Polyploidy—or How Do We Get Seedless Fruit?


By Christine Bradish, Ashland Inc.

It’s quite possible that you have seen a commercial for “23 and Me”, advertising an ancestral DNA test. The “23” represents the DNA in a human being, contained in 23 chromosomes. Now this number is misleading, because humans have two sets of chromosomes, one from their mother, and one from their father. However, “46 and Me” doesn’t have the same ring to it. With few exceptions, most animals have two sets of chromosomes, and we refer to them as diploid (di = two, ploidy = sets of chromosomes in a cell). Physical size isn’t a factor: in comparison to humans (46), mice have 40, elephants have 56, and dogs have 78 chromosomes.

It should come as no surprise that plants are weird and different, even when it comes to their chromosomes. Plants can have multiple sets of chromosomes, which is called polyploidy. Many of your favorite fruits and vegetables are polyploids, and this makes them even more delicious. Polyploidy can occur naturally, where wild species “add together” their DNA. Two good examples of this are wheat and strawberries. Wheat is a hexaploid, which means it has six sets of chromosomes, and strawberries are octoploids—you guessed it—eight sets!

Plant breeders intentionally develop polyploids with desirable traits—for example, seedless watermelons. Normally, watermelons are diploid, and have seeds. By using natural chemicals, such as colchicine, plant breeders can double the number of chromosomes in a plant. Then, the tetraploid (four sets of chromosomes) watermelons are crossed with a standard diploid watermelon to make triploid watermelon seeds.

These triploid watermelon seeds are sterile, because you need pairs of chromosomes to form seeds—this is why animals and plants always have multiples of two chromosomes. You need to plant at least one regular seeded watermelon in your garden as a pollen source for the sterile seedless watermelon seeds. Seedless watermelon seeds are more expensive because it costs a lot of money to maintain tetraploid lines and produce triploid seeds every year.

Bananas are a triploid fruit—having three sets of chromosomes. The tiny seeds in their interior are sterile. Source: Morguefile

In addition to seedless watermelons, bananas are probably the most common triploid food you eat. The next time you eat a banana, look for the small black specks in the middle of the fruit—these are the sterile seeds. Farmers do not have to buy new banana seed every year because the banana fruit grows on a plant that sends up a new shoot each season.

Polyploidy is one more tool that scientists can use to learn about the genetics of crop plants. Plant breeders use traditional plant breeding methods to change polyploidy to make improved crops faster and more efficiently. Polyploidy can be challenging because there are so many more chromosomes to work with, but it is another ‘tool’ in our plant breeding toolbox that we use to grow the most healthy, delicious plants!
