Development of molecular markers linked to a gene controlling fruit acidity in citrus.

Fruit juice pH, titratable acidity, or citric acid content was measured in 6 populations derived from an acidless pummelo (pummelo 2240) (Citrus maxima (Burm.) Merrill). The acidless trait in pummelo 2240 is controlled by a single recessive gene called acitric. Using bulked segregant analysis, three RAPD markers were identified as linked to acitric. RAPD marker OpZ20410, which mapped 1.2 cM from acitric, was cloned and sequenced, and a sequence characterized amplified region (SCAR) marker (SCZ20) was developed. The SCZ20-410 marker allele that is linked to the acitric allele occurs only in pummelo 2240 and other pummelos, and therefore, this SCAR marker should be useful as a dominant or codominant marker for introgressing acitric into mandarins and other citrus species. Using the cloned OpZ20410 band as a hybridization probe revealed a codominant RFLP marker called RFZ20 that mapped 1.2 cM from acitric. Progeny homozygous (acac) for the acitric allele had citric acid content below 10 μM, the minimum level detectable by high pressure liquid chromatography. The citric acid content of fruit juice from progeny predicted to be heterozygous (Acac) for acitric by the above markers was about 30% lower than that of juice from individuals predicted to be homozygous (AcAc) for the normal acid allele. Markers OpZ20410, SCZ20, and RFZ20 were highly polymorphic among 59 citrus accessions, and using one or more of these markers would allow citrus breeders to select seedling progeny heterozygous for acitric in nearly all crosses between pummelo 2240 or its offspring and other citrus genotypes.