#1501 How do seedless fruit reproduce?

How do seedless fruit reproduce? Seedless fruit don’t produce viable seeds, so they can’t reproduce, but they don’t need to because they are produced by breeding two different plants that can produce fruit with seeds.

These days, we can buy fruit with seeds, and we can usually buy seedless varieties of fruit as well. Watermelon are a good example because you can buy watermelon with seeds, and you can buy seedless watermelon. However, even the fruit we buy that do have seeds have been bred to have far fewer seeds than they would have done in the wild. This is not a recent process. Seedless bananas were produced thousands of years ago. Seedless grapes were around in Roman times. Seedless oranges about 200 years ago. And seedless watermelon about 60 years ago.

What is a fruit? A fruit is the ripened ovary of the plant, and it contains the seeds. Its whole purpose is to be eaten by an animal so that the seeds can be scattered. For this reason, fruit have generally evolved to be sweet and bright. However, they are never sweet enough and colorful enough for us, so we have spent thousands of years crossbreeding them to get the sweetest and the most colorful fruit we can. Then, thanks to refrigerated transport and supermarkets, we can eat perfect fruit from pretty much any country at pretty much any time of the year. And yet, we complain about the seeds. So, we have seedless fruit.

The best way to think about how seedless fruit are produced is to think about how mules are produced. Mules are the offspring of a male donkey and a female horse. They are born mostly completely infertile, which means they cannot reproduce. The only way to make more mules is to make a male donkey and a female horse reproduce. And this is how seedless fruit are produced. The parents are not infertile, yet the offspring is. The reason this happens is because of the number of chromosomes. A horse has 64 chromosomes, half of which (32) go to the baby. A donkey has 62 chromosomes, half of which (31) go to the baby. That means a mule ends up with 63 chromosomes. When a living organism has an uneven number of chromosomes, it makes reproduction difficult because chromosomes need to make pairs, and an odd number of means there is one chromosome left over. That means mules cannot make viable sperm or eggs. Mules cannot reproduce, but they have an enormous number of advantages that they inherit from both of their parents. They have intelligence, strength, endurance, speed, agility, resilience, they rarely get ill, they have tough skin, and they live longer than both donkeys and horses.

So, back to seedless fruit. The same odd number of chromosomes does the same thing to plants that it does to mules. When farmers want to make a seedless watermelon, they crossbreed a watermelon plant with 44 chromosomes with a plant that has 22 chromosomes. The resultant plant has 33 chromosomes, which is an odd number and means it is unable to produce seeds. This was first discovered by a plant geneticist called Dr. O. J. Eigsti. He worked out how to double the number of chromosomes in one of the plants so that he would end up with an uneven number of chromosomes in the offspring. It took him a while to perfect the process and, once he had, he had difficultly finding people to grow it and shops to sell it. It found a market after a while, and today, 90% of all watermelon sold in the USA are seedless.

That odd-number-of-chromosomes trick is one way to get seedless fruit, but in other fruits, like many grapes and citrus, the seeds simply start to form and then abort. Then there is the method used for bananas. The banana that we all eat today is called the Cavendish banana, named after William Cavendish, 6th Duke of Devonshire, who received a box of them and really liked them. They are seedless, so you cannot grow more banana plants from the seeds, but they can be reproduced if they are cloned. When a banana plant is growing, suckers are taken from their roots and replanted, growing into new banana trees. This is wonderful in that all of the bananas are pretty much exactly the same and of the same quality because they are basically clones of each other. It is less good because there is no natural diversity among them, and a single disease can decimate the whole crop. And this is what I learned today.

Sources