SEGREGATIONS IN ESCHERICHIA COLI 519 



plates. Single colonies of Y-40 were picked and streaked across a nutrient agar 

 plate. Streaks of similarly treated Y-53 colonies were made from the opposite 

 direction, so that in the center of the plate, cells of the two types were mixed, 

 treated colony by treated colony. The plates were incubated for 24 hours, the 

 mixed growth scraped from the plates, suspended in sterile water and plated 

 into minimal agar. The occurrence of colonies which would not interact to 

 produce prototrophs, as detected by plating into minimal medium, would be 

 an indicator that the combination was heterogeneous for an aberration. Since 

 in these experiments, both "parents" were exposed to treatment, each plating 

 was equivalent to the testing of two chromosomes for the occurrence of an 

 aberration. No marked variation in the yield of prototrophs was noted in 

 tests involving 121 mustard- and 28 x-ray-treated chromosomes. This can 

 scarcely be regarded as an adequate sample in view of the stringent selection 

 imposed by the technique, which might be expected to eliminate any aberra- 

 tion types which are even slightly less vigorous than the normal. This con- 

 sideration is especially relevant in view of the "hemizygous" condition of any 

 aberrations in the probably haploid vegetative cells. These studies will be 

 continued. 



How Many Segregants per Zygote? 



In the experiments detailed in this paper, recombinants were obtained from 

 different cell types which were exposed to each other in an agar medium. 

 Therefore each prototroph recombinant colony seen by the experimenter marks 

 the site of formation of a zygote. The question may immediately be raised 

 whether there are at that site other recombination classes which, by virtue of 

 their biochemical deficiencies, remain dormant within the prototroph colony on 

 the minimal selective medium. This is equivalent to inquiring whether there is 

 but a single viable product of meiosis (as in megasp orogenesis in many higher 

 plants) or more than one, as in the ascomycetes. The solution to this problem 

 would be of special interest in relation to the possible occurrence of four-strand 

 crossing over. In addition, if an appreciable proportion of prototroph colonies 

 consisted of two distinct segregation types, it would be necessary to isolate 

 these types for the collection of segregation data. 



There are at least three ways in which a zygote might yield more than one 

 haploid recombinant. Firstly, the zygote might be capable of proliferation in 

 the diplophase (or sporophyte), leading to the concurrence of several diploid 

 cells, each of which might undergo meiosis independently, and by chance yield 

 several segregation types. Secondly, a single zygote might produce, after 

 meiosis, in addition to the prototroph, the complementary multiple mutant 

 class. Thirdly, in a system of four-strand crossing-over, there might be two 

 supplementary prototroph recombinants differing in the segregation of factors 

 such as Lac and V\ for which the diploid was heterozygous. 



Obviously, the proper investigation of these possibilities requires that one 

 stringently avoid contamination of one colony with another. For this reason, 

 the cell suspensions used were diluted so as to yield only about five to ten 

 recombination colonies per plate. 



157 



