Virginie Ravigné

Live Where You Thrive: Joint Evolution of Habitat Choice and Local Adaptation Facilitates Specialization and Promotes Diversity

We derive a comprehensive overview of specialization evolution based on analytical results and numerical illustrations. We study the separate and joint evolution of two critical facets of specialization—local adaptation and habitat choice—under different life cycles, modes of density regulation, variance‐covariance structures, and trade‐off strengths. A particular feature of our analysis is the investigation of arbitrary trade‐off functions. We find that local‐adaptation evolution qualitatively changes the outcome of habitat‐choice evolution under a wide range of conditions. In addition, habitat‐choice evolution qualitatively and invariably changes the outcomes of local‐adaptation evolution whenever trade‐offs are weak. Even weak trade‐offs, which favor generalists when habitat choice is fixed, select for specialists once local adaptation and habitat choice are both allowed to evolve. Unless trapped by maladaptive genetic constraints, joint evolution of local adaptation and habitat choice in the models analyzed here thus always leads to specialists, independent of life cycle, density regulation, and trade‐off strength, thus raising the bar for evolutionarily sound explanations of generalism. Whether a single specialist or two specialists evolve depends on the life cycle and the mode of density regulation. Finally, we explain why the gradual evolutionary emergence of coexisting specialists requires more restrictive conditions than does their evolutionarily stable maintenance.

Experimental evidence of inbreeding avoidance in the hermaphroditic snail Physa acuta

Inbreeding depression should select for inbreeding avoidance behaviours. Here we test this hypothesis in two populations of the simultaneous hermaphroditic freshwater snail Physa acuta. We recorded the copulatory behaviour of 288 pairs of sib-mates, non-kin mates from the same population, or non-kin mates from two different populations. We find that kin discriminatory behaviours exist in this species, exclusively expressed by individuals playing the female role. We discuss the relevance of our finding in the context of the evolution of recognition systems and the consequences of such a behaviour in natural populations.

Selective interactions between short-distance pollen and seed dispersal in self-compatible species.

Abstract In plants, genes may disperse through both pollen and seeds. Here we provide a first theoretical study of the mechanisms and consequences of the joint evolution of pollen and seed dispersal. We focus on hermaphroditic self-compatible species distributed in structured populations, assuming island dispersal of pollen and seeds among small patches of plants within large populations. Three traits are studied: the rate of among-patch seed dispersal, the rate of among-patch pollen dispersal, and the rate of within-patch pollen movement. We first analytically derive the evolutionary equilibrium state of each trait, dissect the pairwise selective interactions, and describe the joint three-trait evolutionary equilibrium under the cost of dispersal and kin competition. These results are then analytically and numerically extended to the case when selfed seeds suffer from depressed competitiveness (inbreeding depression, no heterosis). Finally individual-based simulations are used to account for a more realistic model of inbreeding load. Pollen movement is shown to generate opposite selection pressures on seed dispersal depending on spatial scale: within-patch pollen movement favors seed dispersal, whereas among-patch pollen dispersal inhibits seed dispersal. Seed dispersal selects for short-distance movements of pollen and it selects against long-distance dispersal. These interactions shape the joint evolution of these traits. Kin competition favors among-patch seed dispersal over among-patch pollen dispersal for low costs of within-patch pollen movement (and vice versa for significant costs of within-patch pollen movement). Inbreeding depression favors allogamy through high rates of within- and among-patch pollen movement. Surprisingly, it may select either for or against seed dispersal depending on the cost of among-patch pollen dispersal. Heterosis favors increased among-patch dispersal through pollen and seeds. But because these two stages inhibit each other, their joint evolution might lead to decreased seed dispersal in the presence of heterosis. Of crucial importance are the costs of dispersal.

Biological invasion and biological control select for different life histories.

