A LOCALIZED LINKAGE MAP OF THE CITRUS TRISTEZA VIRUS-RESISTANCE GENE REGION

A localized genetic linkage map was developed of the region surroundin g the citrus tristeza virus (CTV) resistance gene (designated Ctv) fro m Poncirus trifoliata L., a sexually compatible Citrus relative. Bulke d segregant analysis (BSA) was used to identify potential resistance-a ssociated RAPD fragment markers in four intergeneric backcross familie s that were segregating for CTV resistance. Eight RAPD fragments were found that were consistently linked to Ctv in the four families. Map d istances and locus order were determined with MAPMAKER 3.0, using the results obtained from 59 individuals in the largest family. Also, a co nsensus map was constructed with JOINMAP 1.3, using pooled results fro m the four backcross families. Marker orders were identical, except fo r 1 marker, on these independently developed maps. Family-specific res istance-associated markers were also identified, as were numerous susc eptibility-associated markers. The identification of markers tightly l inked to Ctv will enable citrus breeders to identify plants likely to be CTV-resistant by indirect, marker-assisted selection, rather than b y labor-intensive direct challenge with the pathogen. These markers al so provide a basis for future efforts to isolate Ctv for subsequent ge netic manipulation.