How is watermelon seedless

From the standpoint of a plant, the whole point of fruit is produce seeds, so I wondered what kind of hanky panky was going on to produce seedless watermelons. Turns out that they are like mules, self-sterile hybrids and involve a lot of work. Watermelon plants are usually diploid, like us, meaning they have two sets of chromosomes, the packages of DNA with instructions for life.

Seedless watermelons are triploid. They have three sets of chromosomes. This odd number results in them being sterile and not producing seeds. The way they become triploid is by mating a diploid male with a tetraploid female. Tetraploids have four sets of chromosomes. The way you get tetraploids is by applying a chemical called colchicine which messes with cells as they are dividing.

You add it to diploid seedlings and then some cells become tetraploid. You have to cultivate these over several generations to get enough that produce enough viable seeds with suitable traits. Watermelon plants have male flowers and female flowers. Following these steps generally produces a more than 90 percent germination rate.

High germination rate is important since seed of seedless types is quite expensive compared to seeded varieties. The standard number of chromosomes in watermelon is This is called the diploid number di meaning two, as in dissect — cut in two. With this even number, cell division is highly regular and produces pollen and egg cells with 11 chromosomes that recombine to produce seed with the usual 22 chromosomes.

Through a chemical process, the chromosome number can be doubled from 22 to 44 tetraploid, tetra meaning four. Cell division in plants with 44 chromosomes is, again, highly regular and will produce pollen and egg cells with 22 chromosomes that recombine to produce seed having 44 chromosomes. However, if pollen from a plant with 22 chromosomes is placed on a female flower of a plant with 44 chromosomes, the resulting seed will have 33 chromosomes triploid — three sets of the base number of 11 chromosomes.

This odd number does not produce or rarely produces viable pollen and eggs in the resulting seedlings. Seedless watermelon fruit will have white seed traces, but only occasionally will it have a mature, brown, hard seed.

Since the pollen of these plants is not viable, a diploid, seeded watermelon needs to be planted along with the seedless variety. The diploid will provide good pollen for the bees to move around and pollinate the flowers of the seedless variety. When the cotyledons first emerge from the soil, the growing point is treated with colchicine to stop chromosome division and produce a tetraploid shoot with four sets of chromosomes rather than two.

Colchicine is applied to the seedling growing point in the morning and evening for 3 consecutive days, using 1 drop on small- or medium-seed size plants and 2 drops on large-seed size varieties. Protect workers who are handling colchicine with safety equipment such as gloves. The treatment produces plants that are diploid, tetraploid, or aneuploid, so it is necessary to identify and select the tetraploids in later stages. Some diploid varieties and breeding lines produce a higher percentage of tetraploids than others.

Tetraploids can be detected by the direct method of counting chromosomes of cells under the microscope, or by comparing stem, leaf, flower, and pollen size with diploid controls.

A popular method involves counting the number of chloroplasts in stomatal guard cells using a leaf peel under the microscope. Tetraploids have approximately chloroplasts in each guard cell total on both sides of the stomate , whereas diploids have only in each guard cell total. The method is useful for screening many plants for ploidy level in the seedling stage before transplanting to the main part of the greenhouse or field nursery for self-pollination.

Usually, multiple methods are used, identifying tetraploid seedlings using their phenotype in flats before transplanting, the chloroplast number in the stomatal guard cells of the true leaves in seedling flats and greenhouse pots, and by the appearance of the fruit and seeds at harvest after self-pollination in the greenhouse.

Tetraploids usually have thicker leaves, slower growth, and shorter stems than diploids. Stage 3 involves tetraploid line development.

Tetraploid plants are selected using methods such as leaf guard cell chloroplast number in the T0 generation plants from colchicine treated diploids from the greenhouse flats where they were treated with colchicine. It is then necessary to plant the T1 generation in flats to verify that the plants are tetraploids in that next generation, and transplant the selections to greenhouse pots for self-pollination.

Seeds from those selections T2 can then be increased in larger plantings such as field isolation blocks to get sufficient numbers of seeds per tetraploid line to use in triploid hybrid production.

The fertility and seed yield of tetraploid lines will increase over generations of self- or sib-pollination, probably because plants with chromosome anomalies are eliminated, resulting in a tetraploid line with balanced chromosome number and regular formation of 11 quadrivalents. Seed yield of tetraploid lines in early generations is often only seeds per fruit and sometimes as low as seeds compared to seeds for diploids. Another problem with early generation tetraploids is poor seed germination, making it difficult to establish uniform field plantings.

It may require as much as 10 years of self-pollination before sufficient seeds of tetraploid lines can be produced for commercial production of triploid hybrids. Advanced generations of tetraploid lines usually have improved fertility, seed yield, and germination rate compared to the original lines.

Some companies require more than lbs. Approximately tetraploid plants are required for production of each pound of triploid seeds. Stage 4 is the evaluation of tetraploids usually T3 generation or later as parents of triploid hybrids.

The tetraploids should be evaluated directly for rind pattern, high seed yield, and other traits such as male sterility for reduced hand labor in hybrid seed production.

The major test for tetraploids however, is as female parents in triploid hybrid seed production after making controlled crosses using diploid male parents. The resulting hybrids are tested in yield trials with two rows of triploid plots alternating with one row of diploid plots to assure adequate pollen for fruit set in the triploid hybrids.