The full-sib intercross line (FSIL): a QTL mapping design for outcrossing species

A full-sib intercross line (FSIL) is constructed in an outcrossing species by mating two parents and intercrossing their progeny to form a large inter cross line. For given statistical power, a FSIL design requires only slight ly more individuals than an F2 design derived from inbred line cross, but 6 - to 10-fold fewer than a half-sib or full-sib design. Due to population-wi de linkage disequilibrium, a FSIL is amenable to analysis by selective DNA pooling. In addition, a FSIL is maintained by continued intercrossing so th at DNA samples and phenotypic information are accumulated across generation s. Continued intercrossing also leads to map expansion and thus to increase d mapping accuracy in the later generations. A FSIL can thus provide a brid ge to positional cloning of quantitative trait loci (QTL) and marker-assist ed selection in outcrossers; and is particularly effective in exploiting th e QTL mapping potential of crosses between selection lines or phenotypicall y differentiated populations that differ in frequency, but are not at fixat ion, for alternative QTL alleles. In the course of the power analyses, it i s shown that for F2 and FSIL designs, power is a function of Nd-2 alone, wh ere N is the total size of the mapping population and dis the standardized gene effect; while for half-sib and full-sib populations, power is a functi on of Nd-2 and of the number of families included in the mapping population . This provides a convenient means of estimating power for a wide variety o f mapping designs.