Aphis gossypii/Aphis frangulae collected worldwide: Microsatellite markers data and genetic cluster assignment.

Aphis gossypii is a cosmopolitan aphid species able to colonize hundreds of plant species from various families [1]. It causes serious damage to a wide range of crops and it is considered a major pest of cucurbits and cotton [2]. It reproduces clonally, by obligate parthenogenesis, on secondary hosts present throughout the year in the intertropical area. At higher latitude, some lineages clonally overwinter but part of the population may have a sexual reproduction in autumn on primary host such as Hibiscus syriacus, to generate cold resistant overwintering eggs [3]. It is highly challenging to distinguish A. gossypii from its sister species Aphis frangulae as both are colonizing solanaceous plants as secondary hosts but the primary host of A. frangulae is Frangula alnus[4]. This paper describes a worldwide collection of both species from December 1989 to September 2019. Aphids were collected individually on plants (19 families) or in traps. The location, the morph type and the botanical family of the host plant were registered. DNA was extracted from each aphid and amplified at 8 microsatellite loci [5]. Amplicons were analysed with ABI technology and their size was defined with Genemapper software. We named each unique combination of alleles, called a multilocus genotype (MLG), and then each individual was given its MLG. The matrix of alleles of all MLGs was run for a Bayesian analysis to describe the genetic structure of the diversity collected and then each MLG had a probability to belong to a genetic group [6,7]. Probability of assignation to each genetic group revealed by the analysis was reported to each individual according to its MLG. This dataset can be used to analyze host plant specificities in A. gossypii, genetic diversity in A. gossypii and relative incidence of variants in diverse geographical regions, admixture between two sister species (Aphis gossypii and Aphis frangulae).

Vat, an Amazing Gene Conferring Resistance to Aphids and Viruses They Carry: From Molecular Structure to Field Effects.

We review half a century of research on Cucumis melo resistance to Aphis gossypii from molecular to field levels. The Vat gene is unique in conferring resistance to both A. gossypii and the viruses it transmits. This double phenotype is aphid clone-dependent and has been observed in 25 melon accessions, mostly from Asia. It is controlled by a cluster of genes including CC-NLR, which has been characterized in detail. Copy-number polymorphisms (for the whole gene and for a domain that stands out in the LLR region) and single-nucleotide polymorphisms have been identified in the Vat cluster. The role of these polymorphisms in plant/aphid interactions remains unclear. The Vat gene structure suggests a functioning with separate recognition and response phases. During the recognition phase, the VAT protein is thought to interact (likely indirectly) with an aphid effector introduced during cell puncture by the aphid. A few hours later, several miRNAs are upregulated in Vat plants. Peroxidase activity increases, and callose and lignin are deposited in the walls of the cells adjacent to the stylet path, disturbing aphid behavior. In aphids feeding on Vat plants, Piwi-interacting RNA-like sequences are abundant and the levels of other miRNAs are modified. At the plant level, resistance to aphids is quantitative (aphids escape the plant and display low rates of reproduction). Resistance to viruses is qualitative and local. Durability of NLR genes is highly variable. A. gossypii clones are adapted to Vat resistance, either by introducing a new effector that interferes with the deployment of plant defenses, or by adapting to the defenses it triggered. Viruses transmitted in a non-persistent manner cannot adapt to Vat resistance. At population level, Vat reduces aphid density and genetic diversity. The durability of Vat resistance to A. gossypii populations depends strongly on the agro-ecosystem, including, in particular, the presence of other cucurbit crops serving as alternative hosts for adapted clones in fall and winter. At the crop level, Vat resistance decreases the intensity of virus epidemics when A. gossypii is the main aphid vector in the crop environment.

Insight into the durability of plant resistance to aphids from a demo-genetic study of Aphis gossypii in melon crops.

