Sex Dclermi nation igy 



Although they looked alike in every particular, he concluded that 

 they were sexually different for the following reason. Neither 

 strain by itself is capable of producing zygotes, but when phis 

 and minus strains are brought together sexual branches from the 

 one meet sexual branches from the other and pro<luce abundant 

 zygotes. The natural conclusion is that Blaki-:slee's plus and 

 minus strains represent the male and female conditions, although 

 the sex cannot be distinguished by direct examination. The third 

 type of mycelium he called the neutral strain, for it is incapable 

 of producing zygotes in any combination. 



The answer to the question as to where sex is determined in 

 these forms is as follows. WTien a zygote germinates, one or more 

 sporangia are produced very early, and individuals are multi[)lied 

 by the spores from these sporangia. In Mucor itself the segrega- 

 tion of sex is evidently completed before the formation of spores 

 in this first sporangium, for all of its spores will produce the same 

 strain of mycelium. The sporangium as a whole, therefore, is 

 either male or female. In Phycomyces, however, a dilTerent 

 behavior appears. 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 producing the 

 minus strain, and spores producing the neutral strain. The plus 

 strain then perpetuates only plus strains through its spores, which 

 means that sex is fixed in this case. The minus strain behaves in 

 a similar manner. The neutral strain, however, produces spores 

 of all three types, an interesting situation, for it suggests Mendelian 

 segregation. 



BuRGEFF (8) has performed an interesting operation on this 

 same material. By means of a very careful technique, he grafletl 

 parts of the plus strain on to the minus strain and secured graft 

 hybrids with the characteristics of the neutral strain. In attempt- 

 ing to interpret the foregoing results, it should be remembered that 

 Mucor and its relatives are coenocytic. so that nuclei of two tyi>cs 

 can mingle freely in the mycelium. 



Proceeding further with this material. Bi.akeslee (4) isolated 

 numerous plus and minus strains, and found that they ditTereti 

 in their sexual intensity, as computed in terms of the number 

 of zygotes formed under standard conditions. Evidently some 



