Tuesday, April 6, 2010
Allo + polyploidy = Allopolyploidy
Jerry Coyne describes allopolyploid speciation as speciation that occurs due to the “hybridization of two different species that live in the same area” (Coyne 186). It occurs in plant species with regularity, producing a new species that contains the added number of chromosomes of the parents that produced it. It also happens in animal species; however, it occurs far more rarely. What do plants have that animals don’t that allow them to develop allopolyploid species? If an animal were to develop allopolyploidally, how could it develop a new species, being that no other examples of the allopolyploid animal would be able to mate with the original organism? Since plants develop allopolyploidally relatively often, what might the reasons be that plants with thousands of chromosomes don’t exist, given that one allopolyploid species could mate with another?
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Allopolyploid speciation is a special form of sympatric speciation; during a human lifetime, the species actually forms. Rather than being isolated (same species), the two species that are different have different chromosome number. As a consequence because of this difference, the improper pairing of chromosome number leads to a sterile hybrid (pollen and ovules are unable to be made). But by doubling its own chromosomes somehow (plants), the species would be fertile but a new species once again.
ReplyDeleteFor plants, they are at an evolutionary advantage to produce such speciation; unlike most animals (except fish etc), a single plant may produce millions of eggs and pollen grains. Possibly for decorative leaves to attract bees, etc, these plants often are hybrids. Humans take advantage of this gene combination to produce desirable polyploidy plants that are used for food (such as wheat that adds a healthy perspective to double haploid plants for humans) and/or decoration. For animals, such polyploidy is rare and many are still undetected.
Many polyploidy plants are weeds, which are placed at an adaptive advantage because they are able to withstand extreme/ harsh environmental changes.
For humans (or other mammals) such polyploidy is at a disadvantage because many cases (for humans) have resulted in abnormal growth and shorter life spans; such as trisomy 21 etc (Campbell).
One study on hybrid speciation focuses specifically on hybrid species with a doubling of chromosome number. Allopolyploids are found to be reproductively isolated from ploidy, and have many barriers to cross in order to become successfully hybrid. For plants, it is achieved easier because plants usually have indeterminate growth and somatic chromosome doubling leads to germ-line polyploidy; plants may be perennial allowing the chances of mutations to occur to be higher than for animals; and plants are more often hermaphrodites. Therefore, for animals, such polyploidy is unfavorable. (http://www.ucl.ac.uk/taxome/jim/pap/mallet07_hybspn_Nature.pdf
Sources: Coyne book
http://aknhp.uaa.alaska.edu/staff_home_pages/MattCarlson/SYS%20BOT%20Lectures/Lecture%2012%20-%20Polyploidy.pdf