Gladieux, Pierre; Guérin, Fabien; Didelot, Frédérique, Parisi, Luciana; Caffier, Valérie; Le Cam, Bruno*. Host specialization prevents gene flow between sympatric divergent lineages of the apple scab fungus Venturia inaequalis. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-08. Click here to download the presentation on pdf format.
Plant resistance based on a gene-for-gene relationship can select populations that spread on formerly resistant cultivars over large areas. The breakdown of the Vf resistance gene to scab (Venturia inaequalis) in apple (Malus x domestica) is a case study for the origin and fate of new virulence in an agricultural landscape. Previous studies in North-Western France showed that populations pathogenic on Vf cultivars (Vfcv populations) (1) shared a common origin, (2) have not been selected from populations pathogenic on non-Vf cultivars (nVfcv populations), (3) had a low genetic diversity and were highly differentiated from populations pathogenic on nVfcv populations. For a sexual fungus, observing such a pattern several years after the first reported breakdown was suggesting little recombination between nVfcv and Vfcv populations. This study was conducted to address this hypothesis. We used microsatellite markers and pathogenicity tests on nVfcv and Vfcv strains sampled from the same orchard in different locations over several years. In all locations, the signature of original founder events prevailed: Vfcv populations were genetically depauperate, strongly differentiated from nVfcv ones and no significant evolution in time was observed. Contrary to nVfcv populations, multilocus tests for linkage disequilibrium were not consistent with random mating in Vfcv populations. However, genotyping of ascospores isolated from perithecia on leaves collected on Vf-cultivar and overwintered on the ground allowed excluding asexuality of Vfcv populations as a possible isolating barrier. Rather than a lack of sexual structure, strict host specialization appeared to be a more valuable hypothesis to account for apparent reproductive isolation: pathogenicity tests revealed that Vfcv and nVfcv populations were enable to infect their reciprocal hosts. Our data are consistent with theoretical work showing that for pathogens with restricted dispersal between development on the host and mating, specialization could prevent gene flow between sympatric lineages without requiring assortative mating or habitat preference.
Keywords: Resistance durability, virulence spreading, founder events, host specialization, gene breakdown
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