DEVELOPMENT OF MOLECULAR MARKERS LINKED TO A GENE CONTROLLING FRUIT ACIDITY IN CITRUS

Fruit juice pH, titratable acidity, or citric acid content was measure d in 6 populations derived from an acidless pummelo (pummelo 2240) (Ci trus maxima (Burm.) Merrill). The acidless trait in pummelo 2240 is co ntrolled by a single recessive gene called acitric. Using bulked segre gant analysis, three RAPD markers were identified as linked to acitric . RAPD marker OpZ20(410), which mapped 1.2 cM from acitric, was cloned and sequenced, and a sequence characterized amplified region (SCAR) m arker (SCZ20) was developed. The SCZ20-410 marker allele that is linke d to the acitric allele occurs only in pummelo 2240 and other pummelos , and therefore, this SCAR marker should be useful as a dominant or co dominant marker for introgressing acitric into mandarins and other cit rus species. Using the cloned OpZ20(410) band as a hybridization probe revealed a codominant RFLP marker called RFZ20 that mapped 1.2 cM fro m acitric. Progeny homozygous (acac) for the acitric allele had citric acid content below 10 mu M, the minimum level detectable by high pres sure liquid chromatography. The citric acid content of fruit juice fro m progeny predicted to be heterozygous (Acac) for acitric by the above markers was about 30% lower than that of juice from individuals predi cted to be homozygous (AcAc) for the normal acid allele. Markers OPZ20 (410), SCZ20, and RFZ20 were highly polymorphic among 59 citrus access ions, and using one or more of these markers would allow citrus breede rs to select seedling progeny heterozygous for acitric in nearly all c rosses between pummelo 2240 or its offspring and other citrus genotype s.