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What Do Seedless Watermelons and Mules Have In Common?

Since the 4th of July is right around the corner I thought this article was fitting. Use this article to impress your friends and family while you are enjoying a seedless watermelon, and watching your uncle try not to blow his fingers off with bottle rockets.

So, how are seedless watermelons grown? Where do the seeds come from? To understand, you must go back to basic genetics and how we inherit things from our parents. In most cases plants and animals have two chromosome sets – one set from the mother and another set from the father and this is called “diploid”. Seeded watermelons are like this and get one set of chromosomes from the “mother” – the plant that produces the melon and another set from the “father” – the plant that pollinated the melon producing flower and since male and female flowers are found on the same plant these sets may come from the same plant.

Now it gets a little more complicated. Plant breeders will use a plant derived chemical called colchicine to alter the growing point of seeded watermelons causing them to double and have 4 sets of chromosomes which is called tetraploid. If you use the tetraploid plant as the female or seed parent and pollinate it with pollen from a seeded diploid watermelon, the tetraploid will give half its number of chromosome sets, in this case two and the diploid will give half its number of chromosome sets, in this case one, to the resulting seed which will have three sets of chromosomes and will be triploid. This does not prevent the normal development of viable seeds and fruit so these seeds can be harvested once the fruits are mature and these seeds can be planted to produce a seedless watermelon plant.

The triploid plant that comes up from this seed is sterile because it has an uneven number of sets of chromosomes.  The plant needs sets to pair up and because you have an uneven number of sets that is just not possible. Remember that we said that the plant that grows from the triploid seed is sterile and this means that the female flower that produces fruit cannot form functional egg cells to develop into seeds. The white seed like things that you find in seedless watermelons are just empty seed coats and are not a complete seed. If pollinated the flower can produce fruit albeit without seeds but remember the plants are sterile and do not produce pollen either and so to successfully grow seedless watermelons there must be normal diploid watermelons planted in the field with the seedless plants to pollinate the female triploid flowers. This stimulates fruit development but not seed development, hence seedless watermelons.

Normally if you buy seedless watermelon seed in a pack, it will contain a certain number of a normal diploid seeded variety to serve as pollinators. These seeded varieties usually have a different color rind or shape so that you can keep them separate at harvest. Please note that the seed for the seedless watermelon plants do not germinate as well and grow as vigorously as normal seeded varieties. Therefore most seed companies recommend starting triploid watermelon seedlings as transplants.

This is much like a mule in the animal world that cannot reproduce. Horses have a different number of chromosome sets than a donkey and the cross has an uneven number of chromosome sets which prevents pairing up and so a mule is almost always sterile. Mules are produced when a male donkey is crossed with a female horse or mare. Donkeys and horses are genetically similar enough to mate and produce offspring but different enough for their offspring to be sterile. Notice the phrase above “a mule is almost always sterile”; there have been a few rare cases when a molly (female) mule has had a foal. It’s an event so rare that the Romans had a saying, “cum mula peperit,” meaning “when a mule foals” – the equivalent of “once in a very blue moon” or something that never happens.

For more information about these or other subjects, please contact your local Extension Agent.