Scientists have sequenced and analyzed the genome of the Kordofan melon (Citrullus lanatus subsp. cordophanus), a Sudanese form of melon with nonbitter whitish pulp, and found that the subspecies is the closest relative of domesticated watermelons (Citrullus lanatus subsp. vulgaris) and a possible progenitor.
Domesticated watermelon is among the 10 most important crops in Central Asia, and knowing its geographic origin and potential progenitor would help targeted breeding efforts.
The geographic region of its domestication has long remained unclear with competing hypotheses favoring southern Africa, West Africa, and Northeast Africa, especially the Kordofan region, a former province of Sudan bordering North and South Darfur, and part of the western Sahel savannas.
Besides Citrullus lanatus, the genus Citrullus contains six other species, of which four (Citrullus amarus, C. ecirrhosus, C. naudinianus, and C. rehmii) are native in the Namib-Kalahari region, one (Citrullus mucosospermus) in West Africa (Benin, Ghana, and Nigeria), and one (Citrullus colocynthis) in northern Africa to West India.
All wild species have white pulp that cannot be eaten raw due to the presence of bitter terpene compounds. Only fruits of Citrullus mucosospermus are sometimes not bitter but instead bland tasting; the large, soft seeds of this species are used in West African ‘egusi’ stews.
“Based on DNA, we found that watermelons as we know them today — with sweet, often red pulp that can be eaten raw — were genetically closest to wild forms from west Africa and northeast Africa,” said Professor Susanne Renner, a researcher in the Institute of Systematic Botany and Mycology at the University of Munich and Washington University.
Professor Renner and colleagues found that a Sudanese form with non-bitter whitish pulp, the Kordofan melon, is the closest relative of domesticated watermelons.
The results are consistent with the newly-interpreted Egyptian tomb paintings that suggest the watermelon may have been consumed in the Nile Valley as a dessert about 4,200 years ago.
“It was the Egyptian tomb paintings that convinced me that the Egyptians were eating cold watermelon pulp,” Professor Renner said.
“Otherwise, why place those huge fruits on flat trays next to grapes and other sweet fruits?”
The study also corrects an old taxonomic mistake that lumped watermelons into the same category as Citrullus amarus, also known as the South African citron melon.
“Today’s watermelon comes from a very small genetic stock and is highly susceptible to diseases and insect pests, including various mildews, other fungi, viruses and nematodes,” Professor Renner said.
“So far, we found variation in three disease resistance genes between the Kordofan melon and the domesticated watermelon. Breeders might use these and other insights from the genome.”
“The watermelon is one of the most important tropical fruits, with over 200 million tons produced every year, but it is also very susceptible to disease,” said Dr. Guillaume Chomicki, a researcher in the Department of Animal and Plant Sciences at the University of Sheffield.
“There are specific watermelon diseases, such as the Watermelon mosaic virus and they are also very sensitive to fungal infections. In conventional agriculture, they are frequently treated with fungicides, and insecticides to limit virus transfer.”
“Our analysis clearly shows that the Kordofan melon has more disease resistant genes, and different versions of those too.”
“This means that the genome of the Kordofan melon has the potential to help us breed disease-resistant watermelons and allow non-GM gene editing. Achieving this would be reducing substantially pesticide use in watermelon farming.”
The findings were published in the Proceedings of the National Academy of Sciences.
Susanne S. Renner et al. 2021. A chromosome-level genome of a Kordofan melon illuminates the origin of domesticated watermelons. PNAS 118 (23): e2101486118; doi: 10.1073/pnas.2101486118
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