Comparative assessment of An. gambiae and An. stephensi mosquitoes to determine transmission-reducing activity of antibodies against P. falciparum sexual stage antigens.

BACKGROUND: With the increasing interest in vaccines to interrupt malaria transmission, there is a demand for harmonization of current methods to assess Plasmodium transmission in laboratory settings. Potential vaccine candidates are currently tested in the standard membrane feeding assay (SMFA) that commonly relies on Anopheles stephensi mosquitoes. Other mosquito species including Anopheles gambiae are the dominant malaria vectors for Plasmodium falciparum in sub-Saharan Africa. METHODS: Using human serum and monoclonal pre-fertilization (anti-Pfs48/45) and post-fertilization (anti-Pfs25) antibodies known to effectively inhibit sporogony, we directly compared SMFA based estimates of transmission-reducing activity (TRA) for An. stephensi and An. gambiae mosquitoes. RESULTS: In the absence of transmission-reducing antibodies, average numbers of oocysts were similar between An. gambiae and An. stephensi. Antibody-mediated TRA was strongly correlated between both mosquito species, and absolute TRA estimates for pre-fertilisation monoclonal antibodies (mAb) showed no significant difference between the two species. TRA estimates for IgG of naturally exposed individuals and partially effective concentrations of anti-Pfs25 mAb were higher for An. stephensi than for An. gambiae. CONCLUSION: Our findings support the use of An. stephensi in the SMFA for target prioritization. As a vaccine moves through product development, better estimates of TRA and transmission-blocking activity (TBA) may need to be obtained in epidemiologically relevant parasite-species combination

Arrhenotokous parthenogenesis and mate-finding Allee effect in parasitoids

International audienceMating failures at low density – a common type of Allee effect – can reduce the per capita reproductive rate and, if severe enough, doom small populations to extinction. Parasitoids frequently undergo low densities triggered by their typically cyclic dynamic and/or the bottlenecks they experience when introduced for the biological control of agricultural pests. However, most parasitoid species belong to the order Hymenoptera and reproduce via arrhenotokous parthenogenesis: unfertilized eggs develop into males. A common belief is therefore that parasitoids are immune to the mate-finding Allee effect because mating failures yields more males, which should in turn restore mating success. Yet, meta-analyses of biological control introductions suggest that hymenopteran parasitoids are as extinctionprone as non-parthenogenetic species. Here we developed a population dynamic model to investigate the advantage of arrhenotokous parthenogenesis for parasitoid species that experience a mate-finding Allee effect. We show that: i) the conditions for which mating failures drive parasitoids to extinction are marginally more restricted for parthenogenetic species than for non-parthenogenetic ones; ii) parthenogenesis increases the resistance of parasitoids to accidental decrease in density and to invasions by a competing species whether or not parthenogenetic; and iii) parthenogenesis favors population establishment in new environments, whether or not already occupied. Hence even if arrhenotokous parthenogenesis is not as effective as expected to limit parasitoid extinctions caused by mating failures, it provides a significant advantage for population establishment and persistence

Impact of the mate-finding Allee effect on the competitiveness of diploid versus haplodiploid parasitoids: a theoretical approach

International audienceInsect parasitoids can either be diploid, and require mating prior to ovipositing, or haplodiploid, in which case females can produce male offspring through parthenogenesis. In small or low-density populations, parasitoid females might face a mate-finding Allee effect, that is they might not be able to find a male to mate with. Incidences of low-densities populations may be frequent in parasitoid due to the host-parasitoid interaction, which can lead to cycling dynamics, or because of their use in biological control where they might be introduced in small numbers. The ability of virgin haplodiploid females to produce offspring might favor them when confronted with mate-finding difficulties. Using modeling tool to simulate a host-parasitoid dynamics, we investigated whether haplodiploidy confers a competitive advantage over diploidy in parasitoid populations undergoing mate-finding Allee effect. Our results suggest that the Allee effect impacts parasitoid dynamics by inducing competitive exclusion between diploid and haplodiploid parasitoids. Haplodiploidy is favored when mate-finding difficulties are strong

Sharing a predator: can an invasive alien pest affect the predation on a local pest?

