Jennifer Schultz Nelson
Extension Educator, Horticulture
I credit Dr. Lambert's plant breeding class at the U of I for finally answering a question I had for years-- if seedless watermelons have no seed, where do you get the seed to plant it?
Japanese plant breeders developed the first seedless watermelons over fifty years ago. Understanding the process requires basic knowledge of genetics.
Plant and animal cells have chromosomes, threadlike structures containing genes, the instructions cells need to function. A complete chromosome set is called the "n" number of a species. The number of sets in a cell is the "ploidy" level.
In humans, our ploidy level is diploid, or 2n, meaning two chromosome sets–one set from our mother, another from our father. Seeded watermelons are also 2n, with one set of chromosomes from the "mother"–the plant whose flower produces the melon, another set from the "father"–the plant that pollinated the melon-producing flower.
To make seedless watermelon, seeds from seeded watermelon are treated with the chemical colchicine which doubles the ploidy level of the plants to 4n, which is called tetraploid.
The tetraploid plants are pollinated with pollen from seeded diploid watermelon. When crossed, each parent donates half its number of chromosome sets-- the tetraploid plant gives two sets, and the diploid plant gives one. The seed from this cross is 3n, or triploid. This triploid seed produces seedless watermelons.
The triploid seed develops into a sterile plant that cannot produce seed, much like a mule in the animal world cannot reproduce. The triploid chromosomes cannot pair up and divide as with diploid or tetraploid plants. The pollinated flower behaves normally, setting fruit and beginning to develop seed. True seeds never completely develop, but there may be small white rudimentary seeds that are completely edible and usually go unnoticed.
Seedless watermelon usually commands a premium price at market, but considering the additional labor involved, it isn't a huge price to pay for a seedless slice of summer sweetness.