From Seed to Seed:
Plant Science for K-8 Educators

Digging Deeper

Genetic "Accidents"

Polyploidy. Up until now we have been talking about plants with two sets of chromosomes-that is, those whose somatic cells are diploid. However, some plants have more than two sets of chromosomes, a condition called polyploidy. Polyploid plants often have bigger flowers and fruit, more intensely colored flowers, bigger or thicker leaves, or bigger tubers. Polyploidy is quite a common condition; in fact, it is thought that almost half of the angiosperms (flowering plants) are polyploids.

How does this characteristic come about?

Triploids. Sometimes "errors" occur during cell division. If the chromosomes fail to segregate properly to the daughter cells, the result can be a diploid gamete (with two sets of chromosomes rather than the usual one). If this gamete unites with a normal, haploid gamete, then the resulting zygote will be triploid-that is, it will have three sets of chromosomes. Although this condition can sometimes result in plants with larger flowers or other desirable characteristics, triploid plants are usually sterile. Because the chromosomes occur in threes, rather than in pairs, the chromosomes cannot segregate evenly to the daughter cells. This often results in infertile pollen and reduced sterility in the female plant.

Despite their sterility, triploids are still horticulturally important. Not only are triploid flowers often larger and more colorful than their diploid counterparts, but since they don't produce mature seeds the flowers often last longer as well, and require little or no "deadheading." One interesting triploid is the familiar banana. Diploid bananas do exist, but they have hard seeds. However, in triploid bananas, the seeds abort, leaving only the tiny remnants you see. So don't bother looking for banana seeds-at least not if you want to eat the fruit! The plant must be multiplied from cuttings.

Tetraploids. Plants with four sets of chromosomes are called tetraploids; this is the most common type of polyploidy. Tetraploid plants are often larger than their diploid counterparts, but they are usually less uniform in appearance. One common example is the tetraploid snapdragon, which has significantly larger blooms than the diploid version. Tetraploidy arises from the union of two diploid gametes (or the union of a triploid and a haploid gamete). Because the chromosomes occur in sets of four, they can be evenly divided to daughter cells during cell division; therefore, tetraploid plants are usually fertile. Plant breeders can create tetraploids by treating diploid seeds or young plant tissues with a substance called colchicine, a poisonous alkaloid.

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