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Iconographic evidence from Egypt suggests that watermelon pulp was consumed there as a dessert as early as 4,360 years ago. The oldest known watermelon seeds, about 6,000 years old, were found during an archaeological dig from Neolithic settlements in Libya, but whether these were watermelons with sweet pulp or other forms is unknown. To shed light on this mystery, an international team of scientists generated genome sequences from the Libyan seeds and another set of 3,300-year-old watermelon seeds from Sudan, as well as from worldwide herbarium collections made between 1824 and 2019. Their results shows that the pulp of the 6,000-year-old Libyan watermelon was white and bitter, matching the inference that this plant was used for its nutritious seeds, instead of its pulp.
Citrullus seeds from Uan Muhuggiag, Libya. Scale bar – 1 mm. Image credit: A. Bieniek.
Scientists generally agree that watermelons (Citrullus lanatus) came from Africa, but exactly where and when watermelons with red, sweet flesh were first domesticated from their wild form is debatable.
The most recent data point to watermelon getting its start in the Nile Valley, which is consistent with archaeological evidence.
However, the 6,000-year-old seeds discovered at Uan Muhuggiag, a rock shelter in what is now the Sahara Desert in Libya, seemed at odds with this explanation.
“The oldest seeds of watermelons cannot be securely identified as either belonging to a sweet-pulped domesticated form, or instead to one of the bitter-pulped wild forms,” explained co-senior author Professor Susanne Renner, a researcher in the Department of Biology at Washington University, Saint Louis and the Faculty of Biology, Systematic Botany and Mycology at the University of Munich.
“The seeds of the seven species of the genus Citrullus are basically undistinguishable.”
“Now, having a chromosome-level genome, we can be sure that Neolithic Libyans were using a bitter-fleshed watermelon.”
“We suspect they used the fruits to get at the (numerous!) seeds, which even today are eaten air-dried or roasted or also boiled in soups or stews.”
Papyrus de Kamara illustrating a Citrullus fruit (red circle), interpreted as a wild watermelon; the globose striped fruit is reminiscent of the morphology of the Kordofan melon. Image credit: Renner et al., doi: 10.1073/pnas.2101486118.
In the new study, Professor Renner and her colleagues sequenced DNA from 6,000 and 3,300 year-old watermelon seeds from archeological sites in Libya and northern Sudan.
“These seeds were a riddle because they were thought to be the oldest true watermelon seeds,” said co-senior author Dr. Guillaume Chomicki, a researcher in the School of Bioscience at the University of Sheffield.
“Yet they were from Libya, which was never thought to be the cradle of watermelon domestication.”
The authors also sequenced the genomes from geographically widespread herbarium specimens collected between 1824 and 2019.
They analyzed the data together with resequenced genomes from important germplasm collections.
They discovered that the 6,000-year-old Libyan seeds came from a form of Citrullus that was genetically close to today’s seed-use, bitter-fleshed, egusi-type watermelon (Citrullus mucosospermus), now found in Ghana, Benin, and Nigeria in West Africa.
According to the team, the likely use of the Libyan seeds as a snack matches the traces of cracking from human teeth found in a computer-tomographic study of seeds from the Uan Muhuggiag site.
“An unexpected new insight is that Citrullus appears to have initially been collected or cultivated for its seeds, not its sweet flesh, consistent with seed damage patterns induced by human teeth in the oldest Libyan material,” Dr. Chomicki said.
“This study documents the use of the seeds (rather than the fruit) of a watermelon relative more than 6,000 years ago, prior to the domestication of the watermelon.”
“Watermelons — the wild species, as well as the domesticated form — have very numerous seeds that are tasty and oil-rich,” Professor Renner said.
“Different from the pulp, the seeds never contain the extremely bitter cucurbitacin chemical. Snacking on those easily available nutritious seeds may have been a good thing.”
The findings were recently published in the journal Molecular Biology and Evolution.
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Osca A. Pérez-Escobar et al. Genome sequencing of up to 6,000-yr-old Citrullus seeds reveals use of a bitter-fleshed species prior to watermelon domestication. Molecular Biology and Evolution, published online July 30, 2022; doi: 10.1093/molbev/msac168
Source: sci-news.com
