Thierry Rigaud

Understanding how infection‐related traits vary across parasite populations and host stages is key to predicting host–parasite dynamics in complex life cycles. We tested whether the geographic origin of the acanthocephalan parasite Pomphorhynchus laevis influences infection success in both its intermediate ( Gammarus pulex ) and definitive ( Squalius cephalus ) hosts. Parasite eggs from two watersheds (Seine and Saône) were used to infect naïve gammarids and subsequent larval stages of the parasites were used to infect naïve fish (from the Seine). Parasites from the Saône showed significantly higher infection success and virulence in gammarids than those from the Seine, including reduced survival even in exposed but uninfected individuals, revealing strong population‐level differences in infection traits. When analyses were restricted to low infection intensities, differences in gammarid survival persisted, indicating that these effects were not solely due to parasite load. Fish in the Seine basin were more successfully infected by local parasites, but differences relative to parasites from the Saône remained modest. Overall, our results highlight that population‐level variation in parasite performance can generate asymmetric infection patterns across host stages, emphasizing the importance of considering the full life cycle in studies of parasite evolution and host–parasite dynamics.

Biological invasions lead to substantial changes in global biodiversity. Non‐native species invade local ecosystems, interacting also with already established invaders. The presence of invasive species in ecosystem may facilitate secondary invasions by positive interactions. Alternatively, the success of invasion may be diminished by negative relationships among newly and previously introduced/established non‐native species. However, few studies have determined joint effect of co‐invasive competitors and predators on previous invaders. Our study aimed to estimate potential synergistic effects of the fish predator, Babka gymnotrachelus , and stronger interspecific competitor, Dikerogammarus villosus , on population dynamics of the prior invader, D. haemobaphes , all of Ponto‐Caspian origin and invading many major European river basins. We hypothesized that abundance and distribution of prior invader would be negatively affected by presence of D. villosus and fish, either alone or simultaneously. Using mesocosm and laboratory experiments, we showed that the secondary co‐invaders affected distribution and abundance of D. haemobaphes , depending on ontogenetic stage of the latter. Abundance of juvenile D. haemobaphes was reduced in presence of D. villosus , while presence of predator had no effect. Reversely, adult D. haemobaphes avoided the predators, but not competitors. In the simultaneous presence of both co‐invaders, reactions of juvenile D. haemobaphes have reinforced: they were no longer able to hide against D. villosus and suffered greater abundance decline, pointing to synergistic negative effect of the competitor and predator. We provide experimental evidence demonstrating how negative interactions among invasive species may shape community structure. Moreover, as nowadays dispersal of species is driven by climate change, our results may be extrapolated to native communities, undergoing composition changes due to phenomena other than biological invasions.

The yellow mealworm ( Tenebrio molitor ) is a key species in industrial insect farming due to its high nutritional value, low environmental footprint, and efficient feed conversion. However, intensive high-density rearing increases the risk of pathogen transmission, highlighting the need for sustainable disease management strategies. Enhancing natural immune defences, such as immune priming, may reduce reliance on antibiotics, but the duration, reactivation potential, and physiological costs of such responses remain poorly understood. This study investigated long-term immune priming in T. molitor females by assessing immune parameters 20 and 50 days after a primary challenge with heat-killed Bacillus cereus , with or without a secondary challenge. We measured changes in haemocyte composition, prophenoloxidase (proPO) system activity, and antibacterial activity in the haemolymph, while also evaluating female fecundity to assess potential trade-offs. Our results show that immune priming induced long-lasting changes in immune function, notably enhanced plasmatocyte counts and PO activity, but only in females that received a secondary challenge 50 days after priming. This suggests a memory-like recall response. Antibacterial activity was sustained for at least 50 days post-priming, independent of secondary challenge, reflecting durable inducible humoral immunity. However, this heightened immune activity was negatively correlated with fecundity, indicating a trade-off between immune investment and reproductive output. These findings demonstrate that T. molitor can mount robust, and long-lasting immune responses after bacterial challenges, but at a reproductive cost. While immune priming holds promise for improving disease resistance in mealworm farming, its application must consider impacts on productivity.

