Science in Christian Perspective

 

 

BIOLOGY
Irving W. Knobloch, Ph.D.


The Role of Polyploidy in Evolution

From: JASA 6 (December 1954): 15-16.


 In previous issues we have discussed the roles played by point mutation and chromosome rearrangement in species formation. In this contribution it is proposed to briefly outline some of our knowledge regarding polyploids. First of all, we might distinguish between aneuploidy and polyploidy. The first term is applied to a condition in which there are not complete additional haploid sets of chromosomes. For example a somatic cell of Drosophila should have eight chromosomes. If it has seven or nine or some such odd number, we are dealing with a case of aneuploidy. Polyploidy may be said to exist if an organism has three or more complete haploid sets of chomosomes. If the haploid number of a hypothetical individual were seven, then the normal diploid would show fourteen in each cell. One containing three sets or twenty-one would be a polyploid, in this case a triploid. If one had twenty-eight it would be a tetraploid or if it had thirty-five it would be a pentaploid and so on. These are degrees of polyploidy.

Turning back now to a brief look at aneuploidy, we find that the fruit fly and the Jimson weed have been intensively studied. In the former organism a gain or loss of a chromosome has, in general, a noticeable effect on the phenotype of the fly. For data on the second example we look to the work of Dr. Albert F. Blakeslee. In regard to the investigations on the Jimson weed, Winchester (Genetics, Houghton, Mifflin Co. 1951) says that "In the normal diploid the genes are in a balance which produces the normal phenotype, but with an extra chromosome present that balance is upset. According to this concept, a different phenotype would be expected for every different chromosome present in triplicate. This was found to be true. Blakeslee has, in fact, found twelve differphenotypes which deviate from the normal-". Because of the unbalanced chromosome numbers, however, aneuploidy is not considered one of the major methods of species formation.

Dr. G. L. Stebbins, who has been quoted before, believes that polyploidy is one of the principal methods employed in species formation, particularly in the higher plants. Some statistics on the prevalence of polyploidy, drawn from various sources, may be illuminating. The phenomenon is rarer among animals than among plants being found in the pulmonate mollusks, rotifers, one crustacean, one moth, a weevil, in Paramecium, in the worm Ascaris, in the fruit fly and in the Salmonidae. There may be others, but in any case the list will not be long.

In the plant kingdom we have Cladophora, Chara and Lomentaria.from the algae, Bacterium tumefaciens from the bacteria, and a number of mosses. Further study will, no doubt, reveal any others in these lower groups. In the vascular plants we find examples in the potato, coffee, banana, alfalfa, peanut, sweet potato, tobacco, cotton, wheat, oats, sugar cane, plums, apples, pears, loganberries, strawberries, ornamental cherries, Dahlias, lilies, tulips, daffodils, hyacinths, sugar beets and others. It should be remembered that some species, including some of the above, have both diploid and P01YPloid races. In one, Bromus inermis, there is no diploid race but only polyploids. For example there are tetraploids, hexaploids, octoploidsand decaploids in this one species. The chomosome numbers are 28, 42, 56, and 70 respectively. Taken as a whole, it is estimated that thirty to thirty-five per cent of the angiosperms are polyploids with the highest percentage in perennial herbs and the lowest in woody plants. An interesting sidelight here is that certain plant families have no polyploid members. These are the Pinaceae, Fagaceae, Asclepiadaceae, Caprifoliaceae and Rubiaceae.

Polyploids are divided into two classes-the auto and the allopolyploids. The former might be illustrat:W. ed by an autotetraploid. AAAA in which there are f our similar genes. An allopolyploid such as a allotetraploid could have two sets of genes, one f rom each parent such as AAA'A'. If the number of chromosomes in the cells of a branch of a tomato plant become doubled, as may happen in callus formation, the branch could be the forerunner of an autopolyploid race. If the*chromosomes in a hybrid are doubled, then the resultant plant would be called an allopolyploid.

Because of the double gene effect, it is to be expected that tetraploids would be different in appearance. Some are much larger than the diploids. There is generally a noticeable increase in the size of the guard cells and pollen grains and experienced workers can frequently tell a diploid from a polyploid by looking at the guard cells. It has been found possible to produce polyploids by using the chemical colchicine and some commercial products have resulted such as large snapdragons, tomatoes, buckwheat, maize, wheat, sorghum, petunia, gaillardia and soybeans.

One of the best known polyploids is the primrose, Primula Ke-wensis. This species arose as a hybrid of P. verticillata and P. floribunda, both having n=9 for the chromosome number. The hybrid originally had a 2n number of 18 and was sterile but later it was found that the number had been doubled (2n=36) and the plant was now fertile.

We will conclude by saying that autopolyploidy "creates" more types than it does species. The types, with their larger number of chromosomes, are, in many cases, able to live in ecological niches not suited to the diploids. Allopolyploidy, which is hybridization coupled with chromosome doubling, does, however, seem to have resulted in the production of plants which are so different from the diploids as to merit the designation of "species". The doubling of the chomosomes is what makes many otherwise sterile hybrids, fertile. The next article will discuss the role of hybridization in species formation and possibly the part played by allopolypolidy will become clearer a that time.

East Lansing, Michigan October 29, 1954