DIFFERENT RATES OF SPONTANEOUS MUTATION DURING MITOSIS AND MEIOSIS IN YEAST

Abstract

PONTANEOUS mutability has been studied principally in systems organized ' in biologically distinct ways: in microorganisms, where mutations are recog-nized after mitosis or paramitosis, and in higher organisms, where the mutations that have been studied are mostly meiotic in origin. Data collected in microorganisms permit analyses of mutation rates in terms of mutations per cell per division (LURIA and DELBRUCK 1943; RYAN and WAIN-WRIGHT 1954) or mutations per cell per unit time (NOVICK and SZILARD 1951; NOVICK 1955). In higher organisms the spontaneous mutation frequencies are not difficult to obtain, but the spontaneous mutation rates are very difficult to quantitate. Mitosis and meiosis are systems in which the chromosome behavior and internal environment are different, and therefore a comparative study of these two systems could lead to a better understanding of the mechanisms of origin of spontaneous mutations. Of course, this would only be true if differences were found. There is some evidence that spontaneous mutation frequency in Drosophila is higher during gametogenesis than during the gonia1 stages (MULLER 1958). This conclusion is based on the relatively low frequency of clusters of visible mutations from single individuals (SCHALET 1958) , but a quantitative analysis on a per cell basis would be desirable. Mutations in yeast can be treated quantitatively on a per cell basis during either the vegetative (mitotic) or sporulating (meiotic) stages of its life cycle. Therefore, we undertook a series of experiments to determine in a clear-cut man-ner if suspected differences in spontaneous mutation rates were real. In this study we analyzed reversions to wild type of mutants for biochemical deficiencies. These will be referred to as revertants rather than as back mutants since a back mutation implies considerably more knowledge of intragenic site specificity than the data warrant. MATERIALS A N D METHODS Culture media (a) Y . E. medium 21: The medium has all the growth requirements for yeast present: Costantino yeast autolysate, 3 g; Costantino casein hydrolysate, 3 g;