QTL mapping of citrus freeze tolerance

Freeze limits the cultivated area of citrus. Periodical freeze events in some producing areas caused severe damages and low temperature during extreme climate change often affected the citrus global production in recent years. To develop and cultivate freeze tolerant cultivars is the most effective way to avoid damages caused by freeze. Cold tolerance is considered a quantitative trait in plants. In this research, QTL mapping was used to find loci and molecular markers putatively related with freeze tolerance in citrus. A new linkage map was established by combining the segregating data of markers in a published map and SSR markers newly developed from citrus EST and Clementine BAC-end sequences. New map integrated 452 SSR markers with 9 linkage groups and covered 936.1 cM of citrus genome with an average genetic distance at 2.07 cM between adjacent markers. Freeze tolerance of the mapping population of parents and 68 progenies was tested by the method of electrolyte leakage in the end of December after more than two weeks of low temperature hardening and Semi-lethal low temperature (LT50) was estimated with logistic equation. The distribution of LT50 in the mapping population was not deviated from a normal distribution. Four QTLs were indentified at LOD≥3.0, which could explain 29.3, 23.1, 44.7 and 21.0% of the phenotypic variation. These QTLs were mapped on the linkage group 2, 4, 3 and 1, respectively. The identified QTL areas and nearby markers should be useful for citrus freeze tolerance research and breeding in the future.