CYTOLOGY OF YEASTS . 269 



indistinguishable. Most industrial yeasts reproduce sexually by 

 isogamous, heterothallic conjugation of haploidal yeast cells or 

 ascospores to form the ordinary diploid cells. 



A single yeast cell may be heterozygous and variants may be pro- 

 duced after sporulation as characters segregate. A single-cell yeast 

 culture thus does not necessarily yield a "pure culture" but single- 

 spore cultures are obviously pure. 



Yeasts may be used for genetic studies. If four ascospores are 

 picked and cultured, they may result in different types of growth or 

 of cultural characters due to segregations. But if one of these cul- 

 tures is mixed with another single-spore culture from another source, 

 artificial crosses can be made. One may discover thus which char- 

 acters are dominant, which recessive, and which lethal. Yeasts are 

 found to behave like other plants and animals with regard to the 

 laws of heredity. Also new strains can be developed by making 

 hybrids and phylogenetic relationships may be determined. Several 

 new hybrid yeasts, some of industrial importance, have been so pro- 

 duced and research is now going on in this field. This knowledge 

 also suggests the relationships of genera and species of sporogenous 

 yeasts. Strains closely related hybridize readily and produce fertile 

 progeny, but those distantly related either do not mate or they pro- 

 duce a large percentage of spores which do not germinate. Closely 

 related genera usually produce sterile spores although one fertile 

 intergeneric hybrid has been produced. INIore distantly related gen- 

 era do not mate at all. Both Winge and Laustsen 

 and the Lindegrens have published details of 

 breeding techniques. 



Cytology of Yeasts. The structure of the 

 yeast cell has been studied in some detail. It 

 consists of a protoplast contained in a cell wall. 

 The cell is said to contain a single nucleus 

 although the two bodies which Badian pictures Yig. 112. Diagram 

 and which he interprets as chromosomes may showing the stmc- 

 actually be nuclei. \ Numerous granules and ture of a yeast 

 vacuoles representing various kinds of reserve cell : n, nucleus ; v, 

 foodstuffs are also found. When young, i.e., <^dlncin°g body"; f, 

 when the culture is actively growing, the cell f^^^ vacuole, 



wall is thin and the protoplasm is fairly free 

 of these reserve substances. yAfter growth ceases, the cells develop 

 thicker walls and become filled with granules gnd vacuoles.>K Occa- 

 sional cells in old cultures develop extraordinarily thick walls and 

 become distended with a large amount of reserve material.^ These 



