Scientists are analyzing the wild watermelon--hard, white, small and bitter--for genetic traits that might help develop cultivated forms that are more resistant to diseases.
An international team of more than 60 researchers, led by Zhangjun Fei at Cornell University's Boyce Thompson Institute for Plant Research, sequenced and published the domesticated watermelon genome for the first time in late November in Nature Genetics.
"When you get the sequence, it's just combinations of AGCT, but you need to process them and connect them together into a whole chromosome," Fei said. His team constructed the chromosome and then cross-referenced the domesticated melon's genome with 20 wild watermelon genomes to see which genes had been lost over time.
Fei isolated genetic material that dropped out of the cultivated melon's genome during the selective breeding process, hoping to find genes that protect a watermelon from diseases like fusarium wilt. Of the many genes in the wild watermelon that are missing in the domesticated fruit, Fei identified 10 as candidates for making the fruit more resistant to plant diseases.
Another team member, plant geneticist Amnon Levi at the United States Department of Agriculture's Vegetable Laboratory, said breeding for only a few traits in the domesticated watermelon, to make it big, pink, and sweet, created a genetic bottleneck.
Even in the less common types of watermelons, the heirloom watermelons whose seeds were passed down through family generations, he said genetic diversity is low. According to Levi, there are 300 to 400 heirloom watermelon varieties in the United States. He studied the genomes of those rarer melons to see if they maintained more genetic diversity than the select few sold by major watermelon growers.
"To my surprise I discovered there is very little genetic diversity," Levi said of heirloom varieties in the United States. "Even though size of melon and seeds, and colors, are different, when you look at whole genome, there is very little diversity." This makes the genetic material in the wild African watermelon even more valuable.
Using genetic sequencing to breed new traits into the watermelon is quicker than traditional breeding methods. Fei said the number of years it will take to cultivate more desirable melons with sequencing technology is half that of older techniques. Once his team identifies a marker for disease resistance on the ancient melon's genome, it is simple to check a new strain of melon for the same marker and know whether or not it will hold up to certain plant pests.
"The watermelon is quite sophisticated," Levi said. Breeding for disease resistance, he said, is "just the beginning."