THE GENETICS OF BACTERIA 1 59 



occurs ininiediately after fusion. The status of the vegetative nucleus is also 

 confirmed by the work of Witkin (1951), which is described in Chapter IV, 

 Section G. 



Some bacteria, such as sporing bacilli, streptomyces and the anaerobic 

 actinomyces appear to have a prolonged diploid phase, and an examination 

 of the genetical behaviour ot such forms would greatly assist in the interpreta- 

 tion of their cytological appearances. 



This correlation of conclusions between strictly genetical and strictly 

 cytological studies gives grounds for increased confidence in both, and its 

 importance cannot be too greatly stressed. 



B: SOURCES OF GENETICAL EllDENCE 



(i, 2, 3, 4, 5, 6, 7, y, 10, 15, 16) 



The evidence upon which these genetical conclusions are based is mainly 

 concerned with the nutritional requirements of the bacteria, that is to say, 

 with their synthetic abilities. Problems of variation in resistance to dis- 

 infectants and to the bacteriophage have also been studied. 



Gene recombination has been detected by the appearance of nutritionally 

 unexacting " wild-type " strains of bacteria in mixed populations of nutrition- 

 ally exacting mutants, and has been confirmed by a variety of ancillary 

 experiments. The segregation and recombination of mutant characters shows 

 interactions which may be regarded as indicative of linkage, and some evidence 

 of the hncar arrangement of genes has been obtained. 



The comparative rarity of the recombination process gives reason for 

 believing that conjugation must normally be autogamous, and only occasion- 

 ally sexual. Here also the genetic and the cytological evidence are in agreement. 



With bacteria, as with plants, animals and fungi, an increased mutation 

 rate is induced by irradiation with ultra-violet light or X-rays. The mutation 

 rate is proportional to the dosage, but is not related to the lethal effect. These 

 mutations may become evident immediately after irradiation, or may appear 

 only in the offspring of the treated cells ; presumably after conjugation. 



The suggestion that hereditary factors in higher organisms may reproduce 

 automatically in the cytoplasm o( vegetative cells, independent of nuclear 

 control, has its bacteriological parallel in the transformation of antigenic 



