How do invasive pests affect interactions between members of pre-existing agrosystems? The invasive pest Drosophila suzukii is suspected to be involved in the aetiology of sour rot, a grapevine disease that otherwise develops following Drosophila melanogaster infestation of wounded berries. We combined field observations with laboratory assays to disentangle the relative roles of both Drosophila in disease development. We observed the emergence of numerous D. suzukii, but no D. melanogaster flies, from bunches that started showing mild sour rot symptoms days after field collection. However, bunches that already showed severe rot symptoms in the field mostly contained D. melanogaster. In the laboratory, oviposition by D. suzukii triggered sour rot development. An independent assay showed the disease increased grape attractiveness to ovipositing D. melanogaster females. Our results suggest that in invaded vineyards, D. suzukii facilitates D. melanogaster infestation and, consequently, favours sour rot outbreaks. Rather than competing with close species, the invader subsequently permits their reproduction in otherwise non-accessible resources and may cause more frequent, or more extensive, disease outbreaks.

Environmentally acquired microbial symbionts could contribute to host adaptation to local conditions like vertically transmitted symbionts do. This scenario necessitates symbionts to have different effects in different environments. We investigated this idea in Drosophila melanogaster, a species which communities of bacterial symbionts vary greatly among environments. We isolated four bacterial strains isolated from the feces of a D. melanogaster laboratory strain and tested their effects in two conditions: the ancestral environment (i.e. the laboratory medium) and a new environment (i.e. fresh fruit with live yeast). All bacterial effects on larval and adult traits differed among environments, ranging from very beneficial to marginally deleterious. The joint analysis of larval development speed and adult size further shows bacteria affected developmental plasticity more than resource acquisition. This effect was largely driven by the contrasted effects of the bacteria in each environment. Our study illustrates that understanding D. melanogaster symbiotic interactions in the wild will necessitate working in ecologically realistic conditions. Besides, context-dependent effects of symbionts, and their influence on host developmental plasticity, shed light on how environmentally acquired symbionts may contribute to host evolution.

1. Insect herbivores choose their food according to its protein to carbohydrate ratio, but the reasons why different species have contrasting intake targets remain unclear. According to the growth rate hypothesis, P‐rich insects have higher growth rates. It is therefore expected that P‐rich insects will prefer plants that are high in protein (hence in nitrogen, N) and phosphorus (P).2. To test this hypothesis, the ecological stoichiometry and the interaction network frameworks were combined. The food preferences of 24 plant species by 23 grasshopper species were determined, and the N and P contents of both trophic levels were measured. The weighted mean P and N contents of the consumed plants, which represent the grasshoppers' feeding niche, were highly correlated, indicating that the grasshoppers' diets are spread along a single functional niche axis. The links between the stoichiometry of the plants and their consumers were then tested with the fourth corner analysis, a multivariate technique combining the plant traits, the insect traits and the interaction network.3. In line with the earlier hypothesis, P‐rich grasshoppers consumed plants high in N and P, probably because their growth rate is higher. These findings therefore introduce a mechanism that accounts for interspecific differences in diet preference. They also contribute to an understanding of how herbivore communities might respond to P and N limitation in ecosystems, and how complex interaction networks can influence biogeochemical cycles of N and P.

This article has been peer-reviewed and recommended by Peer Community in Evolutionary Biology ABSTRACT Environmentally acquired microbial symbionts could contribute to host adaptation to local conditions like vertically transmitted symbionts do. This scenario necessitates symbionts to have different effects in different environments. We investigated this idea in Drosophila melanogaster, a species which communities of bacterial symbionts vary greatly among environments. We isolated four bacterial strains isolated from the feces of a D. melanogaster laboratory strain and tested their effects in two conditions: the ancestral environment (i.e. the laboratory medium) and a new environment (i.e. fresh fruit with live yeast). All bacterial effects on larval and adult traits differed among environments, ranging from very beneficial to marginally deleterious. The joint analysis of larval development speed and adult size further shows bacteria affected developmental plasticity more than resource acquisition. This effect was largely driven by the contrasted effects of the bacteria in each environment. Our study illustrates that understanding D. melanogaster symbiotic interactions in the wild will necessitate working in ecologically realistic conditions. Besides, context-dependent effects of symbionts, and their influence on host developmental plasticity, shed light on how environmentally acquired symbionts may contribute to host evolution.

Both local adaptation and adaptive phenotypic plasticity can influence the match between phenotypic traits and local environmental conditions. Theory predicts that environments stable for multiple generations promote local adaptation, whereas highly heterogeneous environments favor adaptive phenotypic plasticity. However, when environments have periods of stability mixed with heterogeneity, the relative importance of local adaptation and adaptive phenotypic plasticity is unclear. Here, we used Drosophila suzukii as a model system to evaluate the relative influence of genetic and plastic effects on the match of populations to environments with periods of stability from three to four generations. This invasive pest insect can develop within different fruits, and persists throughout the year in a given location on a succession of distinct host fruits, each one being available for only a few generations. Using reciprocal common environment experiments of natural D. suzukii populations collected from cherry, strawberry, and blackberry, we found that both oviposition preference and offspring performance were higher on medium made with the fruit from which the population originated than on media made with alternative fruits. This pattern, which remained after two generations in the laboratory, was analyzed using a statistical method we developed to quantify the contributions of local adaptation and adaptive plasticity in determining fitness. Altogether, we found that genetic effects (local adaptation) dominate over plastic effects (adaptive phenotypic plasticity). Our study demonstrates that spatially and temporally variable selection does not prevent the rapid evolution of local adaptation in natural populations. The speed and strength of adaptation may be facilitated by several mechanisms including a large effective population size and strong selective pressures imposed by host plants.

