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If Bananas Were Spheres and Tomatoes Were Cylinders

As designed by nature, the wild tomato looks a lot like a cherry, though not very much like a cherry tomato. It’s small, round, dark and relatively hard, ideally sized and shaped for its primary consuming audience: South American birds and small animals who eat the fruit, then disperse its seeds.

Grocery-store tomatoes, on the other hand, can be monstrosities. Thousands of years of selective breeding and manipulation have created a cornucopia of tomato sizes, colors and shapes. Tomatoes can be as big as softballs or as small as grapes. They come in hues of green, yellow, red and orange, and in shapes even more diverse: spherical and oblong; pearlike, egglike and pepperlike; completely smooth or pointed at one end.

“It’s safe to say that humans have adapted every edible or usable fruit and vegetable,” said Martin Yanofsky, an associate professor of biology at the University of California San Diego who studies the development of flowers and fruits in higher plants.

“We know we can create new, domesticated varieties that contain more of the qualities we desire, whether that’s being bigger, juicier, higher-yielding or something else.”

But while plant breeders, farmers and scientists know how to do things like build a juicier tomato or a plumper, purpler eggplant, they do not really know why fruits and vegetables are shaped the way they are.

What is the difference between apples and oranges? Why is a banana shaped like a banana? The question might seem esoteric, suitable only for researchers with ovate-shaped heads. But the answer, if it can be fully formed, promises profound and widespread applications.

 

Building a better tomato

Earlier this year, Esther van der Knaap, a professor of horticulture and crop science, and colleagues at Ohio State University announced they had identified a gene that helps determine whether a tomato is round or elongated.

Switched on, the so-called SUN gene (named after a variety of tomato called Sun-1642) produces Roma-type tomatoes, oval-shaped with a pointy end. When the gene is switched off or absent, tomatoes tend to be round.

SUN is only the second gene linked to fruit shape. The first, called OVATE, was discovered a few years earlier by Steven Tanksley, a Cornell University plant breeder and former adviser to van der Knaap. OVATE helps make some tomatoes egg-shaped.

“Tomatoes are the model in this emerging field of fruit morphology studies,” van der Knaap said.

“We are trying to understand what kind of genes caused the enormous increase in fruit size and variation in fruit shape as tomatoes were domesticated,” she added. “Once we know all the genes that were selected during that process, we will be able to piece together how domestication shaped the tomato fruit – and gain a better understanding of what controls the shape of other very diverse crops, such as peppers, cucumbers and gourds.”

If researchers more fully understand the genetic blueprints shaping each fruit or vegetable, they can more effectively modify them, said Charles H. Janson, the associate dean of biological sciences at the University of Montana.

“We’re not talking about growing grapefruits as big as basketballs,” Janson said. “That would require fundamental changes to the whole plant, like stronger pedicels where the fruit is attached or maybe even where the fruit actually grows on the plant. These things may be possible in theory, but they’re not likely to happen.

“Whatever changes do occur would be subtler, intended to improve the fruit’s appeal, nutritiousness, uses or availability.”

One possibility would be tomatoes without seeds, said van der Knaap. “Or extremely elongated tomatoes shaped like cucumbers. These fruits would be very advantageous when preparing sliced tomatoes for hamburgers, as less ends would have to be thrown away.”

 

Seed need

For obvious reasons, it’s difficult – perhaps impossible – to know exactly what the original forms of some fruits and vegetables were, particularly if the fruits and vegetables have a long history of domestication. Still, there do seem to be a few basic rules that shape all fruits and vegetables.

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Shape is also determined by how a plant grows in general. Mature banana plants, for example, tend to be tall and upright, producing a similarly structured fruit bunch that grows and hangs from a central pseudostem.

And like flowers, which frequently take shapes, colors and sizes most attractive to a particular pollinator, fruit forms may be determined in part by who or what eats them. A 1983 study by Janson, for example, found that indigenous birds and mammals in a Peruvian tropical forest tended to prefer fruit species that best matched their size, visual abilities and jaw morphology.

Added Mary Ann Lila, faculty director of the Fruit and Vegetable Science Institute at North Carolina State University: “We know that those fruits (bananas, pears) most commonly consumed by animals with gripping appendages are shaped accordingly to be convenient.”

Of course, convenience tends to mean many different things to humans. In nature, a round fruit is the optimum shape for packing the most seeds into a limited space, wrote Tanksley in an expansive paper on the subject of tomatoes in 2004.

Smallness is esteemed because it tends to produce a better seed yield per gram of fruit tissue. No plant wants to pour extra energy and resources into producing fruit bigger or more fleshy than necessary.

“In the wild, tomato fruit are adapted for seed dispersal,” wrote Tanksley. “Ripening fruit, with their soft flesh and ready supply of sugars and other nutrients, attract small rodents and birds. In this regard, small fruit may provide an advantage over large fruit for dispersal because they can be readily removed, carried and dispersed.”

Humans, of course, generally think bigger is better. They also prefer fruits to be abundant and well-shaped for ease of harvest. Think about the choice between harvesting a bushel of apples and a bushel of blueberries, said Tanksley.

To that end, farmers and plant breeders have historically manipulated fruit shapes and sizes, often with dramatic effect. Example: Squarish tomatoes that are easier to machine-harvest and pack.

This is not the same as genetic modification or engineering, a controversial practice in which genes from two or more organisms are combined to create a new entity with modified or novel traits. In traditional selective breeding, researchers tinker with genes only in that species, emphasizing some or diminishing others.

Not all of the resulting shape-shifting is intentional. In genetics, there’s a concept called pleiotropy, which posits that a single gene can influence multiple traits. Tanksley suggests there’s substantial evidence that at least some domesticated fruits selected for greater size were also altered in shape because both traits were controlled by the same selected gene or set of genes.

There’s no doubt humans have assumed a fundamental and permanent role in determining the shape of fruits and vegetables to come. The upside, of course, are fruits and vegetables better suited to our particular tastes and needs.

A downside, said Lila at North Carolina State, is that some foods may be less nutritious or health-promoting because those traits were subsumed in favor of traits promoting characteristics like longer shelf life or shipping ease.

“This is why some current breeding efforts, including ours, look to introduce back some of the wild characteristics without obliterating market appeal,” Lila said.

Without continued human intervention, it’s debatable whether some, perhaps most, fruits and vegetables would persist – at least as we know them now.

“If man stopped selecting, intervening, harvesting,” said van der Knaap, “most of our domesticated plants would simply disappear because they are not fit to survive under natural conditions or make sufficient seed.”

For example, banana cultivars like the ubiquitous yellow Cavendish would likely vanish. Wild, undomesticated banana plants produce large and plentiful seeds in their fruit. Commercial Cavendish plants are sterile. They do not produce viable seeds. New plants are propagated by removing and transplanting part of a living plant’s underground stem.

Stop doing that, however, and bananas – at least the kind most Americans eat – would be in very bad shape.


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