The resistance to virus triggered by aphid inoculation in Vat melon is not systemic

Host plant resistance is an essential mean of controlling virus epidemies in crops. Resistance to viruses in plant belongs to three major families, either recessive or dominant resistance genes, as weil as the antiviral defense system based on RNA silencing. Most NBS-LRR antiviral resistances are triggered by the recognition of the NBS-LRR protein and a virus protein, playing the raie of avirulence factor. The Vat resistance gene in melon is unique among the dominant resistance to virus genes. It is a CC-NBS-LRR gene, it is triggered by the recognition between an aphid avirulence factor, delivered in plant cells by Aphis gossypii puncturing, and the CC-NBS-LRR pratein produced by Vat plants. This resistance is efficient against unrelated viruses transmitted on the non persistent mode (Boualem et al., 2016). We investigated if the resistance to virus triggered by A. gossypii in Vat plants is systemic

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

International audienceAphis 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)

A new view on aphid resistance in melon: the role of Aphis gossypii variability

Resistance to Aphis gossypii in Cucumis melo has been largely studied but A. gossypii variability has never been considered. Resistance to colonization by A. gossypii, clone NM1, and to non persistent virus transmission by this clone is conferred by a NBS-LRR gene called Vat, isolated in the PI 161375 accession. We investigated resistance to A. gossypii with four clones of A. gossypii that belong to two very distinct genotypes, NM1 and C9. The Vat gene conferred a high level of resistance to a NM1 clone and partial resistance to a C9 clone. Four additive QTL and two pairs of epistatic QTL were detected in a recombinant inbred line population derived from the cross ‘Védrantais’ x PI 161375. Half of them clearly have a specific effect on the acceptance by NM1 or C9 genotypes. We observed transgressive lines more resistant to NM1 or C9 in our RIL populations than PI 161375. Moreover, we studied resistance to A. gossypii and to virus transmission by A. gossypii in a set of 21 C. melo accessions. All the accessions resistant to virus transmission by C9 A. gossypii were resistant to virus transmission by NM1 genotype, and most of them were resistant to acceptance and colonization by the NM1 genotype. This is the phenotype of PI 161375, and we hypothesized that these phenotypes were controlled by the Vat gene. Other phenotypes were observed: resistance to NM1 A. gossypii (aphids and virus transmission) and susceptibility to C9 A. gossypii (aphids and virus transmission), independence between aphid resistance and resistance to virus transmission when using the same clone of A. gossypii. These phenotypes may be conferred by the locus Vat (different alleles) or by other locus of aphid resistance

Effects of resistance combinations in melon on Aphis gossypii field populations

National audienceHost-plant resistance has proven its efficiency in controlling pests, including aphids which are one of the major hemipteran pests in crops. Nevertheless, resistance-breaking biotypes have occurred in several plant-aphid systems. The aim of our field study was to investigate the impact of distinct combinations of aphid-resistance Vat gene and QTLs in melons on the density and genetic structure of field populations of the melon aphid species Aphis qossvpii. We set up field trials with melons combining (i) resistance alleles both at the Vat locus and at QTL (Vat/QTL), (ii) resistance allele at the Vat locus and susceptible alleles at QTL (Vat/-) or (iii) susceptible allele at the Vat locus and resistant alleles at QTL (-/QTL). Ali combinations had a homogenous genetic composition except at the Vat locus and QTL (Boissot et al 2010). Twelve trials were conducted in three distant melon producing areas,.: Southeastern France (SE), southwestern France (SW) and French West Indies (FWI), to take into account that aphid populations are geographically structured (Thomas et al 2012). Aphid population densities were significantly lower on Vat/QTL melon plants than on -/QTL plants over trials, revealing a strong effect of the Vat gene in situ. Conversely, no effect of the QTL was observed on aphid density. We also collected about 2800 aphids from these trials that were distributed in two classes whether they were found as isolated apterous individuals on the leaf soon after melon plantation or whether they were collected from a colony later on. Each aphid was genotyped for eight microsatellite loci. Whatever the resistance combination and area, the genetic diversity decreased dramatically during the melon development, from isolated apterous to colonies, which illustrated a strong host plant selective pressure. We also noticed a Vat gene effect in reducing the genetic diversity of both apterous populations in SE and colonies in SE and SW. Furthermore, according to a genetic clustering analysis, the 138 multilocus genotypes identified were separated in two clusters calied the NMl cluster and the Cucurbit luster. Aphids from the NMl cluster were significantly more affected by the Vat gene as apterous individuals were less numerous on Vat/QTL than on -/QTL melons and they did not develop into colonies on Vat/QTL and on Vat/- melons. ln conclusion, this field study did not provide experimental evidence that the combination of qualitative effective major gene and quantitative polygenic resistance to A. qossypii in melon improves control of the aphid populations. QTLs had no effect on demography or on clonai diversity and structure of aphid populations. On the contrary, the effect of the major resistance gene Vat was shown to de pend on the clonai diversity and structure ofthe aphid populations. However, this clonai diversity might substantially vary in time and space according to many ecological factors, which could prevent or at least slow down the spread of a resistance breaking biotype

