The Mysterious Genetics of the Four-Leaf Clove

The Mysterious Genetics of the Four-Leaf Clove

So you lost your job and your life’s a mess. You’re fat, you’re broke, you’re bad at sex. You’re looking at your dog, hoping she doesn’t die next. You need to get lucky. And what’s the best way to get lucky? No, it’s not finding a four-leafed clover on St. Patrick’s Day. That kind of luck doesn’t exist. What you need to do is go impress a bunch of drunk strangers with your knowledge of four-leaf clover genetics!

Like every other trait on every other living thing, a clover’s lucky fourth leaf sprouts from DNA. But the clover genome is surprisingly complicated, and finding the genes responsible—and under what conditions—for the four-leaf mutation is a still-unsolved biological puzzle.

There are over 300 different species of clover, but the type most associated with the rare fourth lucky leaf is the widespread white clover (so named because of the fluffy, delicious-looking white blossoms). 
“It’s like having a cat with an extra claw. We know it has a genetic basis, and a mutation that happens at a slow but regular frequency,” says Wayne Parrott, who studies crop genetics at the University of Georgia. His lab has come closest to finding the genetic roots of the four-leaf mutation.
 “We know more or less where it is on the chromosome,” he says. But the clover seems to have done everything possible to make its genome inscrutable.

This is partly because the plant is so widely distributed. Native on three continents, the white clover’s genome tells the story of a plant that geography tried, and failed, to split into multiple species. The white clover is an allotetrapoloid. Huh? Best to work backward on that one. -Ploid means chromosome, and -tetra means four. That means the white clover has double the amount of chromosomes as humans, mangoes, pill bugs, and most other organisms. This brings us to the allo- prefix, which means that each pair of the white clover’s chromosomes comes from a different species.

Crazy, I know—but clover aren’t aliens. When it was proliferating over the globe, the clover started to split into multiple species, but then they doubled back and started breeding again. And instead of recombining into diploid chromosomes, the clover kept both pairs. Maize and sorghum had some awkward allotetraploid years when they first started splitting about 20 million years ago (both now have just two chromosomes). Pretty exciting family life for a boring ole ground cover, right?

On top of that, white clover don’t have the tidiest genealogies. The plants are promiscuous outbreeders (opposite of inbreeders, sorry Team Lannister). In fact, they are pretty much incapable of breeding with themselves (as many plants do). Combined with the quadruple chromosomes, the white clover’s sex life means that it is incredibly difficult to figure out which genes came from which parent. This means inheritance studies—which figure out if a gene’s expression is due to nature over nurture—are all but impossible.

But Parrott’s group made an admirable accomplishment. They grew 178 plants and recorded—among other leaf traits like pattern and color—any plants with at least one four-leafed stem (A single clover plant sends out many runners). In the study, four-leafed stems favored warm conditions—there were double the amount of outdoor four-leaf clovers in the summer than winter. And in the winter, more lucky shamrocks grew in the greenhouse than outside.

This could mean that three leaves is an adaption for colder or wetter climates. Clovers are a member of the bean family, which Parrott says have been steadily losing leaves over the millennia.
 “If you go back, the ancestors of the bean family had a lot more leaves,” he says. “In the case of beans and clover, you are down to three.” He says that this could be accidental, though says it is noteworthy that if having more leaves were a big advantage, then you’d expect to see more of them. But, he also says that the number of leaves could be entirely accidental. “Not everything happens for a reason,” he says.

With their experimental data in hand, Parrott’s team started looking at the genome. They found at least one gene that correlated closely to the expression of three leaves. These genes were similarly somewhat suppressed in the summer months, leading to more four leaf clovers. “Weather is important for a lot of plant traits,” says Parrott, though he’s reluctant to say it all comes down to time of year. He says one of the reasons that we see more four leafed clovers when it’s warm is because the plant is making more leaves in those times of year. The probability is simply higher.

And clovers can grow many more than four leaves—the Guinness record is 56 leaves on a single stem. Speaking of Guinness, if you drink enough of it today* you’re sure to find plenty of six-leaf clovers, at the very least.

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