1 86 Plant Genetics 



sex cannot be distinguished. The third type of my- 

 celium he called the neutral strain. Although this 

 cannot produce zygotes by itself it does produce them 

 when mated with either a plus or minus strain. Evi- 

 dently, therefore, the sex branches from the neutral 

 strain contain both male and female potentialities. 



The answer to the question as to where and how sex 

 is determined in these forms involves a number of 

 possibilities. When a zygote germinates, one or more 

 sporangia are produced very early, and this raises the 

 question as to the sexes developed. In Mucor itself 

 the segregation of sex is evidently completed before the 

 formation of the spores in the sporangium, for all the 

 spores in a given sporangium are of a given strain, 

 producing either male or female mycelia. The sporan- 

 gium as a whole, therefore, is either male or female. In 

 Phycomyces, however, a different behavior is regular. 

 The zygote produces a sporangium, but the sporangium 

 is not completely of one sex. It produces three types of 

 spores: spores producing the plus strain, spores pro- 

 ducing the minus strain, and spores producing the 

 neutral strain. The plus strain then perpetuates 

 only plus strains through its spores, which means that 

 the sex is fixed in this case. The minus strain behaves 

 in the same way. The neutral strain, however, pro- 

 duces spores of all three types, an interesting situation, 

 for it suggests the Mendelian segregation. The Mucor 

 group deserves to be studied further for suggestions as 

 to sex determination. 



The work of BURGEFF (3) is brief but striking. He 

 took BLAKESLEE'S plus and minus strains and found 

 that they always bred true to their plus and minus 



