Résumé
Host-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.