12-19 THE YEAST CELL 



Hymenomycetes 



The hymenomycete mechanism resembles the plus -minus Rhizo- 

 pus mechanism rather closely since no obvious sex organs exist 

 in these forms. It differs in involving two loci in many species, 

 thus producing a minimiun of four mating types. Furthermore, 

 mutation is common and a multiple series of alleles at these loci 

 further complicates the picture. 



Paramecium 



The mechanism which assures cross-fertilization in the single- 

 celled diploid Paramecium resembles the plus-minus mechanism 

 found in fungi since no sex organs are present, but the heredity 

 seems to be more complex. The fact that the copulating cells are 

 diploid is a still further difference from the most closely compara- 

 ble fungal mechanism. 



Cross -fertilization has considerable survival value since it in- 

 sures the incorporation of a variety of genes in each zygote and 

 makes possible the production of a number of new recombinations. 

 However, there are many organisms, such as the close -pollinated 

 flowers, in which cross -fertilization does not occur or occurs only 

 rarely. Inbreeding has made these genera somewhat less variable 

 and therefore less plastic or adaptable than cross -pollinated forms. 

 I have described (1942) copulation in a micrococcus in which cross- 

 fertilization did not occur. In this case the zygote was invariably 

 homozygous since it was formed by the fusion of two genetically 

 identical gametes, which originated from the division of a single 

 haploid nucleus. The fusion occurred between two adjacent cells 

 after cell division and solution of the intervening cross wall. This 

 type of copulation has survival value in providing an opportunity 

 (1) for unequal crossing over (Chapter 17) which can increase 

 the number of loci or (2) for chromosomal rearrangements to oc- 

 cur through aberrations of the meiotic mechanism. 



I prefer to define sex as Allen does, only in terms of true male 

 and female sex organs. When we use this definition, mating type, 

 self -sterility alleles, and plus-minus factors take on their true 

 significance. They are not essential to the sexual mechanism but 

 are simply means of assuring cross -fertilization. The fact that 

 they may occur either in the absence of sex organs, as in the Hy- 

 menomycetes, or may be superimposed on true male and female 

 sex organs as in Neurospora (fig. 12-3), prove that they are devices 

 of different origin than the sexual mechanism. 



THE ADAPTABILITY OF YEASTS FOR UNITING BIOCHEMISTRY 

 AND EXPERIMENTAL CYTOLOGY 



Genetics has been found useful in biochemical analysis, especi- 

 ally in elucidating the steps which occur in the synthesis or break- 



