The Grand Gala apple gets its bloated size from cells that get bigger and bigger rather than dividing.
A mutant Gala apple is extra big, extra sweet, and extra crispy.
Scientists have probed the apple's genes to figure out what makes it that way.
The findings could help growers breed new and better varieties of apples.
When a mutant apple tree in Tennessee produced unusually enormous and extra crispy fruit, scientists took note.
"To find a mutant in an orchard is not uncommon, but these were 40 percent bigger," said Peter Hirst, a horticulturalist at Purdue University in West Lafayette, Ind. "From 50 feet away, you could tell they were much larger."
Now, Hirst and colleagues have figured out what gives the apples -- called Grand Galas -- their extra-large personalities: A set of mutations that make their cells grow larger than normal. The finding might eventually help growers produce crispy, delicious and giant apples that consumers would choose over other varieties.
"When I take this fruit home, that's the only one my family asks for," Hirst said. "This is another step in trying to understand how apples grow, so we can grow better apples and grow them more profitably."
Along with a colleague, Hurst found three ways that Grand Galas grow to be giants: They speed up their rates of photosynthesis -- which helps them more efficiently turn sunlight into carbohydrate fuel. They grow bigger stalks, which widens the pipeline for transporting nutrients into the fruit. And they are better at incorporating carbohydrates for growth.
On a microscopic level, the researchers found that DNA inside the apple's cells replicates like it normally does when cells divide during fruit growth. But instead of dividing, the cells just got bigger and bigger. The process also made the apples sweeter and crisper than normal: two traits that have made new varieties of apples so popular, including Honeycrisps and SweeTangos -- both recently introduced by breeders at the University of Minnesota.
And while the Grand Gala currently has an odd and uneven shape, it might some day vie for attention in a growing selection of fruit, if only at farmer's markets where customers choose taste over aesthetics.
"It's not like an apple is an apple," Hirst said. "People on the street can name five or 10 varieties of apple. They are used to seeing new varieties. What's out there now is good. To displace them, a new variety has to be even better."
Size is especially important to growers, who can harvest larger fruit more efficiently and, in turn, make more money from them, said Jim Luby, an apple breeding expert at the University of Minnesota, Twin Cities, who helped develop the Honeycrisp, among others.
By identifying genes involved in making apples bigger, he said, the new study might help growers produce bigger apples through breeding, engineering, or more accurate timing of when they apply plant hormones to promote or slow growth. As for the future of apple varieties, the sky is the limit.
"None are just like their parents or just like each other," Luby said. "There's no more limit than there is a limit to how many different kinds of people there are."