Early intracellular punctures by two aphid species on near-isogenic melon lines with and without the virus aphid transmission (Vat) resistance gene

The Var resistance gene (in Cucumis melo L.) inhibits the transmission of non-persistent viruses by Aphis gossypii Glover, but does not affect transmission by Myzus persicae (Suit.). To see whether this difference was behaviourally determined, we investigated the styler penetration behaviour of these two aphid species by recording EPGs (Electrical Penetration Graphs) of 8 and 20 min on two sets of susceptible and resistant isogenic melon lines. During the 20 min EPG study, inoculation with CMV (Cucumber Mosaic Virus) was also investigated. For both sets of isogenic lines, the two aphid species were able to detect the presence of Vat. The mean duration of individual intracellular punctures on the resistant genotypes was significantly reduced for both M. persicae and A. gossypii (-10% and -8% respectively for duration of pattern 'pd' in the 20 min experiment); this reaction appeared faster for M. persicae, a species for which melon was not a suitable host-plant. Therefore, in contrast to Vat's anti-transmission effect, this behavioural effect was not aphid species-specific. Also, the frequency of intracellular punctures on the resistant genotypes was significantly reduced in A. gossypii (but not in M. persicae): on average, this frequency dropped from similar to 0.65 pd.min(-1) on the susceptible genotypes to similar to 0.5 pd.min(-1) on the resistant ones. It was concluded that (intracellular?) chemical cues were detected very early by aphids probing on the resistant genotypes carrying the Vat gene. However, a comprehensive analysis of the behavioural traits of both aphids on the two genotypes could not alone account for the complete inhibition of transmission which was found only to occur for A. gossypii on resistant genotypes. None of the differences detected (between aphid species or between plant genotypes) could account for the Vat phenotype, although they may explain quantitative differences in transmission efficiencies between aphid species. It was thus concluded that Vat effect was primarily chemically mediated. Finally, some intracellular punctures bearing typical subphases have been identified in both aphid species and were designated as 'long potential drops' (pd-L). For A. gossypii, these were observed early after plant contact and their mean duration was twice that of standard intracellular punctures (similar to 8.5 s vs similar to 4.2 s). Although not necessary for CMV inoculation, the duration of such phases was positively correlated with a high transmission efficiency by A. gossypii on the susceptible genotype. The nature of this pattern and a putative mechanism of action of the Var gene are discussed.