Characterization of fs8.1, a major QTL influencing fruit shape in tomato

In two previous quantitative trait locus (QTL) mapping studies conducted in Lycopersicon esculentum x L. pimpinellifolium BC1 and BC2 populations we had localized a major QTL for fruit shape, fs8.1, to a ca. 20 chi interval on the short arm of chromosome 8, flanked by markers TG176 and CT92. At this QTL the allele from the wild species reduces the length of fruit, giving round-shaped fruit. In order to define more precisely the location of fs8.1, near-isogenic Lines (NILs) segregating for the region of interest were developed. The results from substitution mapping show that no recombination occurred between fs8.1 and the marker CD40 in 322 meioses. The gene action for fs8.1 was determined in a BC4F3 population to be partial dominance. The main effect of fs8.1 is exerted on fruit length while fruit diameter is not significantly affected. A highly significant correlation (r = 0.89; P < 0.01) was found between fruit shape and ovary shape indicating that the fs8.1 gene product acts early in ovary development (preanthesis). Implications for the evolution of fruit shape and the feasibility of map-based cloning of this QTL are discussed.