The secret sex life of strawberries

Woman and man, hen and rooster, cow and bull—separate sexes may seem fundamental to nature, but they’re an oddity for most plants. Now, scientists have figured out how strawberries, which have the youngest known sex chromosomes of any plant or animal, made their recent transition to male and female. The unusual “jumping” genes responsible could mean sex differences can change faster in plants than anyone realized.

“For the first time, we now have a view of sex chromosome evolution over space and time,” says Alex Harkess, an evolutionary biologist at Donald Danforth Plant Science Center in St. Louis, Missouri, who was not involved in the study. “It’s not just about the establishment of sex chromosomes, it’s how the sex-determining regions continue to evolve.”

Animals have ancient sex chromosomes with a common origin. But in plants, sex chromosomes have arisen only recently (in the last few million years), and most plants are generally hermaphrodites—which contain both male and female sex organs. Only about 6% have split into different sexes, including garden asparagus, papaya, hops, and marijuana. Strawberries, as one uneducated Ohio farmer discovered in the 1840s, come in three flavors: male, female, and combo.

To find out how those flavors evolved, ecologist Tia-Lynn Ashman at the University of Pittsburgh in Pennsylvania has spent nearly 20 years showing that different locations on the strawberry genome can control sex. But figuring out where those genes were located was like finding a treasure in a hall of mirrors: Unlike humans, which have just two copies of each of our 23 chromosomes, strawberries have a whopping eight copies of seven chromosomes, for a grand total of 56.

Ashman’s first stroke of luck came when she and her team found the first evidence of male- and female-determining regions in an East Coast variety of a common North American wild strawberry (Fragaria virginiana) some 10 years ago. But when they found the same sex region in a closely related Oregon beach wild strawberry, F. chiloensis, it was in an entirely different place on an entirely different chromosome. The same was true of a third variety of strawberry. What were these regions doing in different places?

Ashman says the obvious answer was that the strawberries had developed different sexes independently. But another, less probable, explanation was that a region of DNA had arisen once and was moving around the genome. When the first strawberry genome project came to fruition in 2011, allowing for more detailed genetic studies, evolutionary biologist Aaron Liston of Oregon State University in Corvallis and evolutionary geneticist Jacob Tennessen, now at the Harvard T.H. Chan School of Public Health in Boston, teamed up with Ashman to take a closer look.

In a brute force approach, they sequenced 60 F. virginiana and F. chiloensis plants, evenly divided between males and females, to see whether any DNA was unique to the females. They asked what genetic sequence was present in all females, but absent in all males. Sure enough, all females shared a short sequence that had jumped at least twice as the plants reproduced over many generations, the researchers reported late last month in PLOS Biology .

What’s more, with each jump, the number of female-specific genes on the sequence increased. Those traveling “souvenirs” increased the difference between the sex chromosomes, the researchers speculate. In humans and other animals, such sex-specific differences eventually became extreme. In strawberries, the short jumping sequence contained two genes with potential roles in pollen and fruit development.

“This study provides the first observations suggesting that a sex determining region in a plant can ‘jump’ from an initial location to a new one,” wrote The University of Edinburgh evolutionary biologist Deborah Charlesworth—a pioneer in the evolution of sex chromosomes—in an email. “The study does a heroic job of trying to sort this out.”

The researchers caution that the functions of the two genes and details of how they “jump” still need to be confirmed. And Ashman says the findings set the stage for an even bigger question: Why do these regions bother jumping in the first place? Ashman and Liston will be following up.

But don’t look for the special male and female strawberries at your grocery stores or farmers’ markets any time soon. In a final plot twist, the hybrid descendants of the wild strawberries in the study have all had the sex differences bred out of them.