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How kiwi plants’ Shy Girls and Friendly Boys helped them evolve separate sexes

These two genes are all it takes to determine the sex of a kiwifruit.

ByKatherine J. WuNOVA NextNOVA Next
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Kiwifruit dangling from its vine. This fruit-bearing tree is a female plant that's been pollinated, presumably by a local male. Image Credit: iStock, BarbarosKARAGULMEZ

If your kiwi vine isn’t producing fruit, it might be languishing under substandard care. Or maybe it’s a boy.

Let me explain.

Kiwi plants (genus Actinidia) segregate into two biological sexes—male and female—that are determined by X and Y sex chromosomes. Female plants, with two X chromosomes, are the fruit-bearing ones, and they need to be fertilized by the pollen of genetically XY male plants before they can produce their tangy, bright-fleshed orbs.

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To us humans, who also use an XY sex determination system, that might not sound all that strange. But in the world of flowering plants, where the vast majority of species are hermaphrodites that contain both male and female sex organs in the same individual, kiwifruit is an anomaly.

Now, an international team of researchers may have pinpointed the two genes—a dynamic duo named Shy Girl and Friendly Boy—responsible for determining a kiwi vine’s sex. And their backstory, revealed in two recent studies, is quirkier than one might imagine.

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Kiwifruit by species. A = Actinidia arguta (hardy kiwifruit), C = Actinidia chinensis (golden kiwifruit), D = A. deliciosa (fuzzy kiwifruit), E = Actinidia eriantha, I = Actinidia indochinensis, P = Actinidia polygama (silver vine), S = Actinidia setosa. Image Credit: Crowhurst et al., BMC Genomics, 2008

Only about 5 to 6 percent of flowering plants split themselves into separate sexes. However, many members of this exclusive club—which includes asparagus, papaya, persimmon, marijuana, and kiwi—are only distantly related, suggesting that plants ditched hermaphroditism several times throughout their evolutionary history.

This sexual switcheroo can be a powerful one, because it essentially forces single plants to mingle. With sex organs divvied up, any act of reproduction must involve two individuals, upping the chances of offspring being more genetically diverse. In the case of the kiwi, this probably helped plants rapidly adapt to a wide range of environments in its native China, says Sarah Pilkington, a plant geneticist and kiwifruit expert at the New Zealand Institute for Plant and Food Research Limited and author on the more recent study.

But rejiggering your entire sex determination system is no small feat. To figure out how the gonadal split evolved in kiwis, a team of researchers led by Takashi Akagi, a plant geneticist at Kyoto University and Okayama University in Japan and author on both studies, sequenced the genomes of dozens of male and female kiwi plants.

When compared side by side, the vines’ genomes revealed that male plants contained a gene that appeared to halt the development of would-be-female reproductive parts. Because this spaying agent led to the cloaking of female gonads, Akagi and his colleagues cheekily dubbed the gene Shy Girl, publishing their results last year in the journal Plant Cell.

But even then, Akagi knew the story was likely incomplete. According to a decades-old theory, Shy Girl should have a partner in crime—a second, male-gonad-promoting gene to complement its female-gonad-suppressing effects.

In this evolutionary model, male and female gonads develop like a pair of light switches, operating side by side. Turn both on, and you’ll get an entire species of hermaphrodites. Transitioning to a two-sex system requires two genetic changes: one that turns off the female switch in males, and another that shuts down the male switch in females. Shy Girl was only enough to accomplish the first.

To find the missing link, Akagi, Pilkington, and their colleagues teamed up to wrangle a new set of kiwi vines. This time, the researchers went through the plants’ genomes with a finer-toothed comb, hunting for genes that were expressed at higher levels in males during a critical point in kiwi sexual development. Sure enough, the search bore fruit, revealing a hunk of DNA that actively encouraged male reproductive parts to develop. The team named the gene Friendly Boy—the etymological yang to Shy Girl’s yin.

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When looking into the sex lives of kiwifruit, things can sometimes get hairy. Image Credit: nata_vkusidey

But as the researchers report today in the journal Nature Plants, Friendly Boy didn’t work exactly like Shy Girl did. While Shy Girl put the brakes on female development by shutting off a switch, Friendly Boy exerted its effects because it was an “on” switch. The critical second genetic change, then, was actually the loss of Friendly Boy from the female kiwi genome: Rather than fussing over which way the male switch was turned, female plants dispensed with the entire contraption.

There’s a flip side to this, too. Because only two roadblocks exist between hermaphroditic kiwis and unisex kiwis, messing with this delicately reconfigured system is kind of, well, low-hanging fruit.

Over the years, kiwi farmers have discovered that at least one species—Actinidia deliciosa, an especially hairy cultivar commonly sold in stores—can be bred to yield hermaphrodites, in the form of genetically XY vines that still produce shrunken female fruits. When the researchers scoured these hermaphrodites’ DNA, they discovered that the plants had discarded Shy Girl from their genomes, allowing female gonads to emerge. (Presumably, the converse of this—a plant with a broken copy of Friendly Boy—would leave a vine in a sexual no plant’s land, yielding a “neutered” male, Akagi says.)

The final nail in the coffin, Pilkington says, was one last experiment in which the team plunked a copy of Friendly Boy into the genome of a fast-flowering female kiwi plant. Within months, the seedlings had sprouted hermaphroditic flowers—and, after a round of self-pollination, the plants even began to yield small but fertile fruit.

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Left: A self-pollinating kiwifruit, created when researchers added a copy of Friendly Boy to a female plant. The fruits shown are fertile. Right: A female plant that was only able to bear fertile fruit when pollinated by another vine. Image Credit: Akagi et al., Nature Plants, 2019

“This team has done beautiful science,” says Céline Caseys, a plant biologist at the University of California, Davis who was not involved in either study. “They knew the [first study] was just half the story...and seeing the full story come together is really nice.”

The study’s findings add to the growing list of unusual strategies plants use to split up the sexes. While asparagus seems to adhere to a somewhat similar system, persimmons don’t. Clearly, there’s more than one path to unisexuality, and so far, all appear to be ripe for the picking.

These results also hint at the possibility of genetically manipulating Shy Girl and Friendly Boy in kiwi cultivars to generate self-pollinating, hermaphroditic vines, Akagi says. In theory, this could give breeders a simple way to mass-produce the fruit.

Several plants, including common cultivars of grapes and papaya, are already purposefully bred to yield hermaphrodites. Though none of these modifications have involved direct gene editing so far, this study shows it’s possible, says Ray Ming, a plant biologist at the University of Illinois at Urbana-Champaign who was not involved with the study.

All this highlights just how loose sexual delineations can be—even from a kiwifruit’s point of view. Something to chew on the next time you slice open one of these bad boys (er, girls).

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