20 



SEX IN MICROORGANISMS 



by the parents. Thus diploid cells heterozygous for lactose fermenta- 

 tion, Lac-\-/Lac—, produce mosaic colonies on an indicator medium, 

 as shown in Fig. 2. The dark or "-]-" sectors consist of still heterozy- 

 gous cells, Lac-{-/Lac—, and of Lac-{- haploid segregants; the light 

 or "— " sectors are Lac— haploid segregants. On complete medium, 

 the faster-growing haploid cells soon outstrip the original diploids, but 

 segregation can be effectively prevented on a minimal medium owing 

 to the nutritional requirements of the haploid components. Single 



Fig. 2. Segregating diploid culture plated on indicator agar, showing va- 

 riegated colonies. 



cell studies (Zelle and Lederberg, 1951) have verified that genetic 

 factors from two parents have converged to a single hybrid cell, 

 the essence of sexuality. 



Haploid and diploid cultures have been studied cytologically, 

 especially for comparisons of their nuclear structure, by means of 

 the Piekarski-Robinow technique (osmic fixation; HCl hydrolysis; 

 Giemsa stain; mount in Abopon). This method gives brilliant nuclear 

 preparations, but E. coli appears to be technically unsuitable for 



