Development and characterization of SCAR markers linked to the citrus tristeza virus resistance gene from Poncirus trifoliata.

Twelve new dominant randomly amplified polymorphic DNA (RAPD) fragments associated with a single dominant gene for resistance to citrus tristeza virus (CTV) were identified using bulked segregant analysis of an intergeneric backcross family. These and eight previously reported RAPDs were mapped in the resistance gene (Ctv) region; the resulting localized linkage map spans about 32 cM, with nine close flanking markers within 2.5 cM of Ctv. Seven of 20 RAPD fragments linked with the resistance gene were cloned and sequenced, and their sequences were used to design longer primers to develop sequence characterized amplified region (SCAR) markers that can be utilized reliably in marker-assisted selection, high-resolution mapping, and map-based cloning of the resistance gene. All seven cloned RAPDs were converted successfully into SCARs by redesigning primers, optimizing PCR parameters (especially the annealing temperature), or digesting amplification products with restriction enzymes. Four of the seven remained dominant markers, displaying presence-absence polymorphism patterns; the other three detected restriction site changes or length variations and thus were transformed into codominant markers. Two genomic regions rich in variability were also detected by two codominant SCAR markers.