Biological invaders have long been hypothesized to exhibit the fast end of the life-history spectrum, with early reproduction and a short lifespan. Here, we examine the rapid evolution of life history within the harlequin ladybird Harmonia axyridis. The species, once used as a biological control agent, is now a worldwide invader. We show that biocontrol populations have evolved a classic fast life history during their maintenance in laboratories. Invasive populations also reproduce earlier than native populations, but later than biocontrol ones. Invaders allocate more resources to reproduction than native and biocontrol individuals, and their reproduction is spread over a longer lifespan. This life history is best described as a bet-hedging strategy. We assert that invasiveness cannot be explained only by invoking faster life histories. Instead, the evolution of life history within invasive populations can progress rapidly and converge to a fine-tuned evolutionary match between the invaded environment and the invader.We investigate the variability of exoplanetary radio emission using stellar magnetic maps and 3D field extrapolation techniques. We use a sample of hot Jupiter hosting stars, focusing on the HD 179949, HD 189733 and τ Boo systems. Our results indicate two time-scales over which radio emission variability may occur at magnetised hot Jupiters. The first is the synodic period of the star-planet system. The origin of variability on this time-scale is the relative motion between the planet and the interplanetary plasma that is co-rotating with the host star. The second time-scale is the length of the magnetic cycle. Variability on this time-scale is caused by evolution of the stellar field. At these systems, the magnitude of planetary radio emission is anticorrelated with the angular separation between the subplanetary point and the nearest magnetic pole. For the special case of τ Boo b, whose orbital period is tidally locked to the rotation period of its host star, variability only occurs on the time-scale of the magnetic cycle. The lack of radio variability on the synodic period at τ Boo b is not predicted by previous radio emission models, which do not account for the co-rotation of the interplanetary plasma at small distances from the star.

Investigating the genetic load of an emblematic invasive species: the case of the invasive harlequin ladybird Harmonia axyridis

Introduction events can lead to admixture between genetically differentiated populations and bottlenecks in population size. These processes can alter the adaptive potential of invasive species by shaping genetic variation, but more importantly, they can also directly affect mean population fitness either increasing it or decreasing it. Which outcome is observed depends on the structure of the genetic load of the species. The ladybird Harmonia axyridis is a good example of invasive species where introduced populations have gone through admixture and bottleneck events. We used laboratory experiments to manipulate the relatedness among H. axyridis parental individuals to assess the possibility for heterosis or outbreeding depression in F1 generation offspring for two traits related to fitness (lifetime performance and generation time). We found that inter-populations crosses had no major impact on the lifetime performance of the offspring produced by individuals from either native or invasive populations. Significant outbreeding depression was observed only for crosses between native populations for generation time. The absence of observed heterosis is indicative of a low occurrence of fixed deleterious mutations within both the native and invasive populations of H. axyridis. The observed deterioration of fitness in native inter-population crosses most likely results from genetic incompatibilities between native genomic backgrounds. We discuss the implications of these results for the structure of genetic load in H. axyridis in the light of the available information regarding the introduction history of this species.

Allee effects and the evolution of polymorphism in cyclic parthenogens

Cyclic parthenogens alternate asexual reproduction with periodic episodes of sexual reproduction. Sexually produced free-living forms are often their only way to survive unfavorable periods. When sexual reproduction requires the mating of two self-incompatible individuals, mating limitation may generate an Allee effect, which makes small populations particularly vulnerable to extinction; parthenogenetic reproduction can attenuate this effect. However, asexual reproduction likely trades off with sexual reproduction. To explore the evolutionary implications of such a trade-off, we included recurrent mating events associated with seasonal interruptions in a simple population dynamics model. Following an adaptive dynamics approach, we showed that positive density dependence associated with Allee effects in cyclic parthenogens promotes evolutionary divergence in the level of investment in asexual reproduction. Although polymorphism may be transient, morphs mostly investing into sexual reproduction may eventually exclude those predominantly reproducing in an asexual manner. Asexual morphs can be seen as making cooperative investments into the common pool of mates, while sexual morphs defect, survive better, and may eventually fix in the population. Our findings provide a novel hypothesis for the frequent coexistence of sexual and asexual lineages, notably in plant parasitic fungi.

Host plant range of a fruit fly community (Diptera: Tephritidae): does fruit composition influence larval performance?

BackgroundPhytophagous insects differ in their degree of specialisation on host plants, and range from strictly monophagous species that can develop on only one host plant to extremely polyphagous species that can develop on hundreds of plant species in many families. Nutritional compounds in host fruits affect several larval traits that may be related to adult fitness. In this study, we determined the relationship between fruit nutrient composition and the degree of host specialisation of seven of the eight tephritid species present in La Réunion; these species are known to have very different host ranges in natura. In the laboratory, larval survival, larval developmental time, and pupal weight were assessed on 22 fruit species occurring in La Réunion. In addition, data on fruit nutritional composition were obtained from existing databases.ResultsFor each tephritid, the three larval traits were significantly affected by fruit species and the effects of fruits on larval traits differed among tephritids. As expected, the polyphagous species Bactrocera zonata, Ceratitis catoirii, C. rosa, and C. capitata were able to survive on a larger range of fruits than the oligophagous species Zeugodacus cucurbitae, Dacus demmerezi, and Neoceratitis cyanescens. Pupal weight was positively correlated with larval survival and was negatively correlated with developmental time for polyphagous species. Canonical correspondence analysis of the relationship between fruit nutrient composition and tephritid survival showed that polyphagous species survived better than oligophagous ones in fruits containing higher concentrations of carbohydrate, fibre, and lipid.ConclusionNutrient composition of host fruit at least partly explains the suitability of host fruits for larvae. Completed with female preferences experiments these results will increase our understanding of factors affecting tephritid host range.

