The Intersection of Plants and People

Unraveling Strawberry Genetics

Professor Aaron Liston in Sichuan, China, holding a native wild strawberry. Photo courtesy of Aaron Liston.

Professor Aaron Liston in Sichuan, China, holding a native wild strawberry. Photo courtesy of Aaron Liston.

The strawberry, although a common sight in markets throughout the world, is an unusual fruit. For one thing, it’s not even a true berry.

The red part of the plant that we eat is a fleshy shoot tip, which originally was a platform for the strawberry’s flower. What we recognize as tiny, dry seeds — the achenes (pronounced uh-KEENS) — are the actual fruits of the plant. So, instead of carrying its seed on the inside of the fleshy fruit like its Rosaceae relative the peach, the strawberry’s seeds are turned inside out, freckling its surface with what is the true fruit.

Strawberries are oddly complex for another reason: their genetics. While wild strawberry cousins can be found all over the world, they have varying sets of chromosomes. Both parents of the cultivated, grocery store strawberry have eight sets of chromosomes. Others may have two, four, six or ten sets. More rarely, sets of five, seven and nine have been reported.

“Humans are so boring,” says Dr. Aaron Liston, Professor of Botany at Oregon State University. “We all have the same number of chromosomes. It turns out that having different chromosome numbers is the most effective way to prevent cross-fertilization. You have instant sterility. Plants are just masters of this. Their chromosome numbers are changing quite often and it leads to immediate reproductive isolation.”

Although “reproductive isolation” and “instant sterility” may sound to many humans like horrific deprivations, for plants, being polyploid (having more than one set of chromosomes) appears to be quite useful. Liston and his research team have traveled all over the Northern Hemisphere to study the genetics of wild strawberries, hoping to discover why more chromosomes might be advantageous.

“To have an extra set of chromosomes, it takes a lot of mechanisms to make that work. It’s like doubling the size of your family,” Liston explains. “So the mystery is, well, why bother?”

Most crop plants are polyploid, and most weeds are as well. Liston suggests that for gardeners, the plants that are easier to grow in various conditions are likely polyploid.

“It’s definitely known that plants that have an extra set of chromosomes are often more flexible in their gene expression,” he says. “Basically, they have an extra set for every gene — instead of having just two copies, they suddenly will have four. One of the hypotheses we’re testing is: Does this truly make them more flexible, more able to deal with environmental stress?”

Having extra sets of genes might mean that plants are not as likely to be thrown by, for example, a heat wave in an area that’s usually cool and wet. They can express different genes for various environments. Diploid plants  tend to be more picky about their circumstances, which may mean their survival is more at stake under fluctuating conditions. It’s interesting to note that humans, like most animals, are diploid.

Wild strawberries have spread so effortlessly, Liston wonders why they are never considered a weed, and feels that trying to pinpoint Fragaria’s geographical origin is not as valuable information as trying to decode its prosperity.

One example of this ability to flourish far and wide is the alpine strawberry, or Fragaria vesca, which is native to Europe. Fragaria vesca can be found in Hawaii, Madagascar, the Andes, New Zealand and Australia. Subspecies of vesca, including F. virginiana, are native to North America. Clearly, strawberries, which are found throughout the Northern Hemisphere, are an evolutionary success story, and make an interesting subject for botanists trying to uncover the secrets of plant genetics, especially of chromosomal doubling.

Researchers like Liston wonder: if polyploidy is such a good thing, why haven’t animals evolved to have more sets of chromosomes also?

“Plants are better able to tolerate the initial stress of having the extra set of chromosomes,” Liston says. “They have coping mechanisms we are just beginning to understand, that animals don’t have. If plants can get past that first step, then there are potential benefits to having the extra set of chromosomes, of better survival.”

To give an example of how stressful extra chromosomes are to humans, consider the condition called triploidy. Triploidy — having three sets of chromosomes instead of two — is fatal. Fetuses with this rare condition rarely make it to birth, and when they do, babies with triploidy usually perish shortly afterward.

Something that goes against the idea of polyploidism’s apparent benefits is what plant breeders call “hybrid vigor.” Hybrid vigor occurs when two plants with the same chromosome number are crossed — they tend to be hardier than their parents. Furthermore, trying to cross plants with two different chromosome numbers seldom results in a cultivar that is fertile or will even survive. Like pitting a nine-player baseball team against a team of two, the chances of a decent match occurring is extremely slim.

To make the wild strawberry genetic puzzle more confounding, Liston has found that species in China showed signs of hybrid vigor even when the parents were very closely related and with no genetic diversity between them.

The parents of the cultivated strawberry — the North American Fragaria virginiana and the South American Fragaria chiloensis — each have eight sets of chromosomes, which resulted in a plant with hybrid vigor (Fragaria X ananassa). But to get to eight sets of chromosomes, there had to have been chromosomal doubling that occurred within the strawberry family well before man intervened. In evolutionary terms, the two parent plants only split about 200,000 years before, a mere couple of twigs apart on the family tree. Many researchers believe that virginiana and chiloensis are not just closely related, but should really be considered the same species.

While modern man likes to give himself credit for the modern strawberry’s existence, virginiana and chiloensis did not have to be bred in France for the ancestor of grocery strawberries to be born. The two species meet naturally in the American Northwest, and the most common wild strawberry in western Oregon is the same cross.

“The plants are doing a lot of mixing and matching on their own,” Liston states. “I think that’s been a real insight of the last ten, twenty years just how much of it is going on all around us. People have taken advantage of those natural processes in the plants we’ve selected for cultivation and domestication as well as what we choose for growing in our gardens for ornament…In my mind, it’s not a very big difference. It’s between selecting things that are occurring naturally versus doing it on our own.”

As far as taste, Liston agrees that the genetics of flavor were largely overlooked in the ’50s and ’60s when California strawberry breeders were selecting fruit for durability and size. Liston recently heard a firsthand account of this from Vivian D. Lee, who, as an undergraduate student, researched strawberry history and breeding for a book by renowned geneticist George M. Darrow, who published his weighty manuscript, The Strawberry in 1966.

At UC-Davis in California, Lee watched breeders test strawberries for desirable traits.

“She said that she saw them taking the strawberries and throwing them against a concrete wall,” Liston recounts. “If they bounced, that was good, because they had to ship them cross-country on trucks.”

Strawberry breeders also come up against a brick wall because of chromosome numbers. The musk strawberry, whose flavor and aroma is said to be head-turning, has six sets of chromosomes — not enough to make it a good mate for Fragaria X ananassa. And Fragaria vesca, known and adored by Europeans as fraises des bois, has the standard two sets. Liston says that very precise genetic engineering could take care of these obstacles, but because of public fears about Frankenfood, that’s not likely to happen.

“All reputable scientists that I know of agree that there’s no difference between what’s being done traditionally through cross-breeding and hybridization and what’s being done with genetic engineering,” he says.

A greater use of genetic engineering, he suggests, would be to make strawberries pest-resistant (removing the need for chemical pesticides) or to have greater nutritional value, to help feed the population explosion projected to be nine billion by mid-century. However, Liston believes that there are legitimate concerns about the possible effects of modifying genetics in agriculture.

“There are ecological and environmental consequences to everything people do,” Liston says. “Every time you clear a field, you’re doing natural selection.”

For more information, read Dr. Aaron Liston’s research paper on wild strawberries.

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This entry was posted on September 9, 2014 by in Feature, Uncategorized and tagged , , , , .
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