I clearly remember one summer night when I was a kid, sitting on the back porch of a farmhouse near Manchester, Texas, eating the hearts out of the yellow watermelons grown in the fields out back, throwing the remnants to the dogs–and spitting seeds. Lots of ’em.
Kids today may never have to spit a watermelon seed, though, thanks to the seedless watermelon, which has pretty much driven the seeded variety from the stores.
The ancestor of the modern watermelon was a native African vine, Citrullus lanatus, which grows wild in the Kalahari Desert This vine has been cultivated for thousands of years–4,000-year-old Egyptian hieroglyphics depict a watermelon harvest–and in recent centuries it has spread all over the world, including China by the 10th century (at least), Europe by the 13th, and North America by the 16th (it probably came here with African slaves).
Originally watermelons were small, spherical, early-maturing fruit with bland or bitter white flesh (rather like a modern watermelon’s rind). After a few centuries of genetic modification through domestication and plant breeding, they have become large, elongated, late-maturing fruit (or possibly vegetables, depending on your definition–they’re related to squash, pumpkins and cucumbers) with sweet flesh. There are more than 1,200 cultivated varieties, bred for a variety of characteristics, from disease-resistance to higher sugar content to various colours of flesh.
Per-capita consumption of watermelon in the U.S. has gone up by more than a third over the last 25 years, and the main reason has probably been the increased availability of seedless watermelons. Recent news about watermelon’s health benefits may increase consumption even more: it turns out that watermelon, in addition to being an excellent source of Vitamins A, B6 and C, contains large amounts of lycopene, a plant pigment with powerful antioxidant capabilities that is also found in tomatoes–in fact, eating watermelon provides as much lycopene as drinking fresh tomato juice, with the great advantage (in my opinion) of not being totally disgusting.
The first seedless watermelons were produced in 1939 by Kihara and Nishiyama, researchers at the Japanese National Institute of Genetics.
A normal watermelon is diploid, which means it has two sets of chromosomes (the thread-like structures that carry an organism’s genes). Normally, when one plant pollinates another, the embryo plants in the resulting seeds contain one set of chromosomes from one parent and one from the other, giving them a complete set of two.
But there’s a chemical called colchicine, an alkaloid extracted from the autumn crocus and some other plants, that can cause the chromosomes in dividing plant cells to double. By applying colchicine to the terminal buds of watermelon plants, Kihara and Nishiyama were able to produce offspring with were tetraploid–they had four sets of chromosomes instead of just two.
They then mated the tetraploid plants with ordinary diploid plants. Since the offspring received two sets of chromosomes from one parent and only one from the other, they were triploid–they had three sets of chromosomes. They were also sterile: they’d produce flowers, but no seeds.
Of course, since fruit exists to house seeds, the triploid plants also had no reason to produce watermelons. Fortunately, they could be tricked into it: if their flowers were pollinated by a fertile plant with an even number of chromosomes, they acted as if they’d been fertilized, even though they really hadn’t, and developed fruit (which isn’t, strictly speaking, seedless, but the pale, soft seeds it does contain can be eaten instead of having to be spit out).
In the field, seedless watermelon seeds are produced by sandwiching a row of tetraploid plants between two rows of diploid plants. Seedless watermelons, in turn, are produced by sandwiching a row of seedless watermelon plants between rows of regular watermelon plants, because the triploid plants have to be pollinated by diploid or tetraploid plants to produce fruit.
The original seedless watermelons tended toward poorer germination and low yield. But over time, seed developers have overcome many of these problems through development of better tetraploid lines and careful selection of diploid pollinators. As a result, there are now many new seedless cultivars, suitable for all kinds of conditions. They’re so popular that, here in Saskatchewan, at least, it’s hard to find a seeded watermelon in the grocery store.
When I was a kid, sitting on that porch in Texas, spitting out seeds, I would have thought nothing could be better than a seedless watermelon.
And you know what? I was right.