Resistance breakdown has been observed following the deployment of plant cultivars resistant to pests. Assessing the durability of a resistance requires long-term experiments at least at a regional scale. We collected such data for melon resistance conferred by the Vat gene cluster to melon aphids. We examined landscape-level populations of Aphis gossypii collected in 2004-2015, from melon-producing regions with and without the deployment of Vat resistance and with different climates. We conducted demo-genetic analyses of the aphid populations on Vat and non-Vat plants during the cropping seasons. The Vat resistance decreased the density of aphid populations in all areas and changed the genetic structure and composition of these populations. Two bottlenecks were identified in the dynamics of adapted clones, due to the low levels of production of dispersal morphs and winter extinction. Our results suggest that (i) Vat resistance will not be durable in the Lesser Antilles, where no bottleneck affected the dynamics of adapted clones, (ii) Vat resistance will be durable in south-west France, where both bottlenecks affected the dynamics of adapted clones and (iii) Vat resistance will be less durable in south-east France, where only one of the two bottlenecks was observed.

Evidence of superclones in Australian cotton aphid Aphis gossypii Glover (Aphididae: Hemiptera).

BACKGROUND: Aphis gossypii is an important pest of cotton that has developed resistance to many chemicals used for its control. Any lack of understanding of its genetic structure, resistance status and host plant specialisation hampers effective management. RSULTS: Eight microsatellite markers were genotyped for a collection of Australian A. gossypii field isolates from 55 plant species from major Australian cotton-producing regions. The aphid's pirimicarb resistance status linked to the ACE1 (acetylcholinesterase) S431F mutation was determined by PCR-RFLP. Overall, the genetic diversity was low and there were only 13 multilocus genotype (MLG) groups found in a total of 936 aphids, suggesting asexual reproduction. Three MLGs (Aust-01, Aust-02 and Aust-04) represented 78% of all aphids tested. MLGs Aust-01 (41%) and Aust-02 (18%) were linked to the ACE1 S431F mutation and found on cotton and a range of hosts. Aust-04 (19%) hosted mainly on cotton (but also Asteraceae and Malvaceae) was predominantly susceptible to pirimicarb. Given their abundance and widespread occurrence, these three clones were considered to be superclones. CONCLUSION: The study demonstrated that any strategy to control A. gossypii and manage pirimicarb resistance should target A. gossypii strains of all MLG types residing on any plant species and not just cotton

Evolution and diversity of Arsenophonus endosymbionts in aphids.

Endosymbiotic bacteria are important drivers of insect evolutionary ecology, acting both as partners that contribute to host adaptation and as subtle parasites that manipulate host reproduction. Among them, the genus Arsenophonus is emerging as one of the most widespread lineages. Its biology is, however, entirely unknown in most cases, and it is therefore unclear how infections spread through insect populations. Here we examine the incidence and evolutionary history of Arsenophonus in aphid populations from 86 species, characterizing the processes that shape their diversity. We identify aphids as harbouring an important diversity of Arsenophonus strains. Present in 7% of the sampled species, incidence was especially high in the Aphis genus with more than 31% of the infected species. Phylogenetic investigations revealed that these Arseno-phonus strains do not cluster within an aphid-specific clade but rather exhibit distinct evolutionary origins showing that they undergo repeated horizontal transfers (HT) between distantly related host species. Their diversity pattern strongly suggests that ecological interactions, such as plant mediation and parasitism, are major drivers for Arsenophonus dispersal, dictating global incidence across insect communities. Notably, plants hosting aphids may be important ecological arenas for global exchange of Arsenophonus, serving as reservoirs for HT.

What do spring migrants reveal about sex and host selection in the melon aphid?

