12-3 



THE YEAST CELL 



plating out on specially designed agars. He discovered 15 genes in 

 one linkage group, apparently in a linear order, suggesting that all 

 the loci lie on one chromosome. This discovery gives genetical 

 analysis in the bacteria almost the same validity that it has in higher 

 organisms. Prior to this, most of the genetical work in the bacteria 

 was by plating and selecting variants which resemble mutants known 

 to occur in other organisms. However, there is no way of distin- 

 guishing most bacterial variants from the depletion mutations, but 

 Lederberg's work suggests that regular segregations of some bac- 

 terial mutations can occur. 



The enormous importance of bacteria as pathogenic agents 

 makes any new technique for understanding their life cycles es- 

 pecially valuable. However, the approach to basic theoretical gen- 

 etical problems is technically simpler using yeasts, and with yeasts 

 one has the advantage of unambiguous tetrad analyses. 



HYMENOMYCETES 



The Hymenomycetes include the mushrooms and toadstools. 

 They grow from an underground mycelium to produce a mushroom 

 fruiting body on whose gills (hymenium) the basidia are borne. Gen- 

 erally, up to the time of production of the basidium, the cells are 

 dicaryons with two separate complementary haploid nuclei in each 

 cell. In the basidium, the nuclei fuse, undergo reduction, and pro- 

 duce four basidiospores on each basidium. The four spores from 

 a single basidium can be collected by making a "spore print" and 

 tetrad analysis of the fungi had its origin in the development of 

 this technique by Kniep. Kniep discovered that many toadstools 

 have tetrapolar "sex" and showed that the mating type alleles are 

 Mendelian characters. 



In the classical case, the zygote is heterozygous for two pairs 

 of genes and has the constitution Aa Bb. There are three types of 

 basidia: (1) AB, ab, Ab, aB; or (2) AB, AB, ab, ab; or (3) Ab, Ab, 

 aB, aB. Each basidiospore produces a mycelium which can be 

 mated with a complementary mycelium to produce a dicaryotic 

 (often incorrectly called a diploid) mycelium according to the 

 scheme below. 



