238 



formed independently. However, 

 there are several differences between 

 the two systems. FA must be evoked 

 while the FTP is extractible from 

 healthy cells. The resistance of FA to 

 various chemical treatments has given 

 only negative evidence of its chemical 

 nature. The role of desoxyribonucleic 

 acid in the FTP was verified by its in- 

 activation by desoxyribonucleic acid- 

 ase. Retention of activity by gradocol 

 membranes has given comparable esti- 

 mates for the size (about 0.1 fi) of the 

 FA particles affecting two different 

 characters. On the other hand, while 

 the particle size of the FTP has been 

 variously estimated from an average 

 centrifugal mass of 500,000 (Avery 

 et al., 1944) to an ionizing irradiation 

 sensitive volume equivalent to a mole- 

 cular weight of 18,000,000 with high 

 asymmetry (Fluke et al., 1951), it is 

 considerably smaller than the FA par- 

 ticle. Pneumococci must be sensitized 

 by a complex serum system for ad- 

 sorption of FTP. The low but poorly 

 determined frequency of transforma- 

 tions has been thought to be due to the 

 low competence of the bacteria. In 

 the absence of adsorption experiments 

 a system similar to Salmojiella has not 

 been ruled out. Important information 

 is still lacking in both systems and time 

 may resolve these apparent differences. 



The relationship of transduction in 

 Sahfiofiella to sexual recombination in 

 E. coli is obscure. Transduction has 

 not been found in crossable E. coli nor 

 sexual recombination in Salmonella. 

 These genera are extremely closely re- 

 lated taxonomically but seem to have 

 entirely different modes of genetic 

 exchange. 



Sexual recombination was first dem- 

 onstrated in E. coli, strain K-12. With 

 the development of an efficient screen- 

 ing procedure, two to three per cent 

 of E. coli isolates were proved to cross 

 with strain K-12 (Lederberg, 1951^). 



ZINDER AND LEDERBERG 



The agent of recombination in E. coli 

 is almost certainly the bacterial cell. 

 The cells apparently mate, forming 

 zygotes from which parental and re- 

 combinant cells may emerge following 

 meiosis, in which linkage is a promi- 

 nent feature (Lederberg, 1947). The 

 combination of genomes within a sin- 

 gle cell has been confirmed by the 

 exceptional occurrence of nondisjunc- 

 tions which continue to segregate both 

 haploid and diploid complements 

 (Zelle and Lederberg, 1951). Although 

 lysogenicity plays a critical role in 

 transduction in Sahnonella, all combi- 

 nations of lysogenic and nonlysogenic 

 cultures of E. coli cross with equal 

 facility (Lederberg, E. M., 1951). 



Owing to the lack of recombination 

 of unselected markers, transduction is 

 a less useful tool than sexual recombi- 

 nation for certain types of genetic 

 analysis. However, as FA may corres- 

 pond to extracellular genetic material, 

 such problems as gene reproduction, 

 metabolism, and mutation may be more 

 accessible to attack. Sexual systems 

 usually provide for the reassortment 

 of genetic material and given an im- 

 portant source of variation for the 

 operation of natural selection in or- 

 ganic evolution. Both sexual recombi- 

 nation and transduction, because of 

 their low frequency, allow only 

 limited gene interchange in bacteria. 

 Transductive exchange is limited both 

 in frequency and extent. 



It is too early to assess the role that 

 transduction may have played in the 

 development of the immunologically 

 complex Salmonella species. White 

 (1926) speculated that the many sero- 

 types evolved by loss variation from a 

 single strain possessing all of the many 

 possible antigens. Bruner and Edwards 

 (1948) obtained specific examples of 

 loss variation with contemporary 

 species. Transduction provides a 

 mechanism for transfer of some of the 



