Virus Yellows and Syndrome "Basses Richesses" in Western Switzerland: A Dramatic 2020 Season Calls for Urgent Control Measures.

Massive outbreaks of virus yellows (VY) and syndrome "basses richesses" (SBR) are thought to be responsible for the major loss of sugar beet yields in 2020 in western cantons of Switzerland. Typical yellowing symptoms were visible during field inspections, and control measures were reportedly ineffective or even absent. Both diseases induce yellowing but have distinct etiologies; while VY is caused by aphid-transmitted RNA viruses, SBR is caused by the cixiid-transmitted γ-proteobacterium Candidatus Arsenophonus phytopathogenicus. To clarify the situation, samples from diseased plants across the country were screened for the causal agents of VY and SBR at the end of the season. Beet yellows virus (BYV) and Beet chlorosis virus (BChV) showed high incidence nationwide, and were frequently found together in SBR-infected fields in the West. Beet mild yellowing virus (BMYV) was detected in two sites in the West, while there was no detection of Beet western yellows virus or Beet mosaic virus. The nucleotide diversity of the detected viruses was then investigated using classic and high-throughput sequencing. For both diseases, outbreaks were analyzed in light of monitoring of the respective vectors, and symptoms were reproduced in greenhouse conditions by means of insect-mediated inoculations. Novel quantification tools were designed for BYV, BChV and Ca. A. phytopathogenicus, leading to the identification of specific tissues tropism for these pathogens.

Experimental Assessment of the Economic Injury Level of the Cereal Leaf Beetle (Coleoptera: Chrysomelidae) in Winter Wheat.

The cereal leaf beetle, Oulema melanopus L., is an economically important pest species, reducing crop yields in many cereal growing parts of the world. Available studies on yield depression in winter wheat in relation to the density of O. melanopus revealed inconsistent results, creating uncertainty about economic thresholds for pest control. We present results from a series of controlled field trials over a 4-yr period (2014-2017) in Switzerland to examine the impact of artificial defoliation (0, 20, 40, and 60% of flag leaf area removed in 1-m2 field plots) on yield and yield components in winter wheat. The applied defoliation treatments correspond to field infestations levels of approximately 2-6 larvae per stem. Analysis of the combined data from 11 location-years revealed a significant linear decrease in yield per ear with increasing defoliation intensity. Yield per ear declined at a rate of 1.14% (SE = 0.28) per 10 percentage-point increase in defoliation. Similar results were found in a parallel series of field trials in which a 40% defoliation treatment was applied to eight common wheat cultivars. Interestingly, however, three cultivars showed complete tolerance in yield to defoliation. Data from two trials with natural defoliation by larvae revealed equally low levels of yield loss by feeding of O. melanopus. Based on the experimentally derived yield loss function, we calculated economic injury levels for a range of crop values. These will serve as benchmarks in the development of IPM decision-support tools for managing cereal leaf beetles in winter wheat.

Root exudate metabolites drive plant-soil feedbacks on growth and defense by shaping the rhizosphere microbiota.

By changing soil properties, plants can modify their growth environment. Although the soil microbiota is known to play a key role in the resulting plant-soil feedbacks, the proximal mechanisms underlying this phenomenon remain unknown. We found that benzoxazinoids, a class of defensive secondary metabolites that are released by roots of cereals such as wheat and maize, alter root-associated fungal and bacterial communities, decrease plant growth, increase jasmonate signaling and plant defenses, and suppress herbivore performance in the next plant generation. Complementation experiments demonstrate that the benzoxazinoid breakdown product 6-methoxy-benzoxazolin-2-one (MBOA), which accumulates in the soil during the conditioning phase, is both sufficient and necessary to trigger the observed phenotypic changes. Sterilization, fungal and bacterial profiling and complementation experiments reveal that MBOA acts indirectly by altering root-associated microbiota. Our results reveal a mechanism by which plants determine the composition of rhizosphere microbiota, plant performance and plant-herbivore interactions of the next generation.

Gene flow from foreign provenances into local plant populations: Fitness consequences and implications for biodiversity restoration.

Long-distance transplantation of seed material as done in restoration programs has raised concerns about the risks associated with the introduction of maladapted genotypes that may hybridize with neighboring native conspecifics and decrease local population fitness (outbreeding depression). We studied the consequences of gene flow from foreign provenances into local populations in the common grassland species Plantago lanceolata (Plantaginaceae). Three generations of intraspecific hybrids (F(1), F(2), and backcross to the local plants) were produced by controlled crossings between local plants and plants from geographically or environmentally distant populations. Their performance was compared to that of within-population crosses in a field experiment. Early growth in some interpopulation hybrids was significantly reduced, and this decrease in performance was higher in progeny of crosses with the local population from a different habitat than with geographically distant populations. At the end of the growing season, most fitness-related traits of the interpopulation hybrids were close to the average of their parents. Crosses with low-performing foreign parents therefore resulted in reduced fitness of the hybrids compared to the local plants and dilution of local adaptation. We conclude that the introduction of maladapted populations from distant or ecologically distinct environments might, at least temporarily, decrease the fitness of neighboring local plants.

