Q. I love seedless watermelon—but I wonder how future crops are germinated if they are seedless?
A. The development of seedless watermelons is an instructive story of how plant genetics can be used to produce a product that has high consumer value but that requires specialized seed production techniques.
The process starts back two generations with watermelon parents that have different numbers of chromosomes in their cell nuclei. Most watermelons have one pair of each chromosome. This is termed the 2X or diploid condition. Using special techniques to double the number of chromosomes, parent lines are developed that have two pairs of each chromosome, which is termed tetraploid (4X). These 4X plants are able to grow normally and also produce seeded watermelons.
To produce the seed for planting seedless watermelons, the seed company takes pollen from a normal diploid (2X) flower and crosses it to a flower on the tetraploid (4X) plant. This is generally done by hand. When the 2X flower makes pollen, it undergoes a type of cell division known as meiosis, in which the number of chromosomes is reduced by half, resulting in haploid (1X) pollen cells. The same process goes on in the development of the egg cells inside the female parts of the flower. Normally, when the pollen cell fertilizes the egg cell, each of them contributes one of each chromosome pair, and the diploid (2X) condition is restored. The fertilized egg then continues to develop into the embryo inside the seed.
In the case of seedless watermelon, the female plant is tetraploid (4X), so when its egg cells form following cell division, they have twice the normal number of chromosomes, or 2X. When these egg cells are fertilized by 1X pollen from the 2X male plant, the resulting embryos and seeds have three copies of each chromosome, or are triploid (3X). This does not prevent the formation of viable seeds and fruit, so these triploid seeds can be harvested once the fruits are mature. These are the seeds that are sold to be planted to produce the seedless watermelons.
However, when the triploid watermelon plants produce flowers, the cell division process that would produce the egg cells runs into a problem due to the 3X number of chromosomes. Since there is an odd number of chromosomes, it is not possible for them to be evenly divided among the daughter cells. This results in a failure to form functional egg cells, and therefore no embryos can develop. Sometimes one finds small white seed coats in seedless watermelons, but these are empty and do not contain an embryo.
As an added complication, the fruit normally will not continue to develop unless there is pollination, but the pollen formation process also fails in these triploid plants, so they cannot produce pollen themselves. In order to achieve pollination, a certain fraction of the field is planted with normal (2X) watermelon plants that supply viable pollen. Bees visit these flowers and carry the pollen to the 3X flowers, which stimulates fruit development even though they are not successful in fertilizing the defective eggs. Generally, the fruit shapes or rind colors of the 2X male and 3X female (seedless) fruits are different so that they can be easily distinguished in the field and workers will only harvest the seedless fruits for market.
Similarly, when home gardeners buy a packet of seedless watermelon seeds, it will contain seeds of both the female and the male (often colored so that it can be recognized), and they should be sure to plant some of each type to ensure the production of seedless fruits on the triploid female plants.
Kent Bradford is vice chair of the Department of Plant Sciences at UC Davis and director of the Seed Biotechnology Center.
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