SEX IN BACTERIA— GENETIC STUDIES 



15 



1947) by growing the two strains either separately or together in 

 complete media, then centrifiiging out the cells, washing repeatedly to 

 remove growth-factors, and plating mixtures of the washed cells into 

 minimal .agar. The results were striking in that about 100 colonies 

 developed for each 10^ cells examined, and on reisolation and purifica- 

 tion these maintained their prototrophic character. Similarly treated 

 single cultures of each strain gave no colonies on the minimal medium. 

 This would be expected on the basis of the low frequency of muta- 

 tion of each character to independence (ca. 1 in 10^ cells) since the 

 derivation of a prototroph from a triply deficient strain would then 

 occur with a frequency of 1 in 10-^ cells. 



©^. Y-IO 



^T-L-BrB+M+ 



I per 102' cells? / 



I per lo'^ cells? 

 I PROTOTROPHa • 



MIXTURE 



T-L-B,-|B+M+ \ 



— H >- V 



t+l+b,+ ;b-m- 



T+L+B,+ B+M+ 

 CO. I per lO^cells 



58-161 

 T+L+B,+B-M- 



FiG. 1. Diagrammatic representation of recombination in prototrophs. 



The simplest explanation for these results therefore appeared to 

 be that gene recombination took place to give the prototroph B+ 

 M-\- T-}- L-\- Bi-\-, as shown in Fig. 1. Other possibilities, such as 

 association of cells, or the formation of unsegregated diploid, or of 

 heterocaryotic cells were made unlikely by various experimental 

 tests which established the homogeneity and uniqueness of the derived 

 prototrophs (cf. Tatum and Lederberg, 1947). 



The only alternative to a sexual recombination seemed therefore 

 a type of unilateral change by a non-cellular transforming principle 

 similar to that involved in induced changes of type in the pneumo- 

 coccus (Austrian, 1952). Two Hnes of evidence made this improbable. 



