To produce a tetraploid plant, the alkaloid colchicine is applied to the terminal bud of a branch. All the cells in the developing branch will be tetraploid (4n) with four sets of chromosomes. This includes cells of the stem, leaves, flowers and fruit. Gametes (egg and sperm) produced by a flower on this tetraploid branch will be diploid (2n) with two sets of chromosomes. A flower on the normal diploid (2n) branch will produce haploid (n) gametes containing one set of chromosomes.
Stages of mitosis in the formation of tetraploid cells. The original mother cell is diploid (2n). During anaphase the chromatids separate and move to opposite ends of the cell. Colchicine causes the dissolution (depolymerization) of protein microtubules which make up the mitotic spindle in dividing cells. This leaves the cell with twice as many single chromosomes (four sets rather than two). When this cell divides, each of the two daughter cells will have fours sets of chromosomes, a total of eight chomosomes per cell. [Note: Spindle poisons such as colchicine are used to prevent tumor cells from dividing in certain chemotherapy treatments.]
The botanical term parthenocarpy refers to the development of the ovary of a flower into a fruit without fertilization. [The biological term parthenogenesis refers to the development of an egg without fertilization.] Fruits that develop parthenocarpically are typically seedless. Some seedless fruits come from sterile triploid plants, with three sets of chromosomes rather than two. The triploid seeds are obtained by crossing a fertile tetraploid (4n) plant with a diploid (2n) plant. When you buy seedless watermelon seeds, you get two kinds of seeds, one for the fertile diploid plant and one for the sterile triploid. The triploid seeds are larger, and both types of seeds are planted in the same vicinity. Male flowers of the diploid plant provide the pollen which pollinates (but does not fertilize) the sterile triploid plant. The act of pollination induces fruit development without fertilization, thus the triploid watermelon fruits develop parthenocarpically and are seedless. Most bananas purchased at your local supermarket came from sterile triploid hybrids. The fruits developed parthenocarpically and are seedless.
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