Genetics / D.J. Cove.

  • Cove, D. J.
Date:
1971
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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    4 Patterns of inheritance I: type of mycelium. The differences are therefore inherited, and it appears there must have been a change in the genetic information. Such altered strains are said to be mutant, and the alteration in the genetic information which has occurred is called a mutation. Mutation can occur at any time and so ascospores too may very rarely be mutant. The rate at which mutations occur in Aspergillus varies depending on the type of mutation involved, but a rate of 1 in 10'' is fairly typical. Thus 1 in lO'' conidiospores from a normal green- spored (wild type) Aspergillus mycelium, may give rise to a mycelium which bears yellow rather than green conidiospores. However the mutation rate can be greatly increased by various treatments. These include ultraviolet irradiation, X-rays, y-rays and certain chemicals. Such treatments are said to be mutagenic, and their nature, and the nature of the mutations they produce tell us a great deal about the process of mutation. Chapter 5 (page 64) of this book deals with the mutation process in greater detail. The effect of a mutation on the cell and organism is also very informative and by its study we now know much about how the genetic information is used. Although this too will be considered in more detail in later chapters of this book, it will be helpful for an understanding of the mechanism of inheritance to consider briefly at this point the main effects of mutation. One type of mutation, that which leads to the production of yellow spores has been mentioned already. Other mutations in Aspergillus can cause fawn, chartreuse, pale green or white conidiospores to be produced. It is also possible to obtain mutations which may have different effects on morphology. We can for example obtain strains with a slower growth rate, which in a given time, give rise to a mycelium of a diameter smaller than that of a wild-type strain. Another type of mutation may render the strain resistant to various chemicals normally toxic to the wild type. The most frequently used mutations in Aspergillus are however those called biochemical or sometimes nutritional mutations. The first extensive study of biochemical mutation was by G. W. Beadle and E. L. Tatum in the period around 1940. These workers did not use Aspergillus for their studies but another fungus Neurospora crassa. Both these fungi are able to grow on a relatively simple medium consisting of inorganic salts and a sugar. Beadle and Tatum found that it was possible, by irradiating conidiospores with u.v. or X-rays so as to increase the mutation rate, to obtain strains which were only able to grow provided a biochemical was added to the