Mapping Of Major QTL Regions And Identification Of Putative Candidate Genes For Drought Tolerence In Maize

Maize (Zea mays L.) is an important cereal crop for both human and animal consumption. However, drought stress strongly restricts maize production in most arid and semi-arid areas. New opportunities to improve drought tolerance in maize have emerged with developments in molecular technology, especially QTL mapping with molecular markers and biological statistics. Moreover, it is better to optimize the methodology of QTL mapping by combining functional and comparative genomics, and these methods should be utilized to improve drought tolerance of maize in combination with conventional breeding efforts.In the present study, the software MapMaker/EXP3.0 was adopted to construct the genetic linkage map, using Kosambi function and LOD threshold of 5.0 and 3.0, respectively. A genetic linkage framework map with SSR markers and an expanded linkage map with adding AFLP markers based on the framework map were constructed respectively with DNA samples from 234 F2 individuals derived from the cross X178 and B73. The field evaluation was conducted on the corresponding F2.3 families. The field experiments were arranged under well-watered (WW) and water-stressed (WS) regimes at Linfen city of Shanxi province in 2002 and 2003, as well as at Sanya city of Hainan province, respectively. Anthesis silking interval (ASI), ear setting percentage (ESP) and grain yield (GY) were measured, and their mean values in each location were used for QTL analysis. QTL analysis were performed by interval mapping method of software MapMaker/QTLl.l and composite interval mapping of software QTL Cartograher2.0 with a LOD threshold of 2.5, respectively. Furthermore, digenic epistatic interactions between all pairs of loci were tested by EPISTACY2.0 software. Interactions were considered at significance level of P<0.0001. By compiling the available QTL information for drought tolerance in maize, major QTL for drought tolerance were analyzed.For understanding the expression pattern of genes relevant to drought tolerance, gene differential expressions of maize lines X178 and B73 under water-stressed condition were detected at seedling and flowering stages using cDNA-AFLP techniques, respectively. And dESTs obtained were genetically localized by comparing molecular marker and sequence homology information between maize and rice genomics databases. The major results are as follows:1. A framework linkage map containing 130 co-dominant SSR markers was constructed, which spanned a total of 1537. 1cM with an average interval of 14.81cM, Furthermore the expanded linkage map was constructed by adding 119 AFLP loci based on the framework map, which spanned 1703.7cM of maize genome with an average interval of 7.13cM and the number of interval less than 20cM accounted for 92.1%. All the marker loci were tested for genetic segregation, and some segregation distortion regions were found most on the chromosomes 1, 2, 5, 8, 10. Additionally, a new method based on double polymorphic bands of co-dominant scoring of AFLPs was explored according to the similarity of loci amplified from AFLP enzyme combination.2. Three traits displayed a large quantitative variability, while showed high correlations under two water regimes across three environments. Parent X178 was characterized by higher values than B73 for ESP and GY, and lower for ASI. Significant variance differences for three traits were found among F2:3 families in different environments.