Updated: September 24, 2021 6:05:02 pm
The domestication of plants and animals 10,000 ago is considered one of the key watersheds in human history. Grasses like wheat, barley, rice and millets were one of the first ones to be domesticated, while fruits, due to their lengthy juvenile phases, were domesticated much later. Some fruits such as apples, dates, and pomegranates were domesticated no earlier than 4000-5000 years ago.
An international study has now shed light on the evolutionary history of the domesticated watermelon, by tapping into the genomes of its wild varieties.
Delighted to share our new paper ‘A chromosome-level genome of a Kordofan melon illuminates the origin of domesticated watermelons’ just out in @PNASNews !
— Guillaume Chomicki (@Chomicki_G) May 24, 2021
Archaeological record of fruits is sparse, as people moved to seedless varieties (although that is not the case for watermelons) and adopted grafting and vegetative propagation as a cultivation method. Therefore, genomics has now become a key method to understand evolutionary relationships of cultivated fruits.
The study concluded that the Kordofan melon from Sudan, Citrullus lanatus cordophanus, is the closest relative and most likely wild progenitor of the modern watermelon, Citrullus lanatus vulgaris.
It is thought that the Kordofan melon once spread as far as the Sahara during the Holocene African Humid Period 14.8-5.5 thousand years ago), when Africa was much wetter than it is today.
However, modern populations of the Kordofan melon or descendants of the progenitor exist today only in the Eastern Sahel region. Local farmers in Darfur, Sudan still grow the Kordofan melon for they need a variety that is uniquely suited to the very short rainy season in the region.
While it is unclear which people domesticated the first watermelons, or what their migration patterns were, the Sudanese Kordofan melon was definitely a key resource of watermelon domestication.
Interestingly, other studies have identified the Kordofan melon in ancient Egyptian iconography. Two depictions considered to be that of the Sudanese melon have been identified at the tombs at Meir (2350-2200 BCE) and at Saqqara (2360-2350 BCE). A third depiction also confirmed to be that of the Kordofan melon, comes from that of a papyrus dated to 1069-945 BCE. ‘These archaeological records are consistent with the Kordofan melon being a direct progenitor of the cultivated watermelon,’ the paper states.
By comparing the genome sequences of various wild and cultivated varieties, the study found a significant admixture between Kordofan melon, the egusi melon and other domesticated varieties. This was owing to the proximity of cultivation centres to the extent of the wild populations. All in all, ‘these results support that the Sudanese Kordofan melon could be the direct progenitor of the domesticated melon.’
Genomic approaches, however, have their limitations, as the authors note. Wild varieties, in many cases, might have gone extinct. Or, as in the case of the watermelon, there might have been multiple hybridisation/admixture events.
Did they escape domestication?
Equally likely is the possibility of domesticated varieties becoming feral i.e. ‘escaping’ domestication and getting established in the wild. For instance, in this study, the authors examine whether the relationship between the Kordofan melon and the domesticated watermelon might be the other way around. That is it could be that the Kordofan melon is a feral. However, they concluded that the Kordofan is the progenitor for it holds more genetic diversity than the domesticated.
Making the plant more palatable, either by making it seedless or less bitter, is one of the key outcomes of plant domestication, especially in the case of fruits. In the case of cucurbits, like melons and cucumbers, bitterness arises from a class of chemical compounds called cucurbitacins, which arises from a gene called the Bt gene.
The authors of the study found that a change in the Bt gene, that makes the fruits less bitter is already present in the Kordofan melon, as well as in cucumbers in honey melons. Therefore, the study notes that the loss of bitterness might have already been present in the wild ones which were chosen for cultivation by ancient farmers.
What is behind the red colour of today’s watermelons?
Modern-day watermelons have a modification in a gene called the lcyb gene, that results in the accumulation of the chemical lycopene, which leads to the unmistakable red flesh colour of the watermelon we know. Most watermelons, including the Kordofan, lack this mutation and therefore have a white to a greenish pulp.
Further, the juxtaposition of the Kordofan melon genome with that of the typical modern cultivar shows a marked increase in the frequencies of a variant form of a gene responsible for fruit sweetness. This indicates that ‘fruit sweetness has increased gradually over the course of domestication.’
– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)
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