fw2.2 : A Quantitative Trait Locus Key to the Evolution of Tomato Fruit Size

Abstract

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...

Differences in regulation of carbohydrate metabolism during early fruit development between domesticated tomato and two wild relatives

“…This probably reXects a large diVerence in ovarian cell number and may thereby explain the large diVerence in absolute fruit growth between domestic tomato and the wild species right after the initiation of the cell division phase. A major Quantitative Trait Locus for fruit size is encoded by the fw2.2 gene (Frary et al 2000). The diVerence between the domesticated and the wild alleles is in the regulation of expression rather than in changes in the fw2.2.…”

Common Functions or Only Phylogenetically Related? The Large Family of PLAC8 Motif-Containing/PCR Genes

“…In tomato the fruit weight 2.2 (fw2.2) has been identified as a quantitative trait locus, which is responsible for about 30% of the fruit size (Alpert and Tanksley, 1996;Frary et al, 2000). When ORFX, the corresponding gene, was introduced into a cultivar producing large fruits, tomato fruit size decreased.…”

QTL hotspots in eggplant (Solanum melongena) detected with a high resolution map and CIM analysis

“…Thus it is not surprising that some of the earliest work in gene and genome mapping as well as comparative genomics was done in the Solanaceae family. The construction of the first high-density molecular linkage map in tomato (Solanum lycopersicum) (Tanksley et al 1992) was essential in establishing that species as a forerunner in the realm of quantitative trait locus (QTL) mapping and cloning (Alpert et al 1995;Frary et al 2000). The close relationship of tomato to potato (S. tuberosum), eggplant (S. melongena), and pepper (Capsicum spp.)…”