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Genetics – Seedless Watermelons

A clever geneticist somewhere thought – “but what if we never had to deal with the seeds of life in our watermelon?” And so seedless watermelons were born.

I recall this story from the schoolyard many years ago about grandpa Joe, and his grandson, George. They were walking along the footpath and young George, oh the terrible 7-year-old he is, steps in a rather titanic-sized portion of processed dog food. He could feel his heart sinking at the aroma of the smelly dog detritus embedded in his shoe, thus beginning a tirade of complaints at why their lovely walk had been ruined by him stepping in dog poo. Well Joe, the wise and reasoned man he is, stops and imparts a piece of wisdom on George:

Never let the seeds of life stop you from enjoying the watermelon”

On that note, a clever geneticist somewhere thought – “but what if we never had to deal with the seeds of life in our watermelon?” And so seedless watermelons were born – George and Joe continued their walk and had a lovely day.

The concept of a seedless fruit is incredibly paradoxical. The entire reason for fruit existing is to spread and propagate genes in various and delightful methods. As a result, plants are not very adept at growing seedless fruit. It’s equivalent to tying the shoelace after taking your shoe off, building a table to throw out or buying seedless fruit to grow a fruit tree. But seedless watermelons are fantastic for those of us less inclined to enjoy a seedy mouth.

The whole process is based in genetics. Humans, like watermelons are diploid creatures – that is we both have two sets of chromosomes. However a seedless watermelon is triploid (or has three sets). Having three sets of chromosomes renders the progeny sterile and hence no seeds. Sounds pretty straightforward right?

Sort of.

Your typical watermelon seedling carrying two sets of chromosomes is treated with a natural compound called colchicine, derived from Meadow Saffron. The compound is incredibly toxic (to plants and in high doses to humans, low doses actually treat gout in place of NSAIDs) and interrupts cell division in such a way that some new cells become tetraploid. That is they have four sets of chromosomes. Those seedlings are then grown up and the progeny selected for favourable traits – such as watermelon producing, non toxic etc.

The tetraploid plants are grown out to flower stage. The male flowers are removed and the female flowers, which produce triploid fruit, are retained. The triploid fruit with triploid seeds contained within are then harvested. The triploid seed stock, when grown out to flower stage, lack adequate pollen production for triploid-triploid pollinisation. So by planting a few diploid plants in there with the triploid plants, the triploid plant is pollinated with diploid genes.

Whilst diploid pollen will stimulate fruit development, because the numbers of genes are different, the diploid and triploid genes are not compatible, and hence the fruit fails to produce seeds. And there you have it seedless watermelons! For more detailed information on the topic and some stats about different varieties check out Purdue’s horticulture page.

Chemical manipulation is just one method of genetic modification in a traditional sense. There are many different types of chemical mutagens (things that mess around with genes) out there used commercially. Some even have a huge radioactive source raised and lowered to mutate seeds and then see what happens! Pretty neat stuff.

So when you eat those seedless watermelons you’re pretty much getting some more genes for free. What a bargain.

Guy Coleman is the Founder of AgriEducate

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