Watermelon (Citrullus lanatus) is a flowering vine-like plant, a major global crop, and one of the best parts of summer. However, the original watermelons were small, thick-skinned, and bitter. Fortunately, five thousand years of human ingenuity has transformed this species into the bright red centerpieces of summer that we know and love.
The earliest evidence of watermelon consumption comes from the famed archeological site of Uan Muhuggiag in modern Libya. While seeds at this site (carbon-dated to ~5000 years before present) may have been from wild plants there’s evidence in neighboring Egypt that people were growing watermelon ~3000 years ago. Several Egyptian pyramids contained carefully preserved watermelon seeds along with paintings depicting watermelon crops. Likely, the entombed rulers included these items on their afterlife pack list not because of their sweet taste, which was still lacking, but because they were a great way to rehydrate.
Early watermelon’s effectiveness as a natural canteen may be one reason why the fruit spread so quickly around the world. Ships packed it both for trade and use on long voyages. Watermelons had arrived in Greece by the 1st century where they were prized as both a diuretic and as a way to treat heat stroke (place the wet rind on the heads of the suffer). By the 7th century, watermelons were also being cultivated in India. By the 10th century, they’d arrived in both China and parts of southern Europe. European explorers and colonists brought the plant to North and South America as well as the Pacific islands in the 17th century.
During this long phase of global expansion, watermelon itself was transforming. Each year farmers would carefully select plants with the most desirable characteristics to create next year’s crop. Top of that list was likely taste. Luckily, a single gene in watermelon, CIVST1, strongly influences its taste by redirecting sugar from the leaves to the fruit. Although early farmers would not have known about the physiology or DNA origins of sweetness, they would have been able to trace its relatively simple pattern of inheritance.
Historical evidence indicates that the transition from bitter to sweet watermelon did not take long. Hebrew tax records from 200 AD show watermelon grouped with other sweet fruits like grapes, figs, and pomegranates. Another indicator? Paintings of increasingly pink watermelons. The genes that color watermelon’s interior neighbor the CIVST1 gene. Consequently, in the vast majority of cases, a redder watermelon equals a sweeter watermelon.
Watermelon was given a scientific name and classification by Carl Linnaeus in 1756 that placed it alongside pumpkins and other gourds in the Cucurbita genus. However, in 1836 it was re-assigned to a much smaller genus of dessert vines (Citrullus) by the German botanists Heinrich Schrader. Today, the species is divided into over a hundred varieties including such aptly named ones as Crimson Sweet, Little Darling, and Big Tasty.
Modern farmers and scientists continued to breed for better watermelon. Charles Andrus at the USDA set out to produce a disease-resistant and wilt-resistant watermelon in the 1950’s. After multiple cross-breeding experiments, he created the Charleston Gray. This new variety was resistant to the two most serious diseases of the time (anthracnose and fusarium wilt), easy to stack, and could grow in a wider range of soils and climates. To boot it was tasty and could grow to a weight of 35+ pounds! Around the same time, Japanese scientist Dr. Kihara developed seedless watermelons by crossbreeding a plant whose chromosome number had been doubled (a tetraploid) with a normal diploid watermelon. This created a triploid variety that was sterile and mostly seedless (occasionally you may still find a few white seed coats in the fruit.).
Another innovation from Japan is cubical and pyramid watermelons. These uniquely shaped fruits are created not by a particular set of genes but by the environment. More specifically by encasing the growing fruit in metal or glass boxes! At the grocery store, you may also have come across the much less desirable “Hollow Heart” watermelon that has a crack or hole in its center. These are also created by the environment. In this case, a low pollinator environment. Researchers at the University of Delaware have found that spring pollination with viable pollen is essential to proper cell division in the watermelon’s fruit tissue. To combat hollow heart most watermelon growers have become beekeepers and now maintain a certain number of hives per acre.
Most recently researchers have sequenced the genomes of watermelon and its six sister species. The resulting database may help future breeders find wild genes that could help this crop adapt to a changing climate. “The sweet watermelon has a very narrow genetic base,” says Dr. Amos Levi a geneticist at the USDA. “But there is wide genetic diversity among the wild species, which gives them great potential to contain genes that provide them with tolerance to pests and environmental stresses….We want to see if we can bring back some of these wild disease resistance genes that were lost during domestication.”
Ready to read more and dive into the DNA details? Here are three scientific articles to get you started! Happy watermelon eating/reading!
Wu, S., Wang, X., Reddy, U., et al. Genome of ‘Charleston Gray’, the principal American watermelon cultivar, and genetic characterization of 1,365 accessions in the U.S. National Plant Germplasm System watermelon collection. Plant Biotechnology Journal 17, 2246-2258 (2019.)