We usually consume seedless fruits. Here in France, it is rare to get a supermarket lemon with seeds, or clementines/oranges with pips. Even many grapes do not have seeds. Seedless fruits are produced in huge volumes and now comprise the majority of fruit sold in the developed world, simply because people like the convenience of eating seedless fruits.
Seedless oranges are freaks
All fruits come from angiosperms, which are defined as flowering plants which spread or disperse via ovules (the seeds) developed within an encased ovary (the fruit).
However, due to genetic accidents, some angiosperms can end up losing its ability to produce seeds. In normal circumstances, such a mistake would end up with a dead plant that would have produced no offspring. Fortunately and oddly, this was how seedless oranges came about.
In the beginning of the 1900s, a clump of freak orange trees in Brazil were found to be producing seedless oranges and some of these trees were transported to the US where by using grafting techniques, these original trees were cloned/propagated via cuttings into the vast seedless navel orange orchards around the country.
It is much the same story with seedless grapes, except that their story happened even earlier. Grapes had traditionally been dried and stored for centuries as raisins but a problem was the dried seeds were very hard, making the original raisins difficult to eat. As such, techniques were developed to remove grape seeds before drying, but they all involved puncturing the skin to remove the seeds which then allowed the sweet juice to leak out. Drying such punctured fruits ended up with clumps of very sticky raisins which were messy to handle.
The problem was solved in the 1870s when a vineyard owner called William Thompson obtained some seedless grape cuttings from a company called Elwanger & Barry who may have obtained their stock from Constantinople in Turkey (though the history trail is not very clear). The original grape species was probably a sub-species called Lady deCoverly but Thompson renamed the grape as Thompson Seedless and to this day, it is the primary grape used for producing raisins and sultanas. Being seedless, the dried raisins were both easier to handle and much more palatable to eat.
Oddly, seedless grapes for the dinner table did not become commonplace until over a century later, in the 1980s, mainly because producers of seeded grapes held such a dominant share of the fresh grape market and did not want any new competition.
Seedless grapes, of course, cannot breed by themselves and are therefore cloned/propagated via planting cuttings in a sterile soil medium which usually contains the hormone auxin, found in the roots of plants.
A huge problem with such cloning practices is the loss of biodiversity; any disease which affects a plant is likely to spread and potentially destroy all similarly cloned plants.
Years ago in London, a friend introduced me to a neat trick where she basically stuck an opened bottle of vodka into a hole cut into a watermelon and left in the refrigerator overnight to soak in the vodka. From what I can recall, the resulting watermelon was fantastic. I repeated this trick last year and it was even better as, by chance, I had obtained a seedless watermelon.
I mention this because the development of seedless watermelons is an interesting story. Nowadays, the commercial way of propagating them is by grafting cuttings on various root stocks but there is another fascinating method which is still used, and it explains how we can actually buy seeds to grow seedless watermelons.
But first, a bit of background stuff.
Virgin fruits, gibberellins
Parthenocarpy is the production of fruit without seeds, and is derived from the Greek for “parthenos” which means virgin and “karpos” which means fruit. Curiously, parthenocarpy can sometimes occur naturally and classic examples are bananas and pineapples although the modern versions are those which have been selectively bred by humans. These modern plants can only be propagated via cuttings.
The story of gibberellins is mainly about a plant hormone called gibberellic acid-3 (GA3), and we know about it from a fungal disease called bakanae, which means “foolish seedling”. This fungus has been known for centuries by Japanese farmers to cause rice plants to grow much taller but without producing any rice seeds.
But it turns out that what is bad for Japanese rice farmers is great news for other farmers, for it was discovered that the bakanae fungus (officially gibberella fujikuroi) produces GA3, and GA3 is a very useful, potent plant hormone which is now widely used to promote the yields of commercial crops. For example, using GA3 with added fertiliser grows stronger, higher-yielding plants. GA3 can also be used to stimulate growth in smaller plants, such as courgettes while reducing seed production at the same time, and also help develop better rinds on citrus fruits.
And now, back to seedless watermelons.
Three distinct watermelons
Watermelons originated in Africa as small, tough, bitter fruits but centuries of selective breeding have resulted in the sweet melons we know today. The ingenious story behind the original seedless watermelons involve alterations to the plant genome itself via a process called mutation breeding, whereby plants were deliberately mutated by applying various chemicals.
In 1939, the use of the compound colchicine finally started scientists on the road to the seedless melon. It was not an easy, obvious route though.
Most normal cells are diploids, which mean each cell contains two copies of each chromosome. There are exceptions, such as haploids in mammals, but that is another story. In melons, colchicine interferes with structural proteins in cells causing the chromosomes to fail to separate properly during cell division, resulting in events called ploidy changes or variations in the number of chromosome sets in the new cells.
The original experiments in 1939 resulted in tetraploid watermelon plants; i.e. they had four copies of each chromosome in each cell. When these tetraploids were then bred with ordinary diploid melons, the resulting melon plants were triploids (cells with three sets of chromosomes) – and triploids cannot produce seeds because the reproductive cells require an even number of matching chromosome sets.
The story does not end there. Triploid melons still have to be triggered to bear fruit and this is achieved by pollinating female flowers from triploids with pollen from the male flowers of diploid melons.
So to produce a seedless watermelon, one first needs to create a tetraploid using colchicine, then mate the resulting tetraploid with a diploid (normal melon), and then the resulting triploid will have to be fertilised by a diploid before we achieve a seedless fruit. Therefore, every crop of seedless melons actually involves the participation of melons with three different genomes.
To increase the success rate, scientists have created diploid plants which do not produce female flowers thereby reducing the chances of two diploid melons producing seeded fruits. And the huge seedless watermelons now often seen in shops are probably due to the application of GA3 on the plant during growth.
One can buy seeds from triploid plants but the instructions will always mention a need to have the plant fertilised by another normal melon plant.