If there was a High School Musical for fruit, watermelons would be the sporty types – the jocks. You know the typecasting: a bit thick, but very popular; heavyset but pretty harmless; rarely left out of parties – and they don’t spend too much time exercising their melons trying to figure out the meaning of life.
But if they ever did contemplate the science of their existence, they might discover they are quite the trick of nature – and, when it comes to their seedless variety, some very clever nurture.
The classic weighty watermelon we all know and slurp in summer are from the Citrullus species, predominantly Citrullus lanatus. In fact, botanically, a watermelon is a very big berry called a pepo, so defined because of its thick rind and numerous seeds in its watery flesh, rather than a big pit, or stone.
This definition extends to nearly all of the melon cousins in the Cucurbitaceae family, including rockmelons, cucumbers, pumpkins, zucchini and loofah, which develop in a similar way.
The purpose of any berry is to be a seed incubator, and those seeds begin life as unfertilised ovule. What might our botanical point-guard think when it discovers it possesses the reproductive capabilities of both sexes? Quite enlightened in this case, actually! After a yellow melon flower is pollinated, sperm from other watermelon pollen on the same plant (or a near neighbour) is carried by bees into the ovary. Here, the sperm fertilises the ovule, and the outer wall of the ovary becomes hard and turns into the green rind. Then the swelling begins.
The fleshy red interior is made from the same ovule wall – but it doesn’t harden. This is called the mesocarp and it is rich in sugars. The mesocarp has two functions: to hold the seeds in place and to provide a tasty snack. No, we are not kidding. Animals eat the fruit, and therefore the seeds, and distribute them as they range and poop.
Because the mesocarp contains seeds, it is essentially the placenta of the plant, and it’s one of the biggest plant placentas in the world – the largest watermelon on record was a whopping 159kg.
Where do I come from?
Watermelons are native to Africa – we know they were cultivated in the Nile Valley during pharaonic times, as wall art depicting watermelons has been found in three Egyptian tombs, and watermelon leaves were found arranged on a 3,500-year-old mummy. Some of those leaves and their genome were later sequenced by botanist Susanne Renner, of the University of Munich, Germany. She found that cucurbitacin genes – responsible for making bitter proteins found in some wild melon cousins – had mutated, which probably helped the melon become sweet.
She also found the reason why watermelons are red. In other melons, a particular enzyme breaks down the red pigment lycopene (the same substance that makes tomatoes red), but the ancient Egyptian watermelons had a mutation in the enzyme, so the pigment remained. All of which suggests that Nile Valley farmers had selectively bred watermelons to be sweet and red at least 3,500 years ago.
In the millennia that followed they were taken to northern countries as people traded and migrated, and started showing up in classical paintings.
In very high-schoolesque fashion, they even get caught up in rumours. A common internet misconception is that the prickly globular fruit in famous artworks such as Albert Eckhout’s Still Life with Pineapple, Watermelons, and Other Fruits (Brazilian fruits), painted about 1641, is a precursor of today’s watermelons (but look again – pineapple, anyone?). Another 17th century painting by the artist Abraham Brueghel called Still Life of Fruit and Flowers shows a red, fleshy melon similar to today’s common melons.
And that’s an important point to remember: just like jocks, watermelons come in many varieties, probably making it a bit impolite for us to generalise too much about them. There are about 50 common types and another 200-300 rare varieties. This melon team appears in varied shapes and sizes, with a mix of spheres stretching to cylindrical shapes with rounded, conical ends; inside they are variously flesh-coloured or yellow, orange, red and white, depending on the amount of the lycopene-breaking enzyme they contain.
Germplasm collections – a type of genetic library consisting of living tissue – of the 17th century watermelons depicted in those paintings still exist, and melons grown from these germplasms resemble Eckhout’s melon. But they’re not as sweet.
Today we tend to recognise only the watermelons grown by large-scale agriculture. For example, most watermelons sold in supermarkets these days are seedless, but that’s a rare and heavily controlled trait in the watermelon family, through a technique created by Japanese scientists in 1939. Also, “seedless” is something of a misnomer; these melons do have seeds, but they’re simply not mature. In fact, and pardon the pedantry, the concept of “seedless fruit” is somewhat oxymoronic: fruit is botanically defined as a mature ovule that contains… well, seeds.
But just as the high-school jocks get more than their fair share of attention, so do seedless watermelons, which are created by breeding watermelons with different sets of chromosomes – qualifying them as a hybrid.
How to make a hybrid? In your classic “diploid” watermelon, with seeds aplenty, each chromosome exists in a pair. The two chromosomes are pulled apart into two separate gametes, each containing one single chromosome – known as a haploid – through the process called meiosis. Later, the haploid ovule is fertilised by a haploid sperm (thanks again, bees), and together they make another chromosome pair.
However, seedless watermelons require a hybrid “coach” – the autumn crocus plant (Colchicum autumnale) – to make them perform differently. The crocus produces a chemical called colchicine that alters normal meiosis. Colchicine, when added by human hand, disrupts chromosome segregation, so they don’t get pulled apart; the result is one gamete that has no chromosomes and another that has two. The chromosome-less gametes can’t do anything, so the diploid gametes find another diploid pair and form a tetraploid – four chromosome – embryo.
Goal! The watermelon has doubled its chromosomes and its entire genome. But the game isn’t over yet. Now the tetraploid watermelon can be backcrossed with those diploid melons. Pollen from a diploid watermelon, usually planted in a row right next to it, is carried by a bee onto the flower of a tetraploid watermelon. This means that a haploid pollen is fertilising a diploid ovule, resulting in a triploid – three chromosome – embryo. When the resulting fruit is ready to produce seeds, it will attempt to undergo meiosis. But meiotic division relies on balanced sets of chromosomes pairing. And as any jock knows, three is a crowd. In genetic terms, a group of three is not balanced, causing the meiosis to fail and the seeds to remain sterile and thus small.
This also means that these hybrid varieties can’t reproduce, so diploid and tetraploid melons need to be bred this way every season. Like any good drugs-in-sport accusation, this has caused a bit of a controversy, because sterile watermelons have occasionally been misunderstood (like jocks!) as a genetically modified no-no. But in reality, they’re just a result of selective breeding and hybridisation. A similar chromosomal phenomenon results in the product of a horse and donkey – the humble, sterile mule.
A question of taste
Despite the preponderance of watermelon-flavoured lollies for sale, the aroma is notoriously hard to produce artificially, because it’s so difficult to pin down a single molecule that contributes to the smell. This might be because enzymes are released from cells when the watermelon is cut, and they oxidise and break down other fatty acids in the flesh to make the specific aromas – which means the aroma molecules aren’t just sitting around inside the melon waiting to be smelled (they have to be oxidised for us to even recognise them, so they’re hard to isolate or make).
The current consensus is that the molecules contributing to watermelon aromas are C6 and C9 aldehydes. Interestingly, one of them, (Z)-3-hexenal, is also the chemical behind the aroma of freshly cut grass – familiar turf to sporty types.
With all the nurturing, crossbreeding and attention across so many centuries, that makes the watermelon one of the star pupils of Big Berry High School.
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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