DETERMINING THE RELATIONSHIP BETWEEN VIRULENCE AND AGGRESSIVENESS IN PLASMOPARA HALSTEDII: DISCUSSION (part 2)

The combination of 12-EST derived markers revealed five multilocus genotypes (MLG) between seven P. halstedii pathotypes (Table 3). The two isolates MIL 001 and MIL 002 were different for all genomic markers excepting Pha54. The Neighbour-joining tree showed that isolates DU 1915, DU 1734 and DU 1943 had an intermediary genetic position between the two parental pathotypes of races 100 and 710 (Figure 1). But the distinctiveness of the 7xx races compared to those of 100 or 3xx has recently been shown on the basis of ITS sequence data (Spring et al. 2006). For the three isolates MIL 001, DU 1842 and DU 1767 localized in the same genetic clade, the relation between the two components of pathogenicity was positive. Our results provide evidence suggesting that a clonal lineage (one multilocus genotype) can include several races (Table 3). This suggests that mutations in a clonal lineage may lead to the emergence of new races in the same genetic background: this is the case for races 100, 300 and 304 (Figure 1). Two non-exclusive mechanisms may be responsible for this phenotypic variability in avirulence determinants.

The first of these mechanisms is recombination within the lineage via the homothallic fusion of gametangia. This hypothesis is certainly the most likely, given the high selfing rate of sunflower downy mildew inferred from our analysis. On the other hand, genetic recombination via parasexual events may have generated this variability as it has been shown recently that mitotic recombination between different isolates of P. halstedii may be involved in the generation of new phenotypes within a population (Spring and Zipper 2006). In this case, mitotic recombination within a genetic lineage would be required, and no such phenomenon has ever been described. The importance of mutation events in race evolution has already been highlighted in several Oomycetes plant pathogens such as Bremia lactucae (Lebeda and Petrzelova 2004) and Phytophtora infestans (Andrivon 1994). Although surprising at first sight, the greater importance of mutation rather than of recombination in populations able to outcross may be related to the large number of clonal cycles of fungal multiplication during epidemics. The combination of large populations, due to asexual reproduction of the pathogen, with strong selective pressures induced by resistance genes in the plant is likely to favor the emergence of new virulent races. To better understand the dynamics of P. halstedii populations, it will be necessary to analyse the relationship between the two components of pathogenicity in a large collection of isolates with different races from several parts of the world.

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