The making of a bell pepper-shaped tomato fruit: identification of loci controlling fruit morphology in Yellow Stuffer tomato

The heirloom tomato cultivar Yellow Stuffer produces fruit that are similar in shape and structure to fruit produced by the bell pepper varieties of garden pepper. To determine the genetic basis of this extreme fruit type in tomato, quantitative trait loci (QTL) analysis was performed on an F2 population derived from a cross between Yellow Stuffer and the related species, Lycopersicon pimpinellifolium, which produces a small, round fruit typical of most wild species. F2 plants were analyzed for both fruit size and the degree to which their fruit resembled the bell pepper. Three QTL were determined to influence bell pepper shape and seven QTL influenced fruit mass. The map positions of all three bell shape and six out of seven fruit size QTL appear to be allelic to components of fruit morphology analyzed in this population and to major fruit morphology QTL reported previously, adding support to the hypothesis that the majority of fruit size and shape variation in cultivated tomato is attributable to allelic variation at a limited number of loci. However, novel loci controlling components of fruit morphology, such as elongated fruit shape, bumpiness, number of seed per fruit and flowers per inflorescence were identified in this study as well. The three bell shape loci involved are: bell2.1, bell2.2 and bell8.1, and appear to correspond to locule number2.1 (lcn2.1) and fruit weight 2.2 (fw2.2) and fruit shape 8.1 (fs8.1), respectively. The Yellow Stuffer alleles at lcn2.1 and fw2.2 increase locule number and fruit size, respectively, hence contributing to the overall bell pepper shape. The Yellow Stuffer allele at fs8.1 causes convex locule walls, giving the extended, bumpy shape characteristic of bell peppers. In addition, most fruit size QTL correspond to loci controlling number of flowers per inflorescence and/or stem-end blockiness. Comparisons among previously identified fruit morphology loci in tomato, eggplant and pepper suggest that loci affecting several aspects of fruit morphology may be due to pleiotrophic effects of the same, orthologous loci in these species. Moreover, it appears that the evolution of bell pepper-shaped tomato fruit may have proceeded through mutations of some of the same genes that led to bell pepper-type fruit in garden pepper.