Inheritance of citrus nematode resistance and its linkage with molecular markers

Eleven RAPD markers Linked to a gene region conferring resistance to citrus nematodes in an intergeneric backcross family were identified. Two sequenc e characterized amplified region markers linked to a citrus tristeza virus resistance gene and one selected resistance gene candidate marker were eval uated for their association with citrus nematode resistance. A nematode-sus ceptible citrus hybrid, LB6-2 [Clementine mandarin (Citrus reticulata)xHaml in orange (C. sinensis)], was crossed with the citrus nematode-resistant hy brid Swingle citrumelo (C. paradisixPoncirus trifoliata) to produce 62 hybr ids that were reproduced by rooted cuttings. The plants were grown in a gre enhouse and inoculated with nematodes isolated from infected field trees. T he hybrids segregated widely for this trait in a continuous distribution, s uggesting possible polygenic control of the resistance. Bulked segregant an alysis was used to identify markers associated with resistance by bulking D NA samples from individuals at the phenotypic distribution extremes. Linkag e relationships were established by the inheritance of the markers in the e ntire population. A single major gene region that contributes to nematode r esistance was identified. The resistance was inherited in this backcross fa mily from the grandparent Poncirus trifoliata as a single dominant gene. QT L 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 s ome moderately susceptible hybrids possessed the resistance-linked markers. The markers may be useful in citrus rootstock breeding programs if it call be demonstrated that they are valid in other genetic backgrounds.