International audienceInvasive species can strongly affect biotic interactions in ecosystems, interacting both directly and indirectly with local species. In European tomato greenhouses , the invasive alien pest Tuta absoluta may impact the population dynamics of other pests like whiteflies. Besides inducing damages to the host plant and competing for resources with local pests, this alien species may exert a predator-mediated interaction on local pests sharing common natural enemies. Biocontrol agents usually used against whiteflies may also prey upon T. absoluta and this could alter the dynamics of local pest populations. We evaluated possible resource competition and predator-mediated interactions in a system involving one mirid predator Macrolophus pygmaeus and two pests, T. absoluta and a local whitefly, Bemisia tabaci, on greenhouse tomatoes. Results showed that both resource competition and predator-mediated interactions occurred simultaneously. In the presence of the shared predator, there was a short-term positive effect of T. absoluta on B. tabaci [up to 5.9-fold increase of B. tabaci juveniles (egg ? larvae) after four weeks]. However, in the long-term there was a negative predator-mediated interaction of T. absoluta on B. tabaci, i.e., after ten weeks the density of B. tabaci was 7.3-fold lower in the presence of the invasive pest. We emphasize the critical role of generalist predators in managing both local and invasive alien pest populations and that the strength and direction of predator-mediated indirect interactions can depend on the time scale considered

Host-Parasitoid Dynamics and the Success of Biological Control When Parasitoids Are Prone to Allee Effects

International audienceIn sexual organisms, low population density can result in mating failures and subsequently yields a low population growth rate and high chance of extinction. For species that are in tight interaction, as in host-parasitoid systems, population dynamics are primarily constrained by demographic interdependences, so that mating failures may have much more intricate consequences. Our main objective is to study the demographic consequences of parasitoid mating failures at low density and its consequences on the success of biological control. For this, we developed a deterministic host-parasitoid model with a mate-finding Allee effect, allowing to tackle interactions between the Allee effect and key determinants of host-parasitoid demography such as the distribution of parasitoid attacks and host competition. Our study shows that parasitoid mating failures at low density result in an extinction threshold and increase the domain of parasitoid deterministic extinction. When proned to mate finding difficulties, parasitoids with cyclic dynamics or low searching efficiency go extinct; parasitoids with high searching efficiency may either persist or go extinct, depending on host intraspecific competition. We show that parasitoids suitable as biocontrol agents for their ability to reduce host populations are particularly likely to suffer from mate-finding Allee effects. This study highlights novel perspectives for understanding of the dynamics observed in natural host-parasitoid systems and improving the success of parasitoid introductions

Trophic interactions may reverse the demographic consequences of inbreeding

International audienceExtinctions have no simple determinism, but rather result from complex interplays between environmental factors and demographic‐genetic feedback that occur at small population size. Inbreeding depression has been assumed to be a major trigger of extinction vortices, yet very few models have studied its consequences in dynamic populations with realistic population structure. Here we investigate the impact of Complementary Sex Determination (CSD) on extinction in parasitoid wasps and other insects of the order Hymenoptera. CSD is believed to induce enough inbreeding depression to doom simple small populations to extinction, but we suggest that in parasitoids CSD may have the opposite effect. Using a theoretical model combining the genetics of CSD and the population dynamics of host‐parasitoid systems, we show that CSD can reduce the risk of parasitoid extinction by reducing fluctuations in population size. Our result suggests that inbreeding depression is not always a threat to population survival, and that considering trophic interactions may reverse some pervasive hypotheses on its demographic impact

Arrhenotokous parthenogenesis and mate-finding Allee effect in parasitoids

International audienceMating failures at low density – a common type of Allee effect – can reduce the per capita reproductive rate and, if severe enough, doom small populations to extinction. Parasitoids frequently undergo low densities triggered by their typically cyclic dynamic and/or the bottlenecks they experience when introduced for the biological control of agricultural pests. However, most parasitoid species belong to the order Hymenoptera and reproduce via arrhenotokous parthenogenesis: unfertilized eggs develop into males. A common belief is therefore that parasitoids are immune to the mate-finding Allee effect because mating failures yields more males, which should in turn restore mating success. Yet, meta-analyses of biological control introductions suggest that hymenopteran parasitoids are as extinctionprone as non-parthenogenetic species. Here we developed a population dynamic model to investigate the advantage of arrhenotokous parthenogenesis for parasitoid species that experience a mate-finding Allee effect. We show that: i) the conditions for which mating failures drive parasitoids to extinction are marginally more restricted for parthenogenetic species than for non-parthenogenetic ones; ii) parthenogenesis increases the resistance of parasitoids to accidental decrease in density and to invasions by a competing species whether or not parthenogenetic; and iii) parthenogenesis favors population establishment in new environments, whether or not already occupied. Hence even if arrhenotokous parthenogenesis is not as effective as expected to limit parasitoid extinctions caused by mating failures, it provides a significant advantage for population establishment and persistence

Effet Allee comportemental chez les parasitoïdes : impacts démographiques et conséquences pour la lutte biologique

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