Although interest in Acanthocephala seems to have reached only a small community of researchers worldwide, we show in this opinion article that this group of parasites is composed of excellent model organisms for studying key questions in parasite molecular biology and cytogenetics, evolutionary ecology, and ecotoxicology. Their shared ancestry with free-living rotifers makes them an ideal group to explore the origins of the parasitic lifestyle and evolutionary drivers of host shifts and environmental transitions. They also provide useful features in the quest to decipher the proximate mechanisms of parasite-induced phenotypic alterations and better understand the evolution of behavioral manipulation. From an applied perspective, acanthocephalans’ ability to accumulate contaminants offers useful opportunities to monitor the impacts – and evaluate the possible mitigation – of anthropogenic pollutants on aquatic fauna and develop the environmental parasitology framework. However, exploring these exciting research avenues will require connecting fragmentary knowledge by enlarging the taxonomic coverage of molecular and phenotypic data. In this opinion paper, we highlight the needs and opportunities of research on Acanthocephala in three main directions: (i) integrative taxonomy (including non-molecular tools) and phylogeny-based comparative analysis; (ii) ecology and evolution of life cycles, transmission strategies and host ranges; and (iii) environmental issues related to global changes, including ecotoxicology. In each section, the most promising ideas and developments are presented based on selected case studies, with the goal that the present and future generations of parasitologists further explore and increase knowledge of Acanthocephala.

Many trophically transmitted parasites with complex life cycles manipulate their intermediate host’s antipredatory defenses in ways facilitating their transmission to final host by predation. Some parasites also protect the intermediate host from predation when noninfective during its ontogeny. The acanthocephalan Pomphorynchus laevis , a fish intestinal helminth, infecting freshwater gammarid amphipods as intermediate hosts, is using such a strategy of protection-then-exposure to predation by the definitive host. However, the whole time-course of this sequence of behavioral switch is not yet known, and only one antipredator behavior has been studied to date. Here we show that the protective part of this manipulation begins quite late during the parasite ontogeny, suggesting that the advantages overpass the costs induced by this protective manipulation only at this stage. We confirmed that the refuge use behavior is showing a switch in the few days following the stage mature for the definitive host (switching from overuse to underuse). However, such a switch was not observed for the gammarids activity rate, a behavior also known to make the host less conspicuous to predators when weak. While we predicted a low activity during early development stages, then a switch to high activity, we observed general decrease in activity during the parasite ontogeny. The possible causes for this discrepancy between behaviors are discussed. All these behavioral changes were observed mostly when animals were tested in water scented by potential predators (here brown trout), suggesting condition-dependent manipulation.

In the isopod Armadillidium vulgare , many females produce progenies with female-biased sex ratios, owing to two feminizing sex ratio distorters (SRD): Wolbachia endosymbionts and the f element. We investigated the distribution and population dynamics of these SRD and mitochondrial DNA variation in 16 populations from Europe and Japan. Confirming and extending results from the 1990s, we found that the SRD are present at variable frequencies in populations and that the f element is overall more frequent than Wolbachia . The two SRD never co-occur at high frequency in any population, suggesting an apparent mutual exclusion. We also detected Wolbachia or the f element in some males, which probably reflects insufficient titer to induce feminization or presence of masculinizing alleles. Our results are consistent with a single integration event of a Wolbachia genome in the A. vulgare genome at the origin of the f element, which contradicts an earlier hypothesis of frequent losses and gains. We identified strong linkage between Wolbachia strains and mitochondrial haplotypes, but no association between the f element and mitochondrial background. Our results open new perspectives on SRD evolutionary dynamics in A. vulgare , the evolution of genetic conflicts and their impact on the variability of sex determination systems.

Background: The yellow mealworm beetle, Tenebrio molitor, is a promising alternative protein source for animal and human nutrition and its farming involves relatively low environmental costs. For these reasons, its industrial scale production started this century. However, to optimize and breed sustainable new T. molitor lines, the access to its genome remains essential. Methods: By combining Oxford Nanopore and Illumina Hi-C data, we constructed a high-quality chromosome-scale assembly of T. molitor. Then, we combined RNA-seq data and available coleoptera proteomes for gene prediction with GMOVE. Results: We produced a high-quality genome with a N50 = 21.9Mb with a completeness of 99.5% and predicted 21,435 genes with a median size of 1,780 bp. Gene orthology between T. molitor and Tribolium castaneum showed a highly conserved synteny between the two coleoptera and paralogs search revealed an expansion of histones in the T. molitor genome. Conclusions: The present genome will greatly help fundamental and applied research such as genetic breeding and will contribute to the sustainable production of the yellow mealworm.