The Life Cycle of the Single Cell 13 



In higher animals, reproduction is tied to the sexual fusion of gam- 

 etes. Human beings cannot bud or fragment. They must mate to reproduce 

 their kind. In yeast cells, as in other microorganisms, sexual fusion is 

 separate from reproduction. Yeast grow by asexual means, but under 

 special conditions do employ sexual fusion to produce genetic recom- 

 binants ( i.e., new varieties ) . 



The life cycle of yeast is illustrated schematically in Fig. 4. The 

 diploid individual is an elliptical cell. It reproduces asexually by budding 

 and can do so indefinitely as long as it is supplied with nutrient materials 

 and given conditions beneficial for growth. Under adverse conditions 

 (when starved and unable to bud), the diploid cell undergoes meiosis to 

 produce four haploid individuals. These are surrounded by thick walls 

 and are resistant to drying, heat, ultraviolet radiation, etc. They are called 



Fig. 4. The life cycle of yeast. 



Life cycle of 

 Sacctiaromyces cerevisiae 



Two haploid cells of 

 different sexes 



Schizosoccharomyces 

 octosporus 



Hansenula anomola 



The variety of osci and ascospores 

 seen in different species of yeast 



Spermophthora 

 gossypii 



