Extension of a citrus genetic linkage map for QTL mapping of freeze tolerance

Freeze limits the cultivated area of citrus. Periodically freeze events in some producing areas caused severedamages to citrus harvest and low temperature during extreme climate change often affected the citrusglobal production in recent years. To develop and cultivate freeze tolerant cultivars is the most effective wayto avoid damages caused by freeze. Cold tolerance is considered a quantitative trait in plant. In this research,QTL mapping was used to find loci and molecular markers putatively related with freeze tolerance in citrus. Anew linkage map was established by combining the segregating data of markers in a published map and SSRmarkers newly developed from citrus EST and clementine BAC-end sequences. New map integrated 452 SSRmarkers with 9 linkage groups and covered 923.3 cM of citrus genome with an average genetic distance at2.04 cM between adjacent markers. Freeze tolerance of the mapping population of parents and 68 progenieswas tested by the method of electrolyte leakage in the end of December after more than one month lowtemperature hardening and semi-lethal low temperature (LT50) was estimated with logistic equation. Thedistribution of LT50 in the mapping population was not deviated from a normal distribution. QTL analysis wascarried out with Map QTL5.0 software. Four QTLs were indentified at LOD≥3.0 using Multiple QTL Mapping(MQM) procedure, which could explain 29.3%, 23.1%, 44.7% and 21.0% of the phenotypic variation. TheseQTLs were mapped on the linkage group 2, 4, 3 and 1 respectively. The identified QTL areas and nearbymarkers should be useful for citrus freeze tolerance research and breeding in the future.