Maria-Das-Dores Lafargue

The dagger nematode Xiphinema index has a major economic impact because of its transmission of Grapevine fanleaf virus to grapevines. This vector nematode, which was introduced into Western countries from the Middle East together with the domesticated grapevine, mostly reproduces by meiotic parthenogenesis, but microsatellite multilocus genotype (MLG) analysis has revealed the occurrence of rare sexual reproduction events in field conditions. In a previous 6-year study under controlled conditions, we evaluated the durability of resistance to X. index in accessions derived from a muscadine resistance source and reference accessions. In this previous study, we used an equal-proportion mixture of four lines (from Spain, Italy, Greece, and Iran) representative of X. index diversity as the inoculum, and we collected random samples in 3-, 4-, 5-, and 6-year-old vines. Here, we genotyped the individuals from these samples using the MLG technique, and we analyzed the changes in line frequency and the occurrence of sexual reproduction events between lines over time. The nematode lines differed in aggressiveness and hybrids between lines were detected at a low, but apparently increasing rate. Hybridization events were recovered in all accessions, regardless of resistance status and propagation type. Finally, our data provide the first evidence of sexual reproduction in the nematode X. index under controlled conditions.

Breeding for varieties carrying natural resistance (R) against plant-parasitic nematodes is a promising alternative to nematicide ban. In perennial crops, the long plant−nematode interaction increases the risk for R breaking and R durability is a real challenge. In grapevine, the nematode Xiphinema index has a high economic impact by transmitting Grapevine fanleaf virus (GFLV) and, to delay GFLV transmission, rootstocks resistant to this vector are being selected, using Muscadinia rotundifolia in particular as an R source. To optimize in fine this strategy, the durability has been studied under controlled conditions in F1 and BC1 muscadine-derived resistant accessions previously obtained from either hardwood-cutting or in vitro propagation. After inoculation with a mix, in equal proportions, of four lines representative of the X. index diversity, multiplication on plants has been monitored 3 to 6 years. The nematode reproduction factor remained lower than 1 in resistant plants obtained from hardwood cuttings while it increased at values far beyond 1 in resistant plants of in vitro origin. Data for nematode numbers per gram of roots mostly paralleled those obtained for the reproduction factor. The effect of the propagation type on resistance over years was also evaluated for the ratio female/juvenile and the frequency of males. Altogether our results illustrate that the muscadine-derived resistance based on hardwood cuttings is durable. By contrast, in resistant and reference accessions obtained from in vitro, our data suggest that the increased nematode multiplication might be mainly due to the modification of root architecture consecutive to this propagation method.