Inheritance of citrus nematode resistance and its linkage with molecular markers
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Eleven RAPD markers linked to a gene region conferring resistance to citrus nematodes in an intergen-eric backcross family were identified. Two sequence- characterized amplified region markers linked to a citrus tristeza virus resistance gene and one selected resistance gene candidate marker were evaluated for their association with citrus nematode resistance. A nematode-susceptible citrus hybrid, LB6-2 [Clementine mandarin (Citrus reticulata)×Hamlin orange (C. sinensis)], was crossed with the citrus nematode-resistant hybrid Swingle citrumelo (C. paradisi×Poncirus trifoliata) to produce 62 hybrids that were reproduced by rooted cuttings. The plants were grown in a greenhouse and inoculated with nematodes isolated from infected field trees. The hybrids segregated widely for this trait in a continuous distribution, suggesting possible polygenic control of the resistance. Bulked segregant analysis was used to identify markers associated with resistance by bulking DNA samples from individuals at the phenotypic distribution extremes. Linkage relationships were established by the inheritance of the markers in the entire population. A single major gene region that contributes to nematode resistance was identified. The resistance was inherited in this backcross family from the grandparent Poncirus trifoliata as a single dominant gene. QTL analysis revealed that 53.6% of the phenotypic variance was explained by this major gene region. The existence of other resistance-associated loci was suggested by the continuous phenotypic distribution and the fact that some moderately susceptible hybrids possessed the resistance-linked markers. The markers may be useful in citrus rootstock breeding programs if it can be demonstrated that they are valid in other genetic backgrounds.
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