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Observations on chromosome loss detection by multiple recessive marker expression in strain D61.M of Saccharomyces
1Division of Toxicological Studies, Food and Drug Administration, Washington, DC 20204.
Abstract:
Since chromosomes of fungi are difficult to observe directly, strains have been developed in which chromosome loss can be detected by the use of genetic markers. In the diploid D61.M strain of Saccharomyces cerevisiae, the loss of a copy of chromosome VII that carries 3 dominant wild-type alleles is measured by expression of 3 recessive mutant alleles carried on the other remaining copy of chromosome VII. We have tested the hypothesis that expression of the 3 recessive alleles might be due to 3 simultaneous independent genetic events other than chromosome loss, such as mutation or recombination. We have measured, when possible, the frequencies of expression for each of these recessive alleles, independently and in combination one with another, under both selective and non-selective conditions. Our results show that simultaneous expression of these 3 recessive alleles is attributable to chromosome loss (greater than 98%). Similarly, at least 99% of the nocodazole-induced events are attributable to chromosome loss. In contrast, most if not all of the apparent chromosome loss induced by ethyl methanesulfonate is due to multiple events of mutation or recombination.
Insights
This study in Saccharomyces cerevisiae shows that simultaneous expression of recessive alleles is primarily due to chromosome VII loss. Ethyl methanesulfonate-induced events, however, often result from mutation or recombination, not chromosome loss.
Area of Science:
- Fungal Genetics
- Molecular Biology
- Yeast Genetics
Background:
- Direct observation of fungal chromosomes is challenging.
- Genetic markers are used to detect chromosome loss in yeast strains.
- The D61.M strain of Saccharomyces cerevisiae utilizes recessive alleles to infer chromosome VII loss.
Purpose of the Study:
- To differentiate between chromosome loss and other genetic events (mutation, recombination) causing recessive allele expression.
- To quantify the frequency of recessive allele expression under various conditions.
- To validate the use of specific genetic markers for detecting chromosome loss.
Main Methods:
- Measuring the frequency of individual and combined recessive allele expression.
- Analyzing events induced by nocodazole and ethyl methanesulfonate.
- Comparing experimental results with expected frequencies for mutation and recombination.
Main Results:
- Simultaneous expression of three recessive alleles on chromosome VII is attributable to chromosome loss in over 98% of cases.
- Nocodazole-induced events are linked to chromosome loss with at least 99% accuracy.
- Ethyl methanesulfonate-induced events are largely attributed to multiple mutation or recombination events, not chromosome loss.
Conclusions:
- The genetic marker system in Saccharomyces cerevisiae reliably detects chromosome loss.
- Nocodazole is a specific inducer of chromosome loss.
- Ethyl methanesulfonate induces genetic changes primarily through mutation or recombination, mimicking chromosome loss events.
