Domestication of many plants has correlated with dramatic increases in fruit size. In tomato, one quantitative trait locus (QTL), fw2.2, was responsible for a large step in this process. When transformed into large-fruited cultivars, a cosmid derived from the fw2.2 region of a small-fruited wild species reduced fruit size by the predicted amount and had the gene action expected for fw2.2. The cause of the QTL effect is a single gene, ORFX, that is expressed early in floral development, controls carpel cell number, and has a sequence suggesting structural similarity to the human oncogene c-H-ras p21. Alterations in fruit size, imparted by fw2.2 alleles, are most likely due to changes in regulation rather than in the sequence and structure of the encoded protein.In natural populations, most phenotypic variation is continuous and is effected by alleles at multiple loci. Although this quantitative variation fuels evolutionary change and has been exploited in the domestication and genetic improvement of plants and animals, the identification and isolation of the genes underlying this variation have been difficult.Conspicuous and important quantitative traits in plant agriculture are associated with domestication (1). Dramatic, relatively rapid evolution of fruit size has accompanied the domestication of virtually all fruit-bearing crop species (2). For example, the progenitor of the domesticated tomato (Lycopersicon esculentum) most likely had fruit less than 1 cm in diameter and only a few grams in weight (3). Such fruit was large enough to contain hundreds of seeds and yet small enough to be dispersed by small rodents or birds. In contrast, modern tomatoes can weigh as much as 1000 grams and can exceed 15 cm in diameter (Fig. 1A). Tomato fruit size is quantitatively controlled [for example, (4)]; however, the molecular basis of this transition has been unknown.Most of the loci involved in the evolution and domestication of tomato from small berries to large fruit have been genetically mapped (5, 6). One of these QTLs, fw2.2, changes fruit weight by up to 30% and appears to have been responsible for a key transition during domestication: All wild Lycopersicon species examined thus far contain small-fruit alleles at this locus, whereas modern cultivars have large-fruit alleles (7). By applying a map-based approach, we have cloned and sequenced a 19-kb segment of DNA containing this QTL and have identified the gene responsible for the QTL effect.Genetic complementation with fw2.2. A yeast artificial chromosome (YAC) containing fw2.2 was isolated (8) and used to screen a cDNA library (constructed from the small-fruited genotype, L. pennellii LA716). About 100 positive cDNA clones were identified that represent four unique transcripts (cDNA27, cDNA38, cDNA44, and cDNA70) that were derived from genes in the fw2.2 YAC contig. A high-resolution map was created of the four transcripts on 3472 F 2 individuals derived from a cross between two nearly isogenic lines (NILs) differing for alleles at fw2.2 ( Fig. 2A) (8). The fo...

A high-resolution physical and genetic map of a major fruit weight quantitative trait locus (QTL), fw2.2, has been constructed for a region of tomato chromosome 2. Using an F 2 nearly isogenic line mapping population (3472 individuals) derived from Lycopersicon esculentum (domesticated tomato) ؋ Lycopersicon pennellii (wild tomato), fw2.2 has been placed near TG91 and TG167, which have an interval distance of 0.13 ؎ 0.03 centimorgan. The physical distance between TG91 and TG167 was estimated to be < 150 kb by pulsed-field gel electrophoresis of tomato DNA. A physical contig composed of six yeast artificial chromosomes (YACs) and encompassing fw2.2 was isolated. No rearrangements or chimerisms were detected within the YAC contig based on restriction fragment length polymorphism analysis using YAC-end sequences and anchored molecular markers from the high-resolution map. Based on genetic recombination events, fw2.2 could be narrowed down to a region less than 150 kb between molecular markers TG91 and HSF24 and included within two YACs: YAC264 (210 kb) and YAC355 (300 kb). This marks the first time, to our knowledge, that a QTL has been mapped with such precision and delimited to a segment of cloned DNA. The fact that the phenotypic effect of the fw2.2 QTL can be mapped to a small interval suggests that the action of this QTL is likely due to a single gene. The development of the high-resolution genetic map, in combination with the physical YAC contig, suggests that the gene responsible for this QTL and other QTLs in plants can be isolated using a positional cloning strategy. The cloning of fw2.2 will likely lead to a better understanding of the molecular biology of fruit development and to the genetic engineering of fruit size characteristics.In nature there are numerous examples of quantitative traits that display continuous variation due to the interaction of polygenes, or quantitative trait loci (QTLs), with the environment. Early genetic experiments with beans (1), wheat (2), and tobacco (3) suggested that continuous variation in phenotype could be accounted for by a large number of polygenes, each contributing a small effect. More recently, through the use of molecular linkage maps, a systematic search of entire genomes for QTLs has revealed that in many cases a large proportion of the total phenotypic variance is attributable to a few major QTLs (4-12). Such QTL marker studies have changed the original hypothesis that polygenes each have an equally small effect on the phenotype. Instead, it appears that for traits displaying continuous variation, the phenotype is the result of the action of major QTLs together with the environment and QTLs of lesser effects.Tomato fruit weight is a classic example of a quantitative trait displaying continuous variation (13). Previous studies indicated that between 5 (14) and 20 (15) genes are involved in the inheritance of tomato fruit weight. Recently, molecular marker studies conducted by Paterson et al. (6) found 11 QTLs affecting tomato fruit mass in a cross bet...

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We have shown that a major QTL for fruit weight (fw2.2) maps to the same position on chromosome 2 in the green-fruited wild tomato species, Lycopersicon pennellii and in the red-fruited wild tomato species, L. pimpinellifolium. An introgression line F2 derived from L. esculentum (tomato) x L. pennellii and a backcross 1 (BC1) population derived from L. esculentum x L. pimpinellifolium both place fw2.2 near TG91 and TG167 on chromosome 2 of the tomato highdensity linkage map. fw2.2 accounts for 30% and 47% of the total phenotypic variance in the L. pimpinellifolium and L. pennellii populations, respectively, indicating that this is a major QTL controlling fruit weight in both species. Partial dominance (d/a of 0.44) was observed for the L. pennellii allele of fw 2.2 as compared with the L. esculentum allele. A QTL with very similar phenotypic affects and gene action has also been identified and mapped to the same chromosomal region in other wild tomato accessions: L. cheesmanii and L. pimpinellifolium. Together, these data suggest that fw2.2 represents an orthologous QTL (i.e., derived by speciation as opposed to duplication) common to most, if not all, wild tomato species. High-resolution mapping may ultimately lead to the cloning of this key locus controlling fruit development in tomato.