Chromosome counts in a polyploid series of Saccharomyces.

Direct determination of chromosome number in Saccharomyces is advantageous in the further development of yeast genetics. The enumeration of centromeres by deductions from tetrad analysis sets a minimum chromosome number (Lindegren, 1949; Lindegren and Lindegren, 1951b), but interchromosomal linkage (Shult and Lindegren, 1955) introduces complexities into the deduction of chromosomal numbers from genetic data. Experimental information bearing on the degree of ploidy arises from (1) morphological studies (Winge, 1935; Lindegren, 1949) and the characteristic growth patterns of the different members of the ploidy series (Townsend and Lindegren, 1954), (2) genetical analysis (Lindegren and Lindegren, 1951a; Roman et al., 1951), (3) biochemical tests (Ogur et al. 1952; and Ogur, 1954a, b), and (4) irradiation data (Lucke and Sarachek, 1953), and although they are in general agreement, they do not reveal the chromosome number. The present work describes the direct investigation of the chromosome number of diploid, triploid, and tetraploid Saccharomvees. The problem was made feasible by the convergence of several lines of experience in this laboratory: (1) The perchloric acid-Giemsa technique of chromatin staining has yielded interpretable nuclear structures in yeast. (2) Study of sporulation in yeast has indicated that the perchloric-Giemsa stained structures behave in a manner characteristic of chromosomes in higher organisms during meiosis I yielding discrete, rodlike, countable, chromatinic bodies. (3) The availability of sporulating diploid, triploid, and tetraploid cultures has made it possible to count chromosomes at several levels of ploidy and to estimate the haploid number as