Bulk segregant analysis with molecular markers and its use for improving drought resistance in maize

The usual method to locate and compare loci regulating quantitative traits (QTLs) requires a segregating population of plants with each one genotyped with molecular markers. However, plants from such segregating populations c an also be grouped according to phenotypic expression of a trait and tested for differences in allele frequency between the population bulks: bulk seg regant analysis (BSA). The same probes used for making a genetic map (e.g. isozyme, RFLP, RAPD, etc) can be used for BSA, A molecular marker showing p olymorphism between the parents of the population and which is closely-link ed to a major QTL regulating a particular trait will mainly cc-segregate wi th that QTL, i.e. segregate according to the phenotype if the QTL has a lar ge effect. Thus, if plants are grouped according to expression of the trait and extreme groups tested with that polymorphic marker, the frequency of t he two marker alleles present within each of the two bulks should deviate s ignificantly from the ratio of 1:1 expected for most populations. As chromo somal locations of many molecular markers have now been determined in many species, the map location of closely-linked QTLs can therefore be deduced w ithout having to genotype every individual in a segregating population. Thi s has been used successfully with composite populations of maize to locate QTLs associated with yield under severe drought. An inbred line derived fro m one of the populations selected for higher drought yield has been crossed with a drought-susceptible inbred line to produce a mapping population for QTL analysis of physiological and developmental traits likely to regulate yield under drought. Future work to identify traits having QTLs with flanki ng markers showing significant allele frequency differences in the BSA stud ies will indicate those traits likely to be important in determining yield under drought.