262 



GENETICS AND PLANT BREEDING 



factors, a result that was also consid- 

 ered technicalh' more reliable than the 

 change of a single marker, for which 

 simple mutation must always be con- 

 sidered. I'his caution led to many inde- 

 cisive experiments: indications of ex- 

 change with single markers could not 

 be confirmed in multi-factorial experi- 

 ments. We eventually realized that 

 genetic exchange was taking place in 

 certain combinations of Salmonella 

 strains, but in a different pattern from 

 crossing in E. coli, as only single markers 

 were being exchanged at each event. 

 This realization was promptly followed 

 by the corollary discover)' that filtrates 

 of certain mixed cultures could be freed 

 of intact cells, and still transform indi- 

 vidual traits of a recipient strain. The 

 term "transduction" was introduced at 

 this point for the hypothesis that ge- 

 netic fragments were being transmitted 

 from one strain to another, via cell-free 

 filtrates, as seemed to occur in the 

 pneumococcus too. However, Salmo- 

 nella proved to be far more amenable 

 to genetic study than the pneumococ- 

 cus, and generalizations on this trans- 

 duction could be based on studies with 

 thirty or forty different markers in a 

 relatively short time. The advantages 

 of Salmonella in genetics were, how- 

 ever, compensated for in biochemistry. 

 At first, the most important informa- 

 tion about the transforming substance 

 of Salmonella was that its activity was 

 not destro}ed by desox\ribonuclease, 

 in distinction to the pneumococcus ex- 

 periments. Filtration and sedimenta- 

 tion experiments then connected the 

 activity with particles about 0.1 micron 

 in diameter and hence just beneath 

 microscopic visibilit}'. (For comparison, 

 an E. coli cell is about 1 micron wide 

 and 3 to 5 microns long.) These par- 

 ticles were later identified as bacterio- 

 phage, or bacterial virus. The lack of 

 effect of dcsoxvribonuclcase was then 

 easilv explained, if the true agent of 

 transduction were protected inside the 



skin of the virus particle. ITie inner 

 agent might very well be DNA— but 

 chemical studies so far cannot distin- 

 guish the bacterial genes from the 

 DNA nucleus of the virus itself, and 

 we must rely for this guess on the 

 analog}' with the pneumococcal trans- 

 duction. 



But where did the virus come from 

 in these bacterial filtrates? One of the 

 Salmonella strains proved to be "lyso- 

 genic," that is to sav is infected by a 

 latent virus. The latent virus, or "pro- 

 phage" is ordinarily transmitted as a 

 hereditary' quality, during the multipli- 

 cation of the lysogenic cells. Once it 

 is freed, the virus can infect other cells. 

 Here it may behave alternatively as a 

 typical lethal parasite, and grow rapidly 

 at the expense of the host, or re-enter 

 its latent form, and render the bac- 

 terium lysogenic. 



The conclusion may be simply 

 stated that the pneumococcus and 

 Salmonella both manifest types of ge- 

 netic transduction. In the latter, bac- 

 teriophage acts as passive carrier of 

 the genetic (DNA?) fragments, but 

 the viral nucleus has no other demon- 

 strable relationship with its host com- 

 panions. In both cases, any genetic 

 marker that could be tested for was 

 amenable to transduction. 



APPLICATIONS AND PROSPECTS 



The studv of transduction in bac- 

 teria has been the labor of many scien- 

 tists of whom only a few are listed in 

 the references here. But the stori' has 

 only well begun. Of the thousands of 

 bacteria species, only a few have been 

 examined at all, and each study has 

 revealed a new facet. So far, only a 

 minority of attempts to demonstrate 

 recombination mechanisms (sexual or 

 transductive) in bacteria have been 

 successful. It is a fair caution that the 

 first essential is a selective technique, 

 a workable means of detecting new 

 types even when they are extremely 



