SEXUAL REPRODUCTIONS 12-2 



posed np acceptable solution. Both maize and Drosophila suffer 

 from the handicap that tetrad analysis cannot be made and this fact 

 has left undiscovered a fundamental genetical mechanism which has 

 been brought to light by our work on yeast. 



Tetrad analysis is so great an advantage in genetics that I be- 

 lieve we must accept it as essential to research into the funda- 

 mental mechanism of the hereditary apparatus. The reasons for 

 this statement will be apparent when we consider the arguments 

 in Chapters 23 to 26. 



MUCORS 



Genetics of the fungi takes its origin from the discovery by 

 Blakeslee that copulation between two thalli of opposite mating types 

 precedes the formation of zygospores in Rhizopus. Burgeff col- 

 lected over 20 variants of Phycomyces Blakesleeanus and analyzed 

 the results of hybridizing these mutants. The Phycomycetes are 

 called the siphonaceous fungi because of the absence of crosswalls 

 in the mycelium. The gametangia are multinucleate and when the 

 zygospore is cut off, large numbers of nuclei are enclosed in the 

 young zygospore. Copulations are multiple, and many pairs of nu- 

 clei fuse in the zygospore, while many haploid nuclei are left over; 

 the latter apparently disintegrate. 



Tetrad analysis is not feasible because several diploid nuclei 

 are produced within a single zygospore, and no cells comparable 

 to ascospores, which isolate individual haploid nuclei, are found in 

 Phycomycetes. After hybrid zygospores were formed Burgeff al- 

 lowed them to germinate, and the individual haploid spores which 

 were produced in the aerial sporangium were collected and diag- 

 nosed. Burgeff made and analyzed numerous crosses. From a 

 doubly heterozygous hybrid one expects either two or four types 

 of haploids from each zygote. However, many zygospores produced 

 only one, two, or three of the expected types. He showed that seg- 

 regation in mucors occurred in both the first and the second re- 

 duction division, but he was unable to detect or calculate linkage. 

 Burgeff also discovered the interesting fact that zygospores can 

 be put away in absolute alcohol and preserved in a viable state for 

 a considerable length of time. His 1927 paper is the most ambi- 

 tious attempt to analyze the genetics of the Phycomycetes, and it 

 revealed that tetrad analysis, which is the principal advantage of 

 fungal genetics, was not ifeasible in these organisms. 



SCHIZOMYCETES 



Lederberg, working with Tatum, discovered that it is possible 

 to mate genetically different mutants of the colon bacillus, Escher- 

 ichia coli, by mixing the cultures together in nutrient medium and 



