Sickly Sweet: Give and Take in the History of the Watermelon

The recently completed watermelon genome sheds light on how the summer fruit darling came to be.                                                                                                                

by Jessica McDonald

Watermelon, or Citrullus lanatus, is only half true to its common name. The ‘water’ part is right (over 90% of the fruit is water), but it isn’t in the melon genus. (Photo credit: Tim Sackton/Flickr.)

To my mind, few things are as refreshing as a cool slice of watermelon on a hot summer day. Crispy and sweet, watermelon is one of my favorite foods of the season. So I was thrilled a few weeks back to find that the fruit is also a great accompaniment to (and proud stage for) toothpick-speared basil and gin Jell-O shots. But watermelon has not always been so tasty.

Whether gussied up for the 21-and-over crowd or chopped into the classic wedge, the watermelon of today is the product of several thousand years of artificial selection. With the sequencing of the watermelon genome last fall, scientists now have a wealth of information about how the fruit grows and accumulates sugar and specific nutrients. Importantly, they also sequenced significant portions of three separate subspecies, and a total of 20 different varieties, allowing for comparisons that may help breeders make an even better watermelon rapidly.

What was most interesting to me was a key trade-off the watermelon plant made in the course of selective breeding. As people chose the juiciest and sweetest watermelons to reproduce, the fruit indeed got sweeter—but it also lost a lot of genes dedicated to preventing disease. This is a dangerous compromise for this plant to make, as it makes them susceptible to diseases such as gummy stem blight and Fusarium wilt.

Like watermelon, cucumber is a member of the Cucurbitaceae family, and has relatively few disease resistance genes. (Photo credit: Karen and Brad Emerson/Flickr)

In the case of watermelon, however, the relatively paltry immune system is not just a result of domestication. All sequenced cucurbit plants, which include melon and cucumber in addition to watermelon, have far fewer disease resistance genes than most other crops. For example, apple and wheat genomes contain about 1000, whereas the cucurbits max out at 81 for melon.

It’s unclear if this trend will hold true for other cucurbits, but it does suggest these plants may rely on other defense mechanisms. One possibility is a family of iron-containing enzymes, called lipoxygenases, which are expanded in the watermelon and cucumber genomes and may help ward off hungry pests and invading pathogens.

Farmers of course will be interested in making the plants as hearty as possible, but unconventional survival tactics have thus far worked for the scrappy watermelon. It certainly makes me appreciate that cocktail-laden fruit cube even more.