Abstract
Genetically homogenous strains of Plasmopara halstedii differing in host specificity and fungicide tolerance were used to test the hypothesis that asexual genetic recombination occurs and may account for the high genotype diversity of this homothallic reproducing oomycete, which causes downy mildew in sunflower. Dual inoculation of sunflower seedlings with single zoospore strains of complementary infection characteristics caused sporulation under conditions where inoculation with each strain alone failed to infect. PCR-based investigation with strain-specific primers proved the presence of genetic traits from both progenitors in single sporangia collected from sporangiophores of such infections. Sister zoospores released from these sporangia revealed the genotype of the one or the other parental strain thus indicating heterokaryology of sporangia. Moreover, some zoospores showed amplification products of both parents, which suggests that the generally mononucleic spores derived from genetic recombination. The possibility of parasexual genetic exchange in the host-independent stage of infection and the evolutionary consequences are discussed.
Highlights
Genetic variation in populations of plant pathogenic oomycetes is fundamental for the evolutionary potential, when the host spectrum is limited or breeding for resistance in crop plants narrows the host accessibility and requires permanent adaptation
Homogenous strains of Plasmopara halstedii differing in host specificity and fungicide tolerance were used to test the hypothesis that asexual genetic recombination occurs and may account for the high genotype diversity of this homothallic reproducing oomycete, which causes downy mildew in sunflower
Sexual recombination and interspecific hybridization are predestined events to increase the genetic variation, but many oomycetes reproduce through homothallic gametangiogamy or apomixis [1] and natural hybridization has rarely been proven in oomycetes [2], except for some hemibiotrophs such as Phytophthora [3,4,5] and Pythium species [6]
Summary
Genetic variation in populations of plant pathogenic oomycetes is fundamental for the evolutionary potential, when the host spectrum is limited or breeding for resistance in crop plants narrows the host accessibility and requires permanent adaptation. Parasexual events have been suggested as an alternative explanation for the unexpected high phenotypic diversity in some plant pathogenic oomycetes [7,8]. A conspicuous high phenotypic plasticity has been reported for Plasmopara halstedii, the causal agent of downy mildew in sunflower [10,11,12], sexual reproduction of single zoospore isolates has shown its homothallic nature, a feature that usually constrains genetic diversity [13]. Dual inoculation of sunflower under selection conditions (different host resistance and fungicide tolerance) with two phenotypically different single zoospore
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