A HIGH-RESOLUTION LINKAGE MAP OF THE CITRUS TRISTEZA VIRUS-RESISTANCEGENE REGION IN PONCIRUS-TRIFOLIATA (L.) RAF

Resistance to citrus tristeza virus (CTV) was evaluated in 554 progeny of 10 populations derived from Poncirus trifoliata. A dominant gene ( Ctv) controlled CTV resistance in P. trifoliata. Twenty-one dominant P GR-based DNA. markers were identified as linked to Ctv by bulked segre gant analysis. Of the 11 closest markers to Ctv, only 2 segregated in all populations. Ten of these markers were cloned and sequenced, and c odominant RFLP markers were developed. Seven RFLP markers were then ev aluated in 10 populations. Marker orders were consistent in all linkag e maps based on data of single populations or on combined data of popu lations with similar segregation patterns. In a consensus map, the six closest marker loci spanned 5.3 cM of the Ctv region. Z16 cosegregate d with Ctv. C19 and AD08 flanked Ctv at distances of 0.5 and 0.8 cM, r espectively. These 3 markers were present as single copies in the Ponc irus genome, and could be used directly for bacterial artificial chrom osome library screening to initiate a walk toward Ctv,. BLAST searches of the GenBank database revealed high sequence similarities between 2 markers and known plant disease resistance genes, indicating that a r esistance gene cluster exists in the Ctv region in P. trifoliata.