Genetics / D.J. Cove.

  • Cove, D. J.
Date:
1971
Catalogue details

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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    haploid organisms ^ In addition to this asexual method of reproduction, Aspergillus also reproduces sexually, producing gametes which fuse to form zygotes, which in turn undergo cell division to give rise to another form of spore, the ascospore. Ascospores germinate to give rise to an Aspergillus mycelium. The life cycle of Aspergillus is represented diagrammatically in figure 1. Although there is more to the life cycle than this, and one of the complications will be dealt with later in this book, this is a sufficient account to enable us to consider the behaviour of Aspergillus in simple breeding experiments. gamete gamete fuse to form zygote Aspergillus mycelium A ж V. -> ascospores Figure 1. Simplified life cycle of Aspergillus nidulans. Having outlined the life cycle of Aspergillus, it is now possible to consider in what way breeding experiments can be used to gain an insight into the mechanism of heredity. But the first thing that must be noted is that breeding experiments can only be useful provided we have recognisably diiferent strains. If two identical strains of Aspergillus are crossed together, the vast majority of the progeny which develop from the ascospores are identical to one another, and to the parents. Clearly this tells us nothing about the mechanism of inheritance that we did not know already. However, variant strains can be obtained. These arise spontaneously, but only at a very low frequency. It was stated earlier that conidiospores, the spores involved in the asexual form of reproduction of Aspergillus, almost always produce a mycehum identical to the one upon which they were borne. In fact among these conidiospores can be found a very small pro¬ portion which give rise to recognisably different types of mycelium, which produce conidiospores almost all of which give rise to the new