Jérôme Carletto

Ecological specialization of the aphid Aphis gossypii Glover on cultivated host plants

Many plant‐feeding insect species considered to be polyphagous are in fact composed of genetically differentiated sympatric populations that use different hosts and between which gene flow still exists. We studied the population genetic structure of the cotton‐melon aphid Aphis gossypii that is considered as one of the most polyphagous aphid species. We used eight microsatellites to analyse the genetic diversity of numerous samples of A. gossypii collected over several years at a large geographical scale on annual crops from different plant families. The number of multilocus genotypes detected was extremely low and the genotypes were found to be associated with host plants. Five host races were unambiguously identified (Cucurbitaceae, cotton, eggplant, potato and chili‐ or sweet pepper). These host races were dominated by asexual clones. Plant transfer experiments using several specialized clones further confirmed the existence of host‐associated trade‐offs. Finally, both genetic and experimental data suggested that plants of the genus Hibiscus may be used as refuge for the specialized clones. Resource abundance is discussed as a key factor involved in the process of ecological specialization in A. gossypii.

Genotypic diversity of the cotton-melon aphid Aphis gossypii (Glover) in Tunisia is structured by host plants

The study of intraspecific variation with respect to host plant utilization in polyphagous insects is crucial for understanding evolutionary patterns of insect-plant interactions. Aphis gossypii (Glover) is a cosmopolitan and extremely polyphagous aphid species. If host plant species or families constitute selective regimes to these aphids, genetic differentiation and host associated adaptation may occur. In this study, we describe the genetic structure of A. gossypii collected in six localities in Tunisia on different vegetable crops, on citrus trees and on Hibiscus. The aim was to determine if the aphid populations are structured in relation to the host plants and if such differentiation is consistent among localities. The genetic variability of A. gossypii samples was examined at eight microsatellite loci. We identified only 11 multilocus genotypes among 559 individuals. Significant deviations from Hardy-Weinberg equilibrium, linkage disequilibria and absence of recombinant genotypes, confirmed that A. gossypii reproduces by continuous apomictic parthenogenesis. Genetic differentiation between localities was not significant, whereas a strong differentiation was observed between host plant families (0.175<FST<0.691). The great majority of aphids exhibited one of three predominant multilocus genotypes that were repeatedly and respectively associated to the three plant families, Cucurbitaceae, Solanaceae and Rutaceae, demonstrating host specialization in A. gossypii. These specialized genotypes were simultaneously found with other clones on Hibiscus, suggesting that this perennial host could act as a refuge plant between two vegetable crop seasons.

Genetic diversity of the cotton aphid Aphis gossypii in the unstable environment of a cotton growing area

1 Spatial and temporal habitat heterogeneity represented by annual crops is a major factor influencing population dynamics of phytophagous insect pests such as the cotton aphid Aphis gossypii Glover. We studied the effects of instability of the cotton agroecosystem resulting from the temporary availability of the plant resource and the repeated use of insecticides on the genetic variability of the cotton aphids.

Insecticide resistance traits differ among and within host races in Aphis gossypii.

BACKGROUND The polyphagous cotton-melon aphid Aphis gossypii Glover is structured into geographically widespread host races comprising a few clones specialised on Cucurbitaceae, cotton, eggplant or pepper. To assess insecticide resistance among and within host races, leaf disc bioassays were conducted on aphid clones collected from Cucurbitaceae (genotypes C4 and C9), cotton (genotypes Burk and Ivo), eggplant (genotype Auber) and pepper (genotype PsP4). Molecular diagnostic (PCR-RFLP) and enzyme assays were also performed to detect the basic mechanisms underlying insecticide resistance. RESULTS All six clones were susceptible to acetamiprid (neonicotinoid) or carbosulfan (carbamate). Conversely, all clones were resistant to dimethoate (organophosphate) (RF = 4.1-38.1) and carried mutation S431F in the acetylcholinesterase gene. Auber, PsP4 and Burk also carried mutation A302S in this gene, which possibly conferred moderate resistance (RF = 3.7-6.8) to profenofos and monocrotophos (organophosphates). Auber and Burk were highly resistant (RF = 41.2 and 473 respectively) to cypermethrin (pyrethroid). This resistance was likely associated with point mutation super-kdr (M918L) in the voltage-gated sodium channel gene (para gene) or metabolic detoxification mediated by esterase and oxidase enzymes. CONCLUSION Multiple resistance to a broad range of insecticides and multiple mechanisms of resistance in some clones could explain to some extent the low genetic diversity observed within A. gossypii host races.

Insecticide use and competition shape the genetic diversity of the aphid Aphis gossypii in a cotton-growing landscape

Field populations of the cotton aphid, Aphis gossypii Glover, are structured into geographically widespread host races. In the cotton-producing regions of West and Central Africa (WCA), two genotypes have been repeatedly detected within the cotton host race, one of which (Burk1) is prevalent (>90%) and resistant to several insecticides, as opposed to the second one (Ivo). Here, we conducted whole plant and field cage experiments to test hypotheses for such low genetic diversity, including selection from insecticide treatments, interclonal competition and adaptation to host plant, or climatic conditions. To assess the genetic diversity of immigrant aphids, alatae were trapped and collected on cotton and relay host plants (okra and roselle) in the early cropping season. Individuals were genotyped at eight specific microsatellite loci and characterized by a multilocus genotype (MLG). When independently transferred from cotton (Gossypium hirustum L.) leaf discs to whole plants (G. hirsutum and G. arboreum, roselle and okra), Ivo and Burk1 performed equally well. When concurrently transferred from cotton leaf discs to the same plant species, Ivo performed better than Burk1, indicating that competition favoured Ivo. This was also the case on G. hirsutum growing outdoors. Conversely, Burk1 prevailed when cotton plants were sprayed with insecticides. In experiments where aphids were allowed to move to neighbouring plants, Burk1 was better represented than Ivo on low-populated plants, suggesting that dispersal may be a way to avoid competition on crowded plants. Most cotton aphids collected on cotton or relay host plants in the early cropping season were Burk1 (>90%), indicating high dispersal ability and, probably reflecting high frequency on host plants from which they dispersed. In the agricultural landscape of WCA, the use of broad-range insecticides on both cotton and relay host plants has led to the prevalence of one genotype of A. gossypii resistant to different classes of insecticides. Deployment of widespread and integrated pest management strategies are needed to restore cotton aphid control.