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Autopolyploidy and Somatic Polyploidy

Autopolyploidy describes the multiple occurrence of a set of chromosomes in a cell, a tissue or a whole organism. Autopolyploidy happens regularly in plants in the course of their tissue differentiation, though, to distinguish it from autopolyploidy, it is then called somatic polyploidy or endopolyploidy. The term autopolyploidy is generally used to describe the so-called generative multiplication of the chromosomal set (see below). Quantitative analyses of the DNA amount in nuclei of different maize tissues showed that the multiplication occurs in steps that behave like 2:4:8:16. Endosperm tissue is at first triploid, but nuclei with 6, 12 or 24 times the simple set were also found. The rather small variations of the values is on one hand an indicator of the method's reliability and shows on the other hand that no further changes besides polyploidy occurred in the chromosomal sets.

Polyploidy can be induced with colchicine, an alkaloid of the meadow saffron (Colchicum autumnale) that inhibits mitosis (O. J: EIGSTI, 1937, A. F. BLAKESLEE and A. AVERY, 1937, B. B. NEBEL, 1937). It hampers the development of the nuclear spindle. A mitosis that takes place after treatment with colchicin is called a C-mitosis. It enables an easier detection and identification of chromosomes than a normal mitosis does. During the prolonged metaphase of a C-mitosis, the chromosomes form an X-shaped structure since the chromatids are still connected at the centromere though they repel each other. After some time, the chromatids finally part, but they do not segregate. They become enclosed by a new nuclear membrane and proceed to their interphase state. The number of chromosomes has now doubled, a diploid nucleus has developed into a tetraploid one. After colchicine had been discovered, it did therefore suggest itself to cultivate new species by using polyploidy - at least to give it a try. The experiments were unsuccessful since polyploidy leads not only to an accumulation of positive traits.

Not only diploid but polyploid organisms, too, can produce germ cells. Those of tetraploid organisms are diploid. Since four chromosomes are homologous, quadrivalents are formed during meiosis. Their stability is far smaller than that of bivalents leading to an increased ratio of mistakes and thus to a reduced fertility and in extreme cases even to sterility of the pollen and egg cells. Furthermore, clear species-dependent differences exist. Some species have an undisturbed quadrivalent development while it does not take place at all in others.

Many cultuivated plants are autopolyploid as the following table shows. We will meet a number of autopolyploid wild plants later. In nearly all cases, bivalents are formed during meiosis indicating that the plants behave like diploids in spite of the large extend of polyploidy.

It makes sense to distinguish n from the basic number (x). The degree of ploidy refers always to the basic number. It cannot be determined by examinations of the meiosis of a species alone instead, comparisons with related and more original species have to be performed. The base number is their largest common factor, or in other words, it is the haploid number of the diploid species of a polyploid series. All chromosome numbers in a polyploid series are divisible by the basic number.

Triploids. During the meiosis of triploids, trivalents are formed. In the following anaphaseI, the chromosomes are distributed onto both daughter cells. Only in rare cases, one gets exactly the double amount (2n) of the simple set (1n). Generally, both of them are equipped with incomplete sets (aneuploidy, see next section). This results nearly always in an imbalance of the chromosome composition leading to lethality.

Triploidy causes therefore, with a few rare exceptions, sterility of the pollen (or a strongly reduced fertility). But the generation of triploids itself is relatively simple and is caused by the fertilization of a haploid egg with a diploid pollen or vice versa.

Haploids. Occasionally, a whole plant develops out of a meiosis product (gone), circumventing fertilization. Certain extern factors lead to an increased probability of such events. Preliminary stages of pollen occurring in unripe antheres, for example, can be induced to germinate and differentiate on suitable media. But the yield of such experiments is very low. Haploid plants are smaller than diploids. They develop flowers, but no fruits since no undisturbed meiosis can take place. Just like the mesophyll cells of many diploid plants, haploids are well suited for the production of protoplasts.

Treatment with colchicine transfers them into a diploid state. The cells keep their ability to regenerate and can develop into complete and normal plants. G. MELCHERS (Max-Planck-Institut für Biologie, Tübingen, 1960) suggested therefore to use haploids in greater extends for the research into cultivation. His arguments were:

1. The success or failure of a mutagen can easily be detected, especially when mutants being deficient in leaf pigment synthesis are regarded.

2. Haploid plants reveal all their genetic information or, in other words, their genotype is completely displayed by their phenotype. Resistance to illnesses or unfavourable extern factors can thus be directly recognized and selected.

3. Haploid plants allow the detection of mutants that are unable to pass through the embryonic phases.

How do diploids differ from tetraploids and other polyploids? It is generally known that cultured plants, that are often polyploid, are bigger and have higher yields than the respective wild species. Polyploids are characterized as follows (in accordance with G. L. STEBBINS, 1940, 1950):

1. The water content of a cell becomes larger with increasing size. This leads to a decrease of the osmotic value and consequently to a decreased resistance against frost. The large fruits of many cultivated plants (tomatoes, etc.) are an example. They taste less intense than the respective wild types. The relative lack of flavour is an indication of a stronger dilution of the contents.

2. The decreased growth rate of polyploids is caused by the reduced ratio of cell divisions. The supply of the cells with auxine, a phytohormone is interrupted, the respiratory intensity is reduced and the activity of many enzymes is diminished. The vitamin C content is increased.

3. Certain organs are abnormally large. Their proportions towards each other are changed, the leaves are often thickened. The increase in size is not correlated to the degree of ploidy but passes through an optimum. Tetraploids are often bigger than triploids which again are bigger than diploids. But plants with a higher degree of ploidy are often marked by stunted growth due to chromosomal anomalies that lead to disturbances of the correlation between the sets of chromosomes. Disharmony is characterized by incompatibility.

4. Both the period until flower formation begins and flowering itself are prolonged. This is caused by a generally slowed-down growth and decreased rates of metabolism. The delay can be disastrous for species flowering in late summer or autumn.

5. The number of chloroplasts in the guard cells correlates to the degree of ploidy (A. MOCHIZUKI and N. SUEOKO, 1955; see table below). The following average numbers of chloroplasts in the guard cells of sugar beet were given by T. BUTTERFAß (Max-Planck-Institut für Pflanzengenetik Heidelberg): in haploid cells 8, in diploids 14, in triploids 20, in tetraploids 25, in pentaploids 30, in hexaploids 36, in octoploid cells 50.