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 melonproducing 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

Genetic diversity of the melon aphid Aphis gossypii Glover in different melon growing areas of France

International audienceIn melon, the Vat gene confers resistance to colonization by the Cotton-Melon aphid species, Aphis gossypii Glover. The Vat gene has been present in several melon varieties produced in south-eastern France for the past fifteen years, although as time goes by, the risk of the aphids overcoming such resistance is increasing. The study of the genetic structure of Cotton-Melon aphid populations is important in order to understand the efficacy and durability of methods to control pests through the use of resistant plants. Therefore, as outlined in the present paper, we set up field trials in geographically distant melon-producing areas in France and the French West Indies (FWI) using melon plants with the resistant allele or with the susceptible allele at the Vat locus. Samples of A. gossypii were collected from these crops in the different regions and their genetic diversity analyzed at eight microsatellite loci. We identified 33 multilocus genotypes (MLGs) that were present in multiple copies, of which five were observed in several regions of France whilst two were restricted to the FWI. The genetic diversity was high in south-eastern France, moderate in south-western and western France, and low in Guadeloupe (FWI). Differentiation between pairs of geographical populations in mainland France estimated by multilocus FST was not significant. Some MLGs (NM1 and C9) were significantly less frequent on melon with the resistant allele at the Vat locus than on melon with the susceptible allele at the Vat locus, while the frequencies of others (C6, CUC1, CUC13 and CUC25) increased significantly on melon plants with the resistant allele at the Vat locus

Combinaison gène majeur/QTL : quel intérêt pour la résistance du melon à <em>Aphis gossypii</em>

National audienceThe melon aphid, Aphis gossypii, is a major pest of many crops. Molecular markers have revealed a host race organization of the species and 8 microsatellite markers have allowed the identification of 21 multilocus genotypes (MLGs) among clones collected into colonies on Cucurbits. Aphid resistance have been identified in several melon accessions; it is controlled by a major gene, Vat, and some quantitative trait loci (QTLs). We built melon populations combining different loci of resistance, in a homogenous genetic background. They were cultivated in melon producing areas with contrasted aphid populations. Then, we showed a significant effect of the Vat gene on aphid density but non effect of the QTLs. One hundred and two MLGs were attributed to 2596 collected in trials, 54 MLGs were observed only once, five MLGs were only observed in the French West Indies, 19 MLGs were observed both in South eastern and South western France. Nevertheless, we did not observe a significant effect of the resistance loci on the diversity index of Shanon. Among clones collected into 125 colonies, we only identified 10 MLGs, five out of them were identified in clones collected on Vat-melon. The resistance loci might have an effect on the diversity of the South-eastern populations.Aphis gossypii, puceron ravageur majeur des cultures, est structuré en race d’hôtes. Huit marqueurs microsatellite ont permis d’identifier 21 génotypes multilocus (MLG)parmi les individus développant des colonies sur les Cucurbitacées. De nombreuses sources de résistance ont été identifiées chez le melon, la résistance est contrôlée par un gène majeur, Vat, et des loci à effet quantitatif (QTL). Nous avons construit des populations de melon combinant différemment ces loci dans un fond génétique homogène. Elles ont été cultivées en plein champ dans trois zones de production du melon présentant des populations de pucerons très contrastées. Sur la densité des populations, nous avons observé un effet significatif du gène Vat mais aucun effet des QTL. Parmi les 2596 pucerons collectés dans les essais, 102 MLG ont été identifiées, dont 54 une seule fois. Dix-neuf MLG ont été observés dans le Sud-est et le Sud-ouest ; seulement cinq MLG ont été observés aux Antilles (non identifiées en métropole). On n'observe pas d’effet significatif du gène Vat et des QTL sur l’indice de diversité de Shanon. Parmi 125 clones échantillonnés dans des colonies, nous avons identifié seulement 10 MLG dont 5 dans des clones échantillonnées sur des melons porteurs du gène Vat. L’effet du gène Vat et des QTL sur la diversité des populations serait restreint à la région Sud-est

Vers une lutte durable contre les virus de plantes

National audienc

Genetic data_Wingless Aphis gossypii on melon crops

Genetic data_Wingless Aphis gossypii on melon crop