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Although biological control agents (BCAs) are an effective tool for limiting the impact of invasive alien plants (IAPs), mismatch between the BCA and IAP, exacerbated by future climate change, may affect biocontrol efficacy and the likelihood of there being non-target effects. In a common-environment experiment, we measured leaf consumption, performance (i.e. development time and adult weight), and survival in 11 populations of the leaf beetle Ophraella communa from its native (North America) and introduced (China, Europe) ranges under three temperatures treatments (20 °C, 27 °C, 31 °C) and on two plant species, the target IAP Ambrosia artemisiifolia (common ragweed) and the closely-related crop Helianthus annuus (sunflower), a potential subsidiary non-target host. Leaf consumption, development time, adult weight and survival differed significantly in the different populations without any significant geographical pattern. Specifically, there was variation in levels of survival at different temperatures between populations, with highest levels of leaf consumption and survival in the Fogang (China) population at all temperatures, while survival was highest in other populations under colder and warmer conditions. In general, we observed faster development and a marginal increase in adult weight, but decreasing survival, with increasing temperatures in all populations. The beetles consumed much more common ragweed than sunflower leaves, with no difference evident in the consumption levels on sunflower under cooler or warmer temperature conditions. Moreover, cold temperatures negatively affected the survival of beetles feeding on sunflower, potentially leading to a reduction of fitness on the non-target host. Genetic composition and diversity, assessed using microsatellite markers, showed higher allelic richness and expected heterozygosity in native US populations compared to introduced Chinese populations and lower allelic richness and expected heterozygosity in US and Italian populations at the edge of the distribution compared to core populations. Overall, Chinese populations showed low genetic diversity, likely due to releases after mass rearing. Although there were differences in genetic diversity among populations, we did not find correlations between genetic diversity and phenotypic trait means. Our results indicate that some of the populations examined might be able to cover areas that are heavily infested by common ragweed or are expected be so under climate warming but are predicted to be presently unsuitable for the beetle.

Following its first record in Europe in 2013, the North American ragweed leaf beetle Ophraella communa, used already as a most successful biocontrol agent against common ragweed in China, is spreading rapidly, asking for a detailed analysis of the potential benefit and risk of this introduction for Europe. Here, we report twelve specific and polymorphic tetranucleotide microsatellite markers, which can be used for redrawing its global invasion history and spread across native and introduced ranges. The high level of polymorphism (i.e. from 4 to 18 alleles per locus) and the genetic variation detected within and between one native and two introduced populations provide adequate statistical power for elucidating the beetle’s invasion process.

Classical biological control (also called importation biological control) of weeds has a remarkable track record for efficiency and safety, but further improvement is still needed, particularly to account for potential evolutionary changes after release. Here, we discuss the increasing yet limited evidence of post-introduction evolution and describe approaches to predict evolutionary change. Recent advances include using experimental evolution studies over several generations that combine -omics tools with behavioral bioassays. This novel approach in weed biocontrol is well suited to explore the potential for rapid evolutionary change in real-time and thus can be used to estimate more accurately potential benefits and risks of agents before their importation. We outline this approach with a chrysomelid beetle used to control invasive common ragweed.

Twenty years of genetic studies of marine invaders have shown that successful invaders are often characterized by native and introduced populations displaying similar levels of genetic diversity. This pattern is presumably due to high propagule pressure and repeated introductions. The opposite pattern is reported in this study of the brown seaweed, Sargassum muticum, an emblematic species for circumglobal invasions. Albeit demonstrating polymorphism in the native range, microsatellites failed to detect any genetic variation over 1,269 individuals sampled from 46 locations over the Pacific-Atlantic introduction range. Single-nucleotide polymorphisms (SNPs) obtained from ddRAD sequencing revealed some genetic variation, but confirmed severe founder events in both the Pacific and Atlantic introduction ranges. Our study thus exemplifies the need for extreme caution in interpreting neutral genetic diversity as a proxy for invasive potential. Our results confirm a previously hypothesized transoceanic secondary introduction from NE Pacific to Europe. However, the SNP panel unexpectedly revealed two additional distinct genetic origins of introductions. Also, conversely to scenarios based on historical records, southern rather than northern NE Pacific populations could have seeded most of the European populations. Finally, the most recently introduced populations showed the lowest selfing rates, suggesting higher levels of recombination might be beneficial at the early stage of the introduction process (i.e., facilitating evolutionary novelties), whereas uniparental reproduction might be favored later in sustainably established populations (i.e., sustaining local adaptation). Agence Nationale de la Recherche - ANR-10-BTBR-04; European Regional Development Fund; Fundacao para a Ciencia e a Tecnologia - SFRH/BPD/107878/2015, UID/Multi/04326/2016, UID/Multi/04326/2019; Brittany Region; info:eu-repo/semantics/publishedVersion