Shift in cytotype frequency and niche space in the invasive plant Centaurea maculosa.

Polyploidy is often assumed to increase the spread and thus the success of alien plant species, but few empirical studies exist. We tested this hypothesis with Centaurea maculosa Lam., a species native to Europe and introduced into North America approximately 120 years ago where it became highly invasive. We analyzed the ploidy level of more than 2000 plants from 93 native and 48 invasive C. maculosa populations and found a pronounced shift in the relative frequency of diploid and tetraploid cytotypes. In Europe diploid populations occur in higher frequencies than tetraploids and only four populations had both cytotypes, while in North America diploid plants were found in only one mixed population and thus tetraploids clearly dominated. Our results showed a pronounced shift in the climatic niche between tetraploid populations in the native and introduced range toward drier climate in North America and a similar albeit smaller shift between diploids and tetraploids in the native range. The field data indicate that diploids have a predominately monocarpic life cycle, while tetraploids are often polycarpic. Additionally, the polycarpic life-form seems to be more prevalent among tetraploids in the introduced range than among tetraploids in the native range. Our study suggests that both ploidy types of C. maculosa were introduced into North America, but tetraploids became the dominant cytotype with invasion. We suggest that the invasive success of C. maculosa is partly due to preadaptation of the tetraploid cytotype in Europe to drier climate and possibly further adaptation to these conditions in the introduced range. The potential for earlier and longer seed production associated with the polycarpic life cycle constitutes an additional factor that may have led to the dominance of tetraploids over diploids in the introduced range.

Potential contribution of natural enemies to patterns of local adaptation in plants.

Genetic differentiation among plant populations and adaptation to local environmental conditions are well documented. However, few studies have examined the potential contribution of plant antagonists, such as insect herbivores and pathogens, to the pattern of local adaptation. Here, a reciprocal transplant experiment was set up at three sites across Europe using two common plant species, Holcus lanatus and Plantago lanceolata. The amount of damage by the main above-ground plant antagonists was measured: a rust fungus infecting Holcus and a specialist beetle feeding on Plantago, both in low-density monoculture plots and in competition with interspecific neighbours. Strong genetic differentiation among provenances in the amount of damage by antagonists in both species was found. Local provenances of Holcus had significantly higher amounts of rust infection than foreign provenances, whereas local provenances of Plantago were significantly less damaged by the specialist beetle than the foreign provenances. The presence of surrounding vegetation affected the amount of damage but had little influence on the ranking of plant provenances. The opposite pattern of population differentiation in resistance to local antagonists in the two species suggests that it will be difficult to predict the consequences of plant translocations for interactions with organisms of higher trophic levels.

Testing the evolution of increased competitive ability (EICA) hypothesis in a novel framework.

The "evolution of increased competitive ability" (EICA) hypothesis proposes that escape from natural enemies, e.g., after transcontinental introductions, alters the selection regime because costly defenses no longer enhance fitness. Such an evolutionary loss of defenses enables resources to be directed toward growth or other traits improving performance. We tested the EICA hypothesis in a novel framework in which the natural enemy is the traveler that follows its widespread host by accidental or deliberate (biocontrol) introductions. In a greenhouse experiment we used populations of Senecio vulgaris from North America, Europe, and Australia that differ in the history of exposure to the rust fungus Puccinia lagenophorae. Contrary to what is predicted by EICA, we found no evidence for increased levels of resistance to the rust fungus in plant populations with a longer history of rust exposure (Australia). Similarly, there was no evidence for reduced fecundity in these populations, although vegetative vigor, measured as secondary branching and growth rate, was lower. The maintenance of high rust resistance in populations with no (North America) or only a short history (Europe) of rust exposure is surprising given that resistance seems to incur considerable fitness costs, as indicated by the negative association between family mean resistance and fitness in the absence of disease observed for all three continents. The comparison of population differentiation in quantitative traits with estimates of differentiation in amplified fragment length polymorphic (AFLP) markers suggests that a number of fitness-related traits are under divergent selection among the studied populations. The proposed framework to test changes in the evolutionary trajectory underlying EICA can be employed in an expanded range of systems. These may include investigations on a cosmopolitan weed or crop when an antagonist is expanding its geographic range (such as our study), studies along a chronosequence of introduction time with expected increasing accumulation of natural enemies over time, or comparisons between introduced plant populations that differ in exposure time to biocontrol organisms.

