Abstract
Increases in fruit weight of cultivated vegetables and fruits accompanied the domestication of these crops. Here we report on the positional cloning of a quantitative trait locus (QTL) controlling fruit weight in tomato. The derived allele of Cell Size Regulator (CSR-D) increases fruit weight predominantly through enlargement of the pericarp areas. The expanded pericarp tissues result from increased mesocarp cell size and not from increased number of cell layers. The effect of CSR on fruit weight and cell size is found across different genetic backgrounds implying a consistent impact of the locus on the trait. In fruits, CSR expression is undetectable early in development from floral meristems to the rapid cell proliferation stage after anthesis. Expression is low but detectable in growing fruit tissues and in or around vascular bundles coinciding with the cell enlargement stage of the fruit maturation process. CSR encodes an uncharacterized protein whose clade has expanded in the Solanaceae family. The mutant allele is predicted to encode a shorter protein due to a 1.4 kb deletion resulting in a 194 amino-acid truncation. Co-expression analyses and GO term enrichment analyses suggest association of CSR with cell differentiation in fruit tissues and vascular bundles. The derived allele arose in Solanum lycopersicum var cerasiforme and appears completely fixed in many cultivated tomato’s market classes. This finding suggests that the selection of this allele was critical to the full domestication of tomato from its intermediate ancestors.
Highlights
Rapid morphological diversification among closely related organisms often arise in response to strong selection pressures such as those imposed by domestication
We identified a gene that encodes a poorly characterized protein that controls fruit weight in tomato
The mutation that led to the increase in fruit weight arose early during the cultivation of tomato and is incorporated in all large tomato varieties
Summary
Rapid morphological diversification among closely related organisms often arise in response to strong selection pressures such as those imposed by domestication. The process of domestication is associated with the taming of wild relatives and the selections of types that benefit human use in terms of food production and clothing as well as shelter and companionship. The fully wild ancestor of tomato (Solanum pimpinellifolium) is indigenous largely to the coastal regions of Ecuador and Peru [3, 4]. Selections to the intermediate type, S. lycopersicum var cerasiforme, took place in the Andean mountain region of Ecuador and Northern Peru and further selections to the earliest domesticate type, S. lycopersicum var lycopersicum, took place in Central America [4]. Continued selections after the initial domestication events led to a huge diversity of size and shape with fruit weight increasing as much as a 1,000 fold [5]
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