Shortly after the Mendelian laws had been rediscovered, H. de VRIES observed newly developing forms in the Oenothera populations. They varied from the wild types by an altered stature, differently shaped leaves and a changed pace of development. The changes in the phenotype were kept in the following generations and were thus obviously hereditary. They seemed to be based on changes of the hereditary factors, although no reason for their appearance could be found. Such changes in the hereditary material are called mutations and organisms that display them are called mutants.
The term wild type describes
in a very general manner those phenotypes of a species that are most
common in nature. If a mutant differs from the wild type in a certain
character, then the mutated allele is marked by a suitable
abbreviation, the wild type by a +
Mutations are contrasted by
modifications. The term describes
changes in the phenotype that are caused by environmental influences.
An example: E. S. ROBERTSON and I. C. ANDERSON characterized a mutant
in corn in 1961 that, in a homozygously recessive condition, stood
out at low temperature (<20°C) (v/v) due to a pale colouring
of the leaves (virescent).
At increased temperatures (37°C), the same plants resembled
the wild type. The F1-hybrids (+/v) were dark green like
the wild type and in the F2, a 3:1 ratio was observed in
plants that were grown at 20°C. If, however, the culturing was
done at 37°C, then it resulted in uniformly dark-green plants.
This example illustrates that the expression of a phenotype can be
influenced by environmental factors like temperature.
Modifications are very common, especially in plants. Among the better known textbook examples is the flower colour of a certain variety of Primula sinensis that is red at room temperature and white at increased temperatures. Mutants that can be distinguished only at increased temperatures are also called temperature-sensitive.
Alpine plants and plants in general that grown under intense
radiation, are usually stout. Plants from lowlands and such plants
that got just little amounts of sunlight during growth, show an
unmistakably elongated growth. It is known that the development of
plants is, much more than that of animals, influenced by external
factors. The term photomorphogenesis
verifies, how large the influence of light is. A plant that is
cultivated in the dark can develop neither flowers nor chloroplasts,
although the necessary hereditary preconditions are present. To
activate them, the respective external signal is necessary. The
development of the plant (as that of other organisms, too) is thus
controlled by an interplay of hereditary and environmental
factors.
Today, it is common knowledge that the environment shapes the phenotype, but exerts no directed influence on the genotype.
The effects of environmental influences on the modification of phenotypes of two different genotypes
There exist, however, a number of chemicals and physical factors,
the mutagens that cause
mutations. Among the most important physical factors are the ionizing
radiations. J. H. MULLER verified in 1927 that Roentgen radiation
(X-rays) increases the frequency of mutations in Drosophila.
At nearly the same time, plant mutations were induced in Nicotiana
tabacum (T. H. GOODSPEED), Datura
stramonium (C. S. GAGER and A. F. BLAKESLEE, 1927), Zea
mays and Hordeum vulgare (L. J. STADLER), too.
These mutagens have nearly without exception a negative outcome on the organism, their effect is time- and dose-dependent and often lethal. Among the few survivors, the mutation rate is drastically enhanced as can be seen when compared to the respective control.
Target Hit Kinetics
Diagram to the left: Inactivation kinetics. The three functions correspond to a one hit, two hits and three hits kinetics (from bottom to top). The number of necessary hits can be deduced, if the linear part of the curve is extrapolated (broken line).The point of intersection with the ordinate indicates the number of hits, for example 300:100 = 3. Diagram to the right: Mutation triggering by ionising radiation in Drosophila. The different symbols within the diagram refer to different sources of radiation. (According to K. G. ZIMMER and TIMOFEEFF-RESSOVSKY, 1942).
Only in very rare cases leads the influence of a mutagen to the
generation of a mutant with, compared to the wild type, optimized
characters or an increased life-span. The attempt to breed varieties
with higher yields or such with increased resistances became hence,
despite the large financial effort, a failure. The situation is
different with microorganisms. Due to their high ratio of
propagation, it is rather easy to isolate even the rare advantageous
forms.
Nearly all varieties used in industrial microbiology for the production of various compounds (from citric acid to proteins and antibiotics) are produced by mutagenes. These mutated organisms find ideal living conditions under the controlled conditions of a laboratory, but have nearly no chance to assert themselves against the respective wild type under natural conditions.
In evolutionary biology, it is spoken of fitness, a term that means an individual's relative success in propagation. The fitness of mutants is usually very low.
Mutagenes can display their effect in different ways. Accordingly, different types of mutations exist. In a strictly formal sense it is spoken of:
point mutations (= gene
mutations), where single genes are effected.
chromosome mutations
that are changes of the chromosomal structure and
genome mutations, where
chromosomal sets are changed en bloc, for example
doubled.
Although the production of plant mutants proved to be
unprofitable, it did become the most effective experimental
instrument of modern pure research. The analysis of the most
different mutants gave information about the structure and the
function of hereditary factors as well as about their influence on
the development of an organism. E. BAUR summarized modifications,
mutations and combinations (hybrid formations) in his textbook on
genetics as follows:
Modifications are non-hereditary differences between individuals of a given population (or species) caused by various external factors like light, temperature, nutrition etc. that exert different influences on the respective individuals.
Mutations are hereditary differences between parents and their offspring covering also the vegetatively generated offspring that are not based on the generation of hybrids, but have other causes.
Recombinations are hereditary differences between the individuals of one population and also between all offspring of the same parents that are caused by the segregation and new combination of the hereditary factors.
These three categories cannot be distinguished just by their outer appearance. The cause for the variation of an individual can usually only be found by careful genetic experiments. If such experiments are taken as the backbone of distinction, then the reliable separation of the three categories is nearly always possible.
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