Polyploidy

HAPPY GROUNDHOG DAY!

I am writing this post as background information before I answer a question from Mike at the HTGSupply.com Lansing MI store. A customer asked about colchicine and plant growth. Colchicine is usually used to induce (make) polyploidy plants.  This is done by doubling the number of chromosomes. I have read that Oryzalin also will double the existing chromosome content.

Polyploidy refers to a numerical change in a whole set of chromosomes, meaning you have 3,4 or even up to 12 copies of all the chromosomes. In high school Biology you should have learned two is the ‘normal’ number of chromosomes for most living things. A similar sounding term, aneuploidy is when only a few or parts of chromosomes are present in more than two copies. In humans Down syndrome is caused when individuals have an extra (three total) copy of chromosome 21.

Polyploid organisms have more than two paired sets of all chromosomes. As I said, most living things are diploid, meaning they have two sets of chromosomes. In humans (Since we can easily understand our own reproduction) the pair of chromosomes resulted because one set came from your dad (sperm), and the other from your mom (egg). A polyploid organism would get 2 from either or both parents increasing the number of all chromosomes. Polyploidy occurs in some ‘lower’ animals. It is very rare in mammals but is not uncommon in plants. I have read estimates suggesting 30–80% of living plant species are polyploid. The larger estimate comes from studies showing evidence of ancient polyploidy (paleopolyploidy). For example, corn has 12 chromosomes. But genetic testing suggests that corn does not have two copies of six (12) but really has four copies of 3 (12) of chromosomes.

Polyploidy sometimes occurs in localized tissues of plants due to abnormal cell division. However, it is much more important, biologically speaking when it occurs throughout the whole plant
Polyploid plants arise spontaneously in nature. Polyploidy can also be induced in plants by adding colchicine, oryzalin or other chemicals to seeds. The effects are not well documented and each chemical will most likely have different outcomes.
Most polyploids display heterosis compared to their parental species, and also often display unique growth habits. Heterosis means hybrid vigor. Plant breeders have know for centuries that when you cross two pure breed plants with different traits, their offspring show increased growth and overall vigor. You can see the benefits in the number of crop that are polyploids: Strawberries, sugar cane, blueberries, apple, banana, tobacco, peanut, watermelon, cherry, canola, wheat, cotton, potato and many other plants like orchids are polyploids.
There are two types of polyploids, autopolyploids and allopolyploids. Autopolyploids occur when the chromosomes are from the same species, this is common in potato for example. Allopolyploids occur when the chromosomes are from different species. Polyploids do not always have predictable features, they usually grow larger and faster, but bamboo for example has mostly increased vegetative growth, other plants have only increased seed set etc. It is in the breeding of polyploids that we select the traits that are most beneficial.

Click here to read more about what creating polyploids can do.

Good Growing,
Dr. E.R. Myers