Ascidians have a recent history of species introductions globally, often with strong ecological impacts. Comparisons of per capita effects of invaders and comparable natives are useful to assess such impacts. Here, we explore ingestion rates (IR) and clearance rates (CR) of Ciona intestinalis and Ciona robusta, co-occurring native and non-native ascidians, respectively, from Brittany, France. IR was positively related to food concentration, with the invader responding more strongly to increasing food concentration. CR also differed by species, with the invader demonstrating higher values. C. robusta exhibited a higher functional response (Type I) than did C. intestinalis (Type II). Relative impact measured using seasonal abundance and IR revealed that C. robusta has a much greater impact than C. intestinalis at all food concentrations tested, though the former has a constrained distribution which limits its regional impact. Nevertheless, when abundant, we expect C. robusta to exert a greater impact on algal foods.

Biological introductions bring into contact species that can still hybridize. The evolutionary outcomes of such secondary contacts may be diverse (e.g. adaptive introgression from or into the introduced species) but are not yet well examined in the wild. The recent secondary contact between the non-native sea squirtCiona robusta(formerly known asC. intestinalistype A) and its native congenerC. intestinalis(formerly known asC. intestinalistype B), in the western English Channel, provides an excellent case study to examine. To examine contemporary hybridization between the two species, we developed a panel of 310 ancestry-informative SNPs from a population transcriptomic study. Hybridization rates were examined on 449 individuals sampled in 8 sites from the sympatric range and 5 sites from allopatric ranges. The results clearly showed an almost complete absence of contemporary hybridization between the two species in syntopic localities, with only one first generation hybrid and no other genotype compatible with recent backcrosses. Despite the almost lack of contemporary hybridization, shared polymorphisms were observed in sympatric and allopatric populations of both species. Furthermore, one allopatric population from SE Pacific exhibited a higher rate of shared polymorphisms compared to all otherC. robustapopulations. Altogether, these results indicate that the observed level of shared polymorphism is more probably the outcome of ancient gene flow spread afterwards at a worldwide scale. They also emphasise efficient reproductive barriers preventing hybridization between introduced and native species, which suggests hybridization should not impede too much the expansion and the establishment of the non-native species in its introduction range.

Human-mediated dispersal interplays with natural processes and complicates understanding of the biogeographical history of species. This is exemplified by two invasive tunicates, Ciona robusta (formerly Ciona intestinalis type A) and C. intestinalis (formerly Ciona intestinalis type B), globally distributed and sympatric in Europe. By gathering new mitochondrial sequences that were merged with published datasets, we analysed genetic patterns in different regions, with a focus on 1) their sympatric range and 2) allopatric populations in N and S America and southern Europe. In the sympatric range, the two species display contrasting genetic diversity patterns, with low polymorphism in C. robusta supporting the prevalent view of its recent introduction. In the E Pacific, several genetic traits support the non-native status of C. robusta. However, in the NE Pacific, this appraisal requires a complex scenario of introduction and should be further examined supported by extensive sampling efforts in the NW Pacific (putative native range). For C. intestinalis, Bayesian analysis suggested a natural amphi-North Atlantic distribution, casting doubt on its non-native status in the NW Atlantic. This study shows that both natural and human-mediated dispersal have influenced genetic patterns at broad scales; this interaction lessens our ability to confidently ascertain native vs. non-native status of populations, particularly of those species that are globally distributed.

International audience; Biological invasions can promote secondary contacts between related species and thus provide excellent case studies for investigating the joint ecological and evolutionary trajectories of interfertile taxa. Here, we studied two tunicates living in sympatry, and sometimes in syntopy, in the English Channel, Ciona intestinalis species A (presumed native to the NW Pacific) and species B (native to the N Atlantic). In addition to monitoring their co-existence over time, we examined the level of interspecific gene flow, a process that may increase the invasiveness of the non-native species. The sampling scheme was repeated twice a year for 3 years (six distinct generations) to determine the relative abundance of the two species in 11 localities along the coasts of the English Channel and the Iroise Sea (covering 1600 km) in Brittany, France. We examined 23,000 individuals, including 5315 specimens for which reproductive status was determined. Four species-diagnostic molecular markers traced interspecific gene flow on a random subset of 3048 individuals. Regardless of the sampling date, the two species co-occurred in most of the study sites, with species A showing higher frequency in the autumn. The regional pattern of seasonal variation in relative abundance of the two congeners appears to correspond to different thermal growth optima. Abrupt variations in environmental parameters can act synergistically and may favor the non-native species locally. Despite syntopy, synchronous gamete production and successful in vitro interspecific crosses, only 4.3 % individuals showed an admixed genome (i.e. footprint of present-day or past introgression events), most of them with a species A maternal lineage, of which only one was a putative first generation hybrid. Altogether, efficient barriers seem to prevent interspecific crosses between the two interfertile congeners in the wild: present-day hybridization may have less impact than competitive interactions on the fate of the two study species over their sympatric range.