Reliable confidence intervals in quantitative genetics: narrow-sense heritability.

Many quantitative genetic statistics are functions of variance components, for which a large number of replicates is needed for precise estimates and reliable measures of uncertainty, on which sound interpretation depends. Moreover, in large experiments the deaths of some individuals can occur, so methods for analysing such data need to be robust to missing values. We show how confidence intervals for narrow-sense heritability can be calculated in a nested full-sib/half-sib breeding design (males crossed with several females) in the presence of missing values. Simulations indicate that the method provides accurate results, and that estimator uncertainty is lowest for sampling designs with many males relative to the number of females per male, and with more females per male than progenies per female. Missing data generally had little influence on estimator accuracy, thus suggesting that the overall number of observations should be increased even if this results in unbalanced data. We also suggest the use of parametrically simulated data for prior investigation of the accuracy of planned experiments. Together with the proposed confidence intervals an informed decision on the optimal sampling design is possible, which allows efficient allocation of resources.

Climate vs. soil factors in local adaptation of two common plant species.

Evolutionary theory suggests that divergent natural selection in heterogeneous environments can result in locally adapted plant genotypes. To understand local adaptation it is important to study the ecological factors responsible for divergent selection. At a continental scale, variation in climate can be important while at a local scale soil properties could also play a role. We designed an experiment aimed to disentangle the role of climate and (abiotic and biotic) soil properties in local adaptation of two common plant species. A grass (Holcus lanatus) and a legume (Lotus corniculatus), as well as their local soils, were reciprocally transplanted between three sites across an Atlantic-Continental gradient in Europe and grown in common gardens in either their home soil or foreign soils. Growth and reproductive traits were measured over two growing seasons. In both species, we found significant environmental and genetic effects on most of the growth and reproductive traits and a significant interaction between the two environmental effects of soil and climate. The grass species showed significant home site advantage in most of the fitness components, which indicated adaptation to climate. We found no indication that the grass was adapted to local soil conditions. The legume showed a significant home soil advantage for number of fruits only and thus a weak indication of adaptation to soil and no adaptation to climate. Our results show that the importance of climate and soil factors as drivers of local adaptation is species-dependent. This could be related to differences in interactions between plant species and soil biota.

Evolution in invasive plants: implications for biological control.

Evidence is increasing that invasive plants can undergo rapid adaptive evolution during the process of range expansion. Here, we argue that evolutionary change during invasions will also affect plant-antagonist interactions and, thus, will have important implications for biological control programmes targeted at invasive plants. We explore how altered selection in the new range might influence the evolution of plant defence (resistance and tolerance) and life history. The degree to which such evolutionary processes might affect biological control efficacy is largely unexplored. We hope that, by testing the hypotheses that we propose here, a closer link can be established between biological control and evolutionary biology, to the benefit of both disciplines.

Long-term persistence in a changing climate: DNA analysis suggests very old ages of clones of alpine Carex curvula.

Carex curvula is a very slow-growing rhizomatous sedge that forms extensive stands in the European an alpine belt. The recruitment of sexual progeny is extremely rare and propagation occurs predominantly through clonal growth. The randomly amplified polymorphic DNA (RAPD) technique was used to analyse clonal structure in a small patch (2.0x0.4 m sampling transect plus some additional samples) of a high-alpine population of the species. Amplification of the DNA of 116 tiller samples from the patch with eight ten-base primers yielded a total of 95 bands, of which 73 were polymorphic. Based on the RAPD amplification profiles a total of 15 multilocus genotypes (putative clones) were identified. Due to the high number of polymorphic loci the number of genetic markers delineating individual clones was high (range: 16-39 markers) which suggests that our estimates of clonal diversity are precise. More than half of the sampled tillers were identified as belonging to a single clone which formed a relatively homogeneous disc intermingling with other clones only at its margin. Based on the maximum diameter of this large clone of more than 7000 tillers and estimates of annual expansion growth of rhizomes (0.4 mm year-1), the age of the clone was calculated to be around 2000 years. This demonstrates that clones of C. curvula may persist on a single spot over long periods with quite diverse alpine climates ranging from rather mild periods in the Middle Ages to cool periods during the so called "little ice age" in the last century. Our results suggest caution with plant migration scenarios based on shifting isotherms where late-successional clonal species, which dominate the alpine vegetation all over the world, are concerned.

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability.

Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.