fw2.2: A quantitative trait locus key to the evolution of tomato fruit size

Domestication of many plants has correlated with dramatic increases in frui t size. In tomato, one quantitative trait Locus (QTL), fw2.2, was responsib le for a Large step in this process. When transformed into large-fruited cu ltivars, a cosmid derived from the fw2.2 region of a small-fruited wild spe cies reduced fruit size by the predicted amount and had the gene action exp ected for fw2.2. The cause of the QTL effect is a single gene, ORFX, that i s expressed early in floral development, controls carpel cell number, and h as a sequence suggesting structural similarity to the human oncogene c-H-ra s p21. Alterations in fruit size, imparted by fw2.2 alleles, are most Likel y due to changes in regulation rather than in the sequence and structure of the encoded protein.