Virulence and molecular diversity of parthenogenetic root-knot nematodes, Meloidogyne spp.

Root-knot nematodes (RKN) are sedentary endoparasites causing severe damage to a wide variety of crops, including tomato. Among them, the parthenogenetic species Meloidogyne arenaria, M. incognita and M. javanica are of particular economic importance. The genetic diversity and relationships of 17 populations belonging to these three major species, either avirulent or virulent against the tomato Mi resistance gene, were investigated in order to determine whether (a)virulence of the nematodes could be related to their molecular fingerprints. Genomic polymorphisms between populations were assessed by using amplified fragment length polymorphism (AFLP) markers, and data were treated by means of a multiple correspondence analysis. A total of 1550 polymorphic amplified DNA fragments were identified and used to compute the relationships between the populations. As expected, the three RKN species were clearly distributed into distinct groups, but combination of data for virulence phenotypes and DNA markers showed that clustering of populations was not associated with their (a)virulence against the tomato Mi resistance gene. Such a lack of correlation indicates that most of the observed DNA polymorphism is independent of virulence, which is presumably under host selection. This result demonstrates that virulent populations do not share a common origin, and strongly suggests that they might have appeared late after the establishment of these clonal lineages, as the result of independent mutational events.