HIGH-RESOLUTION MAPPING AND ISOLATION OF A YEAST ARTIFICIAL CHROMOSOME CONTIG CONTAINING FW2.2 - A MAJOR FRUIT WEIGHT QUANTITATIVE TRAIT LOCUS IN TOMATO

A high-resolution physical and genetic map of a major fruit weight qua ntitative trait locus (QTL), fw2.2, has been constructed for a region of tomato chromosome 2. Using an F-2 nearly isogenic line mapping popu lation (3472 individuals) derived from Lycopersicon esculentum (domest icated tomato) x 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 e stimated to be less than or equal to 150 kb by pulsed-field gel electr ophoresis of tomato DNA. A physical contig composed of six yeast artif icial chromosomes (YACs) and encompassing fw2.2 was isolated. No rearr angements or chimerisms were detected within the YAC contig based on r estriction fragment length polymorphism analysis using YAC-end sequenc es and anchored molecular markers from the high-resolution map. Based on genetic recombination events, fw2.2 could be narrowed down to a reg ion less than 150 kb between molecular markers TG91 and HSF24 and incl uded 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. T he 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 clonin g strategy. The cloning of fw2.2 will likely lead to a better understa nding of the molecular biology of fruit development and to the genetic engineering of fruit size characteristics.