Adaptation of genetically monomorphic bacteria: evolution of copper resistance through multiple horizontal gene transfers of complex and versatile mobile genetic elements

Copper‐based antimicrobial compounds are widely used to control plant bacterial pathogens. Pathogens have adapted in response to this selective pressure. Xanthomonas citri pv. citri, a major citrus pathogen causing Asiatic citrus canker, was first reported to carry plasmid‐encoded copper resistance in Argentina. This phenotype was conferred by the copLAB gene system. The emergence of resistant strains has since been reported in Réunion and Martinique. Using microsatellite‐based genotyping and copLAB PCR, we demonstrated that the genetic structure of the copper‐resistant strains from these three regions was made up of two distant clusters and varied for the detection of copLAB amplicons. In order to investigate this pattern more closely, we sequenced six copper‐resistant X. citri pv. citri strains from Argentina, Martinique and Réunion, together with reference copper‐resistant Xanthomonas and Stenotrophomonas strains using long‐read sequencing technology. Genes involved in copper resistance were found to be strain dependent with the novel identification in X. citri pv. citri of copABCD and a cus heavy metal efflux resistance–nodulation–division system. The genes providing the adaptive trait were part of a mobile genetic element similar to Tn3‐like transposons and included in a conjugative plasmid. This indicates the systems great versatility. The mining of all available bacterial genomes suggested that, within the bacterial community, the spread of copper resistance associated with mobile elements and their plasmid environments was primarily restricted to the Xanthomonadaceae family.

Changes in phytophagous insect host ranges following the invasion of their community: Long-term data for fruit flies

Abstract The invasion of an established community by new species can trigger changes in community structure. Invasions often occur in phytophagous insect communities, the dynamics of which are driven by the structure of the host assemblage and the presence of competitors. In this study, we investigated how a community established through successive invasions changed over time, taking the last invasion as the reference. The community included four generalist and four specialist species of Tephritidae fruit flies. We analyzed a long‐term database recording observed numbers of flies per fruit for each species on 36 host plants, over 18 years, from 1991 to 2009. Community structure before the last invasion by Bactrocera zonata in 2000 was described in relation to host plant phylogeny and resource availability. Changes in the host range of each species after the arrival of B. zonata were then documented by calculating diversity indices. The flies in the community occupied three types of niches defined on the basis of plant phylogeny (generalists, Solanaceae specialist, and Cucurbitaceae specialists). After the arrival of B. zonata, no change in the host range of specialist species was observed. However, the host ranges of two generalist species, Ceratitis quilicii and Ceratitis capitata, tended to shrink, as shown by the decreases in species richness and host plant α‐diversity. Our study shows increased host specialization by generalist phytophagous insects in the field following the arrival of an invasive species sharing part of their resources. These findings could be used to improve predictions of new interactions between invaders and recipient communities.

Fungal adaptation to contemporary fungicide applications: the case of Botrytis cinerea populations from Champagne vineyards (France)

In addition to being one of the most acute problems impeding chemical control of fungal diseases, the evolution of fungicide resistance is an emblematic case of local adaptation to spatially heterogeneous and temporally variable selection pressures. Here we dissected the adaptation of Botrytis cinerea (the causal agent of grey mould) populations on grapes to several fungicides. We carried out a 2‐year survey (four collection dates) on three treated/untreated pairs of plots from vineyards in Champagne (France) and monitored the frequency of four resistant phenotypes that are unambiguously associated with four distinct genotypes. For two loci under selection by currently used fungicides (MDR1 and MDR2), the frequencies of resistant mutations at vintage were greater in treated plots compared to untreated plots, showing that the effect of selection is detectable even at the plot scale. This effect was not detectable for two other loci under selection by previously used fungicides (BenR1 and ImiR1). We also found that treatment with currently used fungicides reduced B. cinerea effective population size, leading to a significant decrease in genic diversity and allelic richness in treated vs. untreated plots. We further highlight that even under ample drift and migration, fungal populations can present an efficient response to selection. Finally, for the four studied loci, the costs of fungicide resistance were estimated by modelling the decrease in the frequency of resistant mutations in the absence of treatment. We discuss the importance of these estimates for defining strategies for limiting or counteracting the local adaptation of pests to fungicides.