BACKGROUND: Host plants exert considerable selective pressure on aphids because the plants constitute their feeding, mating and oviposition sites. Therefore, host specialisation in aphids evolves through selection of the behavioural and chemical mechanisms of host-plant location and recognition, and through metabolic adaptation to the phloem content of the host plant. How these adaptive traits evolve in an aphid species depends on the complexity of the annual life cycle of that species. The purpose of this field study was to determine how winged spring-migrant populations contribute to the evolution and maintenance of host specialisation in Aphis gossypii through host-plant choice and acceptance. We also assessed whether host-specialised genotypes corresponded exclusively to anholocyclic lineages regardless of the environmental conditions. RESULTS: The spring populations of cotton-melon aphids visiting newly planted melon crops exhibited an unexpectedly high level of genetic diversity that contrasted with the very low diversity characterising the host-specialised populations of this aphid species. This study illustrated in natura host-plant-selection pressure by showing the great differences in genetic diversity between the spring-migrant populations (alate aphids) and the melon-infesting populations (the apterous offspring of the alate aphids). Moreover, an analysis of the genetic composition of these alate and apterous populations in four geographic regions suggested differences in life-history strategies, such as host choice and reproductive mode, and questioned the common assertion that A. gossypii is an anholocyclic species throughout its distribution area, including Europe. CONCLUSIONS: Our results clearly demonstrate that the melon plant acts as a selective filter against the reproduction of non-specialised individuals. We showed that olfactory cues are unlikely to be decisive in natura for host recognition by spring-migrant aphid populations that are not specialised on Cucurbitaceae. The agroecosystem structure and history of the four studied regions may have partially shaped the genetic structure of the spring-migrant populations of A. gossypii. Cucurbitaceae-specialised genotypes corresponded exclusively to anholocyclic lineages, regardless of the environmental conditions. However, some genotypes that were genetically close to the host-specialised genotypes and some genotypes that probably originated from wild plants had never been previously sampled; both were holocylic.

Evolutionary history of aphid-plant associations and their role in aphid diversification.

Aphids are intimately linked with their host plants that constitute their only food resource and habitat, and thus impose considerable selective pressure on their evolution. It is therefore commonly assumed that host plants have greatly influenced the diversification of aphids. Here, we review what is known about the role of host plant association on aphid speciation by examining both macroevolutionary and population-level studies. Phylogenetic studies conducted at different taxonomic levels show that, as in many phytophagous insect groups, the radiation of angiosperms has probably favoured the major Tertiary diversification of aphids. These studies also highlight many aphid lineages constrained to sets of related host plants, suggesting strong evolutionary commitment in aphids' host plant choice, but they fail to document cospeciation events between aphid and host lineages. Instead, phylogenies of several aphid genera reveal that divergence events are often accompanied by host shifts, and suggest, without constituting a formal demonstration, that aphid speciation could be a consequence of adaptation to new hosts. Experimental and field studies below the species level support reproductive isolation between host races as partly due to divergent selection by their host plants. Selected traits are mainly feeding performances and life cycle adaptations to plant phenology. Combined with behavioural preference for favourable host species, these divergent adaptations can induce pre- and post-zygotic barriers between host-specialized aphid populations. However, the hypothesis of host-driven speciation is seldom tested formally and must be weighed against overlooked explanations involving geographic isolation and non-ecological reproductive barriers in the process of speciation.

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.

The use of random amplified polymorphic DNA (RAPD) analysis for studies of genetic variation in populations of Coccinella septempunctata in Belgium.

The movement and dispersion of Coccinella septempunctata and its efficacy as aphid control agent over large areas is not really understood because of the difficulty in identifying the origins of predators. To quantify the genetic diversity within the species and monitor the spatial foraging, populations were sampled from Belgium and analysed for RAPD DNA variation. Twenty decamer primers generated more than hundred polymorphic RAPD bands and pairwise distances were calculated between populations according to Nei and Li, then used to construct a radial neighbour-joining dendrogram and examine intra- and inter-population variance coefficients, by analysis of molecular variation (AMOVA). This study shows that while a number of factors can complicate the use and interpretation of RAPD fragments as genetic markers, RAPD analysis can be a valuable technique for studies of intra-specific genetic variation in C. septempunctata.