,17,18). T...

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.

One of our current challenges is to quantify the mechanisms, capacity, and longevity of C stabilization in agricultural lands. The objectives of this study were to evaluate the long‐term (10 yr) role of C input in soil organic carbon (SOC) sequestration and to identify underlying mechanisms of C stabilization in soils. Carbon input and SOC sequestration, as governed by crop management strategies, were assessed across 10 Mediterranean cropping systems. Empirically derived relationships between yield and aboveground plus belowground crop biomass as well as estimates of C contributions from crop residues and manure amendments were used to quantify cumulative C inputs into each cropping system. Soil samples were separated into four aggregate size classes (>2000, 250–2000, 53–250, and <53 μm) and into three soil organic matter (SOM) fractions within the large (>2000 μm) and small (250–2000 μm) macroaggregates. Aggregate stability increased linearly with both C input (r2 = 0.75, p = 0.001) and SOC (r2 = 0.63, p = 0.006). Across the 10 cropping systems, annual soil C sequestration rates ranged from −0.35 to 0.56 Mg C ha−1 yr−1 We found a strong linear relationship (r2 = 0.70, p = 0.003) between SOC sequestration and cumulative C input, with a residue‐C conversion to SOC rate of 7.6%. This linear relationship suggests that these soils have not reached an upper limit of C sequestration (i.e., not C saturated). In addition, C shifted from the <53‐μm fraction in low C input systems to the large and small macroaggregates in high C input systems. A majority of the accumulation of SOC due to additional C inputs was preferentially sequestered in the microaggregates‐within‐small‐macroaggregates (mM). Hence, the mM fraction is an ideal indicator for C sequestration potential in sustainable agroecosystems.

There are both costs and benefits for host plants that associate with microbes in the rhizosphere. Typically, an individual plant associates with multiple microbial genotypes varying in mutualistic benefit. This creates a potential tragedy of the commons where less-mutualistic strains potentially share in the collective benefits, while paying less of the costs. Therefore, maintaining cooperation over the course of evolution requires specific mechanisms that reduce the fitness benefits from "cheating." Sanctions that discriminate among partners based on actual symbiotic performance are a key mechanism in rhizobia and may exist in many rhizosphere mutualisms, including rhizobia, mycorrhizal fungi, root endophytes, and perhaps free-living rhizosphere microbes. Where they exist, sanctions may take different forms depending on the system. Despite sanctions, less-effective symbionts still persist. We suggest this is because of mixed infection at spatial scales that limit the effects of sanctions, variation among plants in the strength of sanctions, and conflicting selection regimes.215