Mark Twain called the cherimoya and its cousin the sugar apple “the most delicious fruit known to men.” Though little more than exotic edibles to most Americans, such fruits of the Annona family have been cultivated by people in Central and South America for generations. Even in pre-Columbian times, Annona fruits were enjoyed for their sherbetlike texture and a flavor that resembles a mixture of banana and pineapple. But they also contain numerous hard seeds that make the fruit difficult to eat. And even though seedless fruits such as grapes and watermelons have been cultivated for thousands of years, botanists have not been able to identify exactly why the seeds fail to form.
Then one day a Spanish sugar-apple farmer identified a strange, seedless fruit and brought it to the attention of botanists in Madrid. The scientists consulted Charles Gasser, a molecular biologist at the University of California, Davis, and in a paper published in the Proceedings of the National Academy of Sciences USA, the two labs identified the genetic mutation that enables the plant to produce fruit without seeds. “This study gives us the molecular basis for seedlessness, which is the first time this has been done for a fruit plant,” Gasser says.
With this knowledge, Gasser observes, humans may be able to create other varieties of seedless fruits, such as cherimoyas and tomatoes, that have so far defied conventional breeding techniques. Seeds are crucial to fruit formation because they typically emit hormone signals that bring the fruit into being. Occasionally fruit-bearing plants, such as the banana, will contain a genetic mutation that allows fruit formation without seed development. In others, such as watermelons, a small part of the seed remains intact and sets off the hormone cascade that tells the fruit to develop. Now we have a new piece of the puzzle.