You say tomato, I say retrotransposon

Written byElie Dolgin

| 2 min read

The oblong shape of some tomatoes arose from a gene duplication caused by a selfish genetic element, according to a linkurl:study;http://www.sciencemag.org/cgi/content/abstract/319/5869/1527 published today in Science. Before tomatoes were linkurl:cultivated;http://www.the-scientist.com/article/display/54040/ and grown around the world, wild tomatoes were a little-known, small, round South American fruit. But go down to the market today and you'll find juicy, ripe tomatoes of all shapes and sizes. When you bite into some of the pear-shaped varieties, you are not just eating tomatoes with a longer shape; new research shows it also has slightly longer DNA owing to the duplication of the radiantly named gene sun. linkurl:Esther van der Knaap;http://oardc.osu.edu/vanderknaap/ of Ohio State University in Wooster has been hunting the source of the elongated shape for years. In 2001, she narrowed her search to a locus on the short arm of chromosome 7 (tomatoes have 12 chromosomes), and in 2004, she mapped the gene to a 24.7 kilobase insertion found only in an elongated tomato variety but not in the round, ancestral type. In her most recent study, van der Knaap's team pinpointed the sun gene within the insertion, and showed that the extra DNA was actually a duplicated stretch of chromosome 10. The flanking sequences around the insertion also showed "the key hallmarks" of a retrotransposon, van der Knaap told The Scientist. linkurl:Parasitic genetic elements;http://www.the-scientist.com/article/display/12080/ known as retrotransposons replicate themselves throughout the genome, often dragging along other fragments of DNA in the process. But these DNA elements usually disrupt gene function. So it was surprising to find that the linkurl:retrotransposon-mediated;http://www.the-scientist.com/article/display/22857/ duplication led to an overexpression of the sun gene, which the authors suggest might ramp up levels of auxin, a key plant hormone. "It's a very unusual thing we found," said van der Knaap. Nor was it very old; over the entire inserted region, only 3 nucleotides differed, indicating that the duplication likely happened very recently. Steve Tanksley of Cornell University, who was van der Knaap's PhD supervisor but was not involved in the research, described the study as an "interesting piece of the puzzle" toward understanding how genetic changes underlie morphological changes. But Antonio Granell of the Polytechnic University of Valencia, Spain, said the duplication was probably "a very particular case," noting that other genes might be involved in other elongated tomato varieties. He also said there was "almost no evidence" that auxin was responsible for the tomato's shape-shift, and cautioned that more work into sun's molecular mechanism was needed. Even if the duplication is unique, van der Knaap said she has unpublished results suggesting that the sun gene is active in a number of different tomato varieties, all displaying the same signature long and pointed shape. So next time you go down to the market, be sure to enjoy a snack of the oblong tomatoes!

You say tomato, I say retrotransposon

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