Biology subjects

Ashu, F.

Publications and source records attributed to Ashu, F..

6 recordsLinked to original sources

bioRxiv2025

Behavioral divergence between a generalist and a specialist mosquito despite minimal differentiation across chemosensory receptor gene families

Oviposition site choice is a major determinant of habitat selection in insects, yet its behavioral and genetic bases remain poorly understood. Although model organisms such as Drosophila melanogaster provide well-established systems for studying oviposition, disentangling foraging from egg-laying decisions remains challenging because females use the same substrates for feeding and reproduction. Here, we identify a pair of sister mosquito species that exhibit contrasting oviposition strategies and provide a promising system for investigating how divergence in sensory information processing shapes behavioral shifts between generalist and specialist taxa. Using binary-choice assays, we tested oviposition preferences in 1,046 gravid females from both field and laboratory populations. Anopheles gambiae females were highly selective and deposited nearly all eggs in a single water source, consistent with specialist behavior. In contrast, females of its sibling species, An. coluzzii, adopted a generalist strategy marked by a more even distribution of eggs across available substrates. To identify chemosensory receptor genes that diverge between the cryptic species and may underlie these behavioral differences, we analyzed amino acid substitutions across odorant, ionotropic, and gustatory receptor families using whole-genome sequencing data. Although differentiation was limited at the gene-family level, several individual divergent loci associated with carboxylic acid, amine, and other volatile detection pathways emerged as candidate drivers of species-specific oviposition behavior. These findings suggest that shifts in sensory perception during egg laying may facilitate behavioral and ecological specialization, and that phenotypic divergence in environmental sensing can arise before substantial genome-wide differentiation.

evolutionary biology↗

bioRxiv2025

Standing genetic variation underlies divergence of nicotinic acetylcholine receptor subunits among cryptic species of the Anopheles gambiae complex

Arthropod species differ in insecticide susceptibility, yet how pre-existing polymorphism at target sites shapes variable responses within and among populations remains poorly understood. Recently diverged taxa provide ideal systems to test how target-site divergence modulates species sensitivity. Using whole-genome sequencing data from 573 mosquitoes representing six cryptic species of the Anopheles gambiae complex, we analyzed standing genetic variation across all 11 nicotinic acetylcholine receptor (nAChR) subunit genes to establish a baseline for natural diversity before the large-scale deployment of nAChR-targeting insecticides in Africa. We detected no previously reported resistance alleles from agricultural pests and found no evidence of selective sweeps or loss-of-function mutations across the nAChR gene family. Patterns of polymorphism were consistent with strong purifying selection. Most nonsynonymous variants were rare, predicted to be tolerated by SIFT (score [≥] 0.05), present almost exclusively in heterozygotes, and occurred outside ligand-binding and transmembrane domains. However, the 6 subunit exhibited relaxed constraint, with two high-frequency substitutions (I198M and D202E) that defined haplotypes segregating by species. The derived alleles represented ancient polymorphisms, showed evidence of introgression, and were fixed in populations with reduced larval susceptibility to spinosad. Our findings show that modest standing variation can shape divergence at insecticide target sites within a highly constrained gene family and underscore the need to monitor interspecific variation during the deployment of nAChR-targeting insecticides.

evolutionary biology↗

bioRxiv2024

Testing Anopheles larvae and adults using standard bioassays reveals susceptibility to chlorfenapyr (pyrrole) while highlighting variability between species

A standard test is available for assessing the susceptibility of adult Anopheles mosquitoes to chlorfenapyr, a new active ingredient in insecticide-treated nets. However, for a new insecticide with a unique mode of action, testing both larvae and adults using different routes of exposure is crucial to a comprehensive evaluation of susceptibility and to identifying potential selection pressures that may drive resistance. We followed WHO guidelines to assess the lethal toxicity of chlorfenapyr and monitor Anopheles susceptibility. Based on the median lethal concentration (LC50), larvae of the pyrethroid-susceptible colonized strain An. coluzzii Ngousso were 16-fold more susceptible to chlorfenapyr than immature stages of another susceptible colony: An. gambiae Kisumu. Larval bioassays indicated 99.63 {+/-} 0.2% mortality after 24 h at a discriminating concentration of 100 ng/ml in Anopheles gambiae and An. coluzzii larvae collected from seven locations in urban and rural areas of Yaounde, Cameroon. By contrast, exposing emerging female adults from these populations to the recommended discriminating concentration (100 {micro}g Active Ingredient (AI)/bottle) in bottle bioassays revealed variable mortality after 72 h, with values below the threshold of susceptibility (98%) in several tests. Anopheles coluzzii larvae and adults were fully susceptible, but mortality rates were slightly lower in An. gambiae adults compared to larvae (94 {+/-} 1.5% vs 100%, Fishers exact test, p < 0.001). Piperonyl butoxide antagonized the activity of chlorphenapyr in An. gambiae adults. 100 ng/ml provides sufficient discriminative power for assessing the susceptibility of An. gambiae and An. coluzzii larvae to chlorfenapyr. Testing An. gambiae adults with 100 {micro}g AI/bottle is likely to reveal inconsistent mortality values making it difficult to detect any emergence of resistance. Exploring different tests and accounting for variability between species are key to a reliable monitoring of Anopheles susceptibility to chlorfenapyr.

zoology↗

bioRxiv2023

Vegetable oil surfactants are synergists that can bias neonicotinoid susceptibility testing in adult mosquitoes.