Both adaptive phenotypic plasticity and local adaptation can influence the match between phenotypic traits and local environmental conditions. Theory predicts that coarse-grained environments, which are stable for multiple generations, promote local adaptation, while fine-grained environments, in which individuals encounter more than one environment in their lifetime, favor adaptive phenotypic plasticity. When the heterogeneity of the environment is spatially and/or temporarily intermediate, with periods of environmental stability from one to only a few generations, the relative contributions of local adaptation and adaptive phenotypic plasticity in enabling individuals’ phenotypes to match the environments they encounter remains unclear. Here, we used Drosophila suzukii as a model system to evaluate the relative influence of genetic and plastic effects on this match in heterogeneous environments with an intermediate grain. This pest insect can develop within different fruits, and persists throughout the year in a given location on a succession of different host fruits, each one being available for only a few generations. Using reciprocal common environment experiments of natural D. suzukii populations collected from cherry, strawberry and blackberry, we found that both oviposition preference and offspring performance were higher on medium made with the fruit from which the population originated, than on media made with alternative fruits. This pattern remained after two generations in the laboratory, suggesting that genetic effects predominate over plastic effects. Our study demonstrates that spatially and temporally variable selection does not prevent the rapid evolution of local adaptation in natural populations. The speed and strength of adaptation may be facilitated by several mechanisms including a large effective population size and strong selective pressures imposed by host plants.Impact SummaryNatural populations often exhibit good “fit” to the environment they are in. However, environments change over both time and space, and following change, the fit between a population and its environment may be poor. A question of long-standing interest to evolutionary biologists is, how do populations track changing environments to maintain fitness? Two main mechanisms are known: (i) plastic shifts, or adaptive phenotypic plasticity, in which traits immediately change in response to environmental change, and (ii) genetic shifts in the form of local adaptation, in which traits change over time through differences in fitness of individuals harboring different genetic variants. Plasticity is common when environments change over the course of an individual lifetime, while adaptation is common when environments change over the course of multiple generations. However, many environments change at an intermediate pace, and it is unclear whether plasticity or adaptation are more vital to maintaining fitness under such conditions.Drosophila suzukii is well-suited to evaluating the relative importance of plasticity and adaptation in response to an intermediate pace of environmental change. This species experiences an environment that shifts every 1-4 generations as host fruits change over time and space. Here, we studied natural populations of D. suzukii collected from different hosts. Using reciprocal common environment experiments, we evaluated their fitness on their source and alternative hosts.Drosophila suzukii populations were most fit on their source host, successfully tracking an intermediate pace of environmental change. We developed a new statistical method to quantify the contributions of adaptive plasticity and local adaptation in determining fitness. We found that fitness was maintained via local adaptation to each new host in succession. This study provides a novel statistical tool that can be applied to other systems, and highlights that spatially and temporally variable selection does not prevent local adaptation and, on the contrary, illustrates how rapid the adaptive process can be.

Floral scent is an important trait in plant–pollinator interactions. It not only varies among plant species but also among populations within species. Such variability might be caused by various non–selective factors, or, as has been shown in some instances, might be the result of divergent selective pressures exerted by variable pollinator climates. Cypripedium calceolus is a Eurasian deceptive orchid pollinated mainly by bees, which spans wide altitudinal and latitudinal gradients in mainly quite isolated populations. In the present study, we investigated whether pollinators and floral scents vary among different latitudes. Floral scents of three C. calceolus populations in the Southern Alps were collected by dynamic headspace and analyzed by gas chromatography coupled to mass spectrometry (GC/MS). These data were completed by previously published scent data of the Northern Alps and Scandinavia. The scent characteristics were compared with information on pollinators recorded for present study or available in the literature. More than 80 scent compounds were overall recorded from plants of the three regions, mainly aliphatics, terpenoids, and aromatics. Seven compounds were found in all samples, and most samples were dominated by linalool and octyl acetate. Although scents differed among regions and populations, the main compounds were similar among regions. Andrena and Lasioglossum species were the main pollinators in all three regions, with Andrena being relatively more abundant than Lasioglossum in Scandinavia. We discuss natural selection mediated by pollinators and negative frequency–dependent selection as possible reasons for the identified variation of floral scent within and among populations and regions.

statement Bacterial symbionts of Drosophila influence yeast maintenance through fly metamorphosis, a novel observation that may have consequences for the evolution of insect-yeast-bacteria interactions. 3 Abstract 1 Interactions between microbial symbionts of metazoan hosts are emerging as key features of 2 symbiotic systems. Little is known about the role of such interactions on the maintenance of 3 symbiosis through host's life cycle. We studied the influence of symbiotic bacteria on the 4 maintenance of symbiotic yeast through metamorphosis of the fly Drosophila melanogaster. 5To this end we mimicked the development of larvae in natural fruit. In absence of bacteria 6 yeast was never found in young adults. However, yeast could maintain through 7 metamorphosis when larvae were inoculated with symbiotic bacteria isolated from D. 8 melanogaster faeces. Furthermore, an Enterobacteriaceae favoured yeast transstadial 9 maintenance. Because yeast is a critical symbiont of D. melanogaster flies, bacterial influence 10