Euploidy and Plant Speciation
Euploidy and Plant SpeciationOne goal of plant breeding has been to develop allopolyploids that have new traits that are not seen in other species. The one beneficial allopolyploid developed to date is Triticale. This amphidiploid was developed from the pollination of wheat (Triticum, 2n=42) with rye (Secale, 2n=14). The goal of this experiment was to combine the rugged phenotype of rye with the high yielding characteristics of wheat. The final chromosomal composition was 2n=56 chromosomes.
Allopolyploidy has now been demonstrated to have been a major genetic event during plant speciation. The analysis of plant genomes has provided insight into how these evolutionary events occurred and the rate at which evolution can take place. The three plants genera that will be discussed are Brassica, wheat and Spartina.
Brassica SpeciationThree Brassica species form a triad from which three other species in the same genera were derived. B. oleracea (broccoli and cauliflower) has a haploid chromosome number of n=9 and the haploid number for B. campesteris (turnip) is n=10. Another Brassica species, B. napus has a haploid number of n=19. This species appears have to been derived by the hybridization of B. oleracea with B. campesteris followed by a doubling of the chromosomes to produce the new species. (This species is currently a major source of research because of the quality of its oil.) The figure below shows how amphidiploidy has generated two other related Brassica species.
Insert Brassica figure here
Wheat SpeciationWheat has played a major role in the development of the world civilization. The domestication of wheat was a major event in world civilization because it allowed humans to change from nomadic hunter gathers to permanent residents of specific locations. The following is the current suggested development of modern bread wheat.
Triticum urartu (AA) X Aegilops speltoides (BB) | | | \_/ Triticum turgidum (AABB) X Triticum tauschii (DD) | | | \_/ Triticum aestivum (AABBDD)Archaeological evidence has shown that Triticum turgidum (AABB) was being grown in both Mesopatamia (Tigris and Euphrates River Valley) and in the Nile River Valley 10,000 years ago. Because wild T. tauschii is found only in the mountain region of southern Russia, western Iran and northern Irag it is thought that the hybridization that produced T. aestivum occurred in these region. It has been suggested that this occurred as recently as 8,000 years ago which coincides with the development of collective settlements by man. The wheats that were developed by the above hybridization scheme are each cultivated today.
Cultivated T. turgidum is called durum wheat. North Dakota is essentially the only state in the US that grows durum wheat. This wheat is processed and used for pasta. Bread, cookie and pastry wheats are cultivated varieties of T. aestivum. North Dakota is also a leading producer of these wheats, and North Dakota is often the #1 producer for all types of wheat.
Spartinia SpeciationThe last allopolyploid example is the recent development of a new saltmarsh grass species. In the early nineteenth century seed of American saltmarsh grass (Spartina alterniflora) was accidentally transported to the southern coast of England and the northern coast of France. The grass began growing in the same location that European saltmarsh grass (S. maritima) was grown. Soon a new species of saltmarsh grass appeared called Townsend's grass (S. townsendii). The growth pattern of this species was more vigorous and soon it had crowded out the other two native species. These characteristics were recognized and soon it was introduced into Holland to stabilize the dikes and subsequently into other locations for the same reason.
Chromosomal analysis suggested that Townsend's grass was an amphidiploid because its chromosomal number, 2n=122, could be derived from the American (2n=62) and European (2n=60) chromosome numbers. Apparently a hybridization occurred on the beaches followed by a chromosomal doubling to produce the current species. An important point to consider is how quickly speciation can occur from allopolyploid. Clearly, the Townsend's grass species appeared and became established within 100 years because of its vigorous growth.
It has been estimated that about 50% of all angiosperm (flowering plants) are polyploid. The following are some examples of common cultivated plants that are autopolyploids.
For some plant species a series of successive ploidy levels are seen. To describe these species it is necessary to introduce the final symbol X. X is the base number of chromosomes for a specific series of species. For rose 2n=14 and the base number of chromosomes (X) is 7. So for diploid rose 2n=2X=14. For the series though, the tetraploid rose species are 2n=4X=28 chromosomes, the pentaploid roses are 2n=5X=35 chromosomes and the hexaploid rose is 2n=6X=42 chromosomes. Fern species exhibit some of the largest chromosome numbers and these are a result of polyploidy. Adder's tongue fern (Ophiglossum) has a base number of 120 chromosomes. The diploid species is 2n=2X=240 chromosomes. One related species has 2n=10X=1200 chromosomes. This demonstrates the high end of the number of chromosomes that are found in eukaryotic species.
Copyright © 1997. Phillip McClean