The Genetic Foundation
Watermelons, like many other organisms, carry their genetic information in structures called chromosomes. Most watermelons found in nature are “diploid,” meaning their cells contain two complete sets of chromosomes. Specifically, a normal diploid watermelon plant has 22 chromosomes, organized into 11 pairs. These two sets allow the plant to undergo normal cell division and produce viable seeds, ensuring the continuation of its species.
The concept of “polyploidy” is central to creating seedless watermelons. Polyploidy is a condition where an organism has more than two sets of chromosomes. Seedless watermelons are specifically “triploid,” meaning they possess three sets of chromosomes, totaling 33. This odd number of chromosome sets is the key to their seedless nature.
During sexual reproduction, cells typically halve their chromosome number to create reproductive cells. However, with an uneven number of chromosome sets, as in triploid watermelons, this process (meiosis) cannot occur properly. Chromosomes cannot pair up evenly, leading to the formation of non-functional reproductive cells and preventing the development of mature, hard seeds within the fruit.
Creating the Parent Plants
The journey to a seedless watermelon begins by transforming a normal diploid watermelon into a “tetraploid” plant with four sets of chromosomes. This change is induced by treating young diploid seedlings with colchicine.
Colchicine, an alkaloid from the autumn crocus, interferes with cell division when applied to a seedling’s growing tip. It prevents chromosome separation during anaphase, causing the cell to double its chromosome number without dividing. This results in cells with four sets of chromosomes. Growers carefully apply colchicine to ensure only the desired parts of the plant undergo this change.
This chemical induction creates a tetraploid watermelon plant with 44 chromosomes. While this tetraploid plant itself produces seeds, these seeds are different from those of a typical watermelon. These tetraploid plants are then used as a parent line in the cross-pollination process that ultimately yields seedless watermelons.
The Cross-Pollination Process
The production of seedless watermelons culminates in a specific cross-pollination between two distinct parent plants. Seeds for seedless watermelons are created by crossing the tetraploid watermelon plant with a normal diploid watermelon plant. The tetraploid plant typically serves as the female parent, providing ovules with two sets of chromosomes.
Pollen from the diploid male parent, carrying one set of chromosomes, then fertilizes the ovules from the tetraploid female parent. This union results in an embryo with three sets of chromosomes, forming a “triploid” seed. These triploid seeds are the ones planted by farmers to grow seedless watermelons.
When these triploid plants mature and produce fruit, the uneven number of chromosome sets prevents them from forming viable, hard seeds. The fruit develops with either very small, soft, edible white seed coats, or no discernible seeds.
Triploid plants still require pollination from a normal seeded watermelon plant to stimulate fruit development. Growers often plant a small percentage of seeded watermelon plants alongside triploid varieties to provide the necessary pollen.
Addressing Common Questions
A frequent question regarding seedless watermelons concerns their genetic origin, specifically whether they are genetically modified organisms (GMOs). Seedless watermelons are not considered GMOs. They are developed through traditional plant breeding methods involving cross-pollination, with an initial chemical treatment to alter chromosome numbers.
The process relies on inducing polyploidy, a natural biological phenomenon. This is distinct from genetic engineering, which directly alters an organism’s DNA. The method used for seedless watermelons has been employed for decades, similar to how mules are bred from horses and donkeys, resulting in a sterile hybrid.