BackgroundThe standard operating procedure for testing the susceptibility of adult mosquitoes to clothianidin, a neonicotinoid, recommends using a vegetable oil ester as surfactant. However, it has not yet been determined if the surfactant is an inert ingredient or if it can act as a synergist and bias the test. Methodology/Principal FindingsUsing standard bioassays, we tested the synergistic effects of a vegetable oil surfactant on a spectrum of active ingredients including four neonicotinoids (acetamiprid, clothianidin, imidacloprid and thiamethoxam) and two pyrethroids (permethrin and deltamethrin). Three different formulations of linseed oil soap used as surfactant were far more effective than the standard insecticide synergist piperonyl butoxide in enhancing neonicotinoid activity in Anopheles mosquitoes. At the concentration used in the standard operating procedure (1% v/v), vegetable oil surfactants lead to more than 10-fold reduction in lethal concentrations, LC50 and LC99, of clothianidin in a multi-resistant field population and in a susceptible strain of Anopheles gambiae. At 1% or 0.5% (v/v), the surfactant restored susceptibility to clothianidin, thiamethoxam and imidacloprid and increased mortality to acetamiprid from 43 {+/-} 5.63% to 89 {+/-} 3.25% (P<0.05) in resistant mosquitoes. By contrast, linseed oil soap had no effect on the level of resistance to permethrin and deltamethrin suggesting that the synergism of vegetable oil surfactants may be specific to neoniconoids. Conclusions/SignificanceOur findings indicate that vegetable oil surfactants are not inert ingredients in neonicotinoid formulations, and their synergistic effects undermine the ability of standard testing procedures to detect early stages of resistance.

evolutionary biology↗

bioRxiv2023

Chronic exposure of mosquito larvae to pesticide residues endangers a new generation of agrochemicals repurposed for malaria prevention.

Agrochemicals have been successfully repurposed to control mosquitoes worldwide, but pesticides used in agriculture challenge their effectiveness by contaminating surface waters and helping larval populations develop resistance. Thus, knowledge of the lethal and sublethal effects of residual pesticide exposure on mosquitoes is critical for selecting effective insecticides. Here we implemented a new experimental approach to predict the efficacy of agricultural pesticides newly repurposed for malaria vector control. We mimicked insecticide resistance selection as it occurs in contaminated aquatic habitats by rearing field-collected mosquito larvae in water containing a dose of insecticide capable of killing individuals from a susceptible strain within 24 h. We then simultaneously monitored short-term lethal toxicity within 24 h and sublethal effects for 7 days. We found that due to chronic exposure to agricultural pesticides, some mosquito populations are currently pre-adapt to resist neonicotinoids if those were used in vector control. Larvae collected from rural and agricultural areas where neonicotinoid formulations are intensively used for insect pest management were able to survive, grow, pupate and emerge in water containing a lethal dose of acetamiprid, imidacloprid or clothianidin. These results emphasize the importance of addressing exposure of larval populations to formulations applied in agriculture prior to using agrochemicals against malaria vectors.

evolutionary biology↗

bioRxiv2023

Enhancing the efficacy of neonicotinoids against mosquitoes and overcoming resistance issues.

BackgroundNeonicotinoids are potential alternatives for targeting pyrethroid-resistant mosquitoes, but their efficacy against malaria vector populations of Sub-Saharan Africa has yet to be investigated. Here we tested and compared the efficacy of four neonicotinoids alone or in combination with a synergist against two major vectors of Plasmodium. ResultsUsing standard bioassays, we first assessed the lethal toxicity of three active ingredients against adults of two susceptible Anopheles strains and we determined discriminating doses for monitoring susceptibility in wild populations. We then tested the susceptibility of 5532 Anopheles mosquitoes collected from urban and rural areas of Yaounde, Cameroon, to discriminating doses of acetamiprid, imidacloprid, clothianidin and thiamethoxam. We found that in comparison with some public health insecticides, neonicotinoids have high lethal concentration, LC99, reflecting their low toxicity to Anopheles mosquitoes. In addition to this reduced toxicity, resistance to the four neonicotinoids tested was detected in An. gambiae populations collected from agricultural areas where larvae are intensively exposed to crop-protection neonicotinoids. However, adults of another major vector that occurred in urbanized settings, An. coluzzii, were fully susceptible to neonicotinoids except acetamiprid for which 80% mortality was obtained within 72 h of insecticide exposure. Importantly, the cytochrome inhibitor, piperonyl butoxide (PBO), was very effective in enhancing the activity of clothianidin and acetamiprid providing opportunities to create potent neonicotinoid formulations against Anopheles. ConclusionThese findings suggest that to successfully repurpose agricultural neonicotinoids for malaria vector control, it is essential to use formulations containing synergists such as PBO or surfactants to ensure optimal efficacy.

evolutionary biology↗