Bacterial Mutation and Conjugation 



313 



+ + r 



+ + - - s 



BMPTV, xBMPTV, P 



--++s + + — r 

 BMPTV, xBMPTV, 



+ + + + 

 Prototrophs BMPT 



86% 



V, 



14% 



+ + + + 

 Prototrophs BMPT 



79% 

 s 

 V, 



21% 

 r 

 V, 



figure 23—6. Genetic recombinations obtained in reversed crosses in- 

 volving unselected markers. 



The type of nutrient medium used in the 

 experiments described permitted the detec- 

 tion of certain recombinants and not others. 

 In the first recombination experiment, only 

 selective markers — only those alleles which 

 provided nutritional independence— were 

 selected for detection. Thus, though the 

 prototroph T + L+B 1 + B+Pa+C + was detect- 

 able, it was not possible to test for the oc- 

 currence of the complementary polyauxo- 

 troph J-L^B x ~B~Pa'C~ . Since no test 

 was made for the multiple auxotroph, one 

 could doubt its occurrence. It is entirely 

 reasonable that the immediate product of 

 mating is a zygote containing part or all of 

 the genotypes of both of the parental cells. 

 Although it is assumed that integration can 

 take place as it does in transformation (if 

 DNA passes from one conjugant to another) , 

 no evidence that it does has been presented 

 so far. In other words, the possibility re- 

 mains, at least with respect to the genes 

 showing recombination, that the recombinant 

 produced by conjugation may be partially or 

 completely diploid. 



Although it is not feasible to find zygotes 

 microscopically, one can look for them ge- 

 netically. After mating, certain clones be- 

 have as though they are mosaic for a num- 

 ber of markers. When a single cell from 

 such clones is isolated, grown, and tested, 



it is found 3 that some of its progeny possess 

 the original genotype of either parent and 

 that others are recombinants. Clearly, the 

 single-cell isolates were derived from more 

 or less persistent heterozygotes — individuals 

 diploid for various markers. Thus, the hap- 

 loid recombinants within a clone offer un- 

 ambiguous proof that they were derived from 

 a true zygote. Haploid recombinants are 

 also called segregants, although this term 

 does not imply that they were the result of 

 a meiotic process. One concludes, there- 

 fore, that the (partially or completely) dip- 

 loid zygote produced after bacterial conjuga- 

 tion usually has a temporary existence — just 

 as in the case of transformation — which 

 terminates in the production of haploid 

 progeny. In other words, all the genes in 

 a nuclear body of E. coli are usually haploid. 

 At the same time that certain genetic 

 markers are used selectively — to detect re- 

 combinant prototrophs, for example — other 

 markers may be present that are not subject 

 to selection. Such genes are called unse- 

 lected markers. E. coli is available in two 

 genetic forms: V{ is resistant to infection 

 by the bacterial viruses Tj and T, ; ; IV is 

 sensitive to infection by these viruses. In- 

 fection is followed by death of the bacteria. 



3 By J. Lederberg and M. Zelle. 



