328 Pierre Fredericq 



The character of susceptibiUty to coUcins behaves in these 

 crosses like any other genetic marker, obeying the general 

 rules of recombination (cf. Lederberg, 1955). Susceptibility 

 and resistance to a given colicin are allelic characters which 

 are linked to other markers and segregate with them accord- 

 ing to the selected markers and to the F polarity of the 

 parents (Fredericq and Betz-Bareau, 1952). 



Let us take, for example, a cross where the F+ parent is 

 M — TLBj-f (methionine-dependent but threonine-, leucine- 

 and thiamin-independent) and is resistant to colicins E, K 

 and V; and where the F~ parent is conversely M+TLB^— 

 and is susceptible to colicins E, K and V. Most of the proto- 

 trophic recombinants are susceptible to colicin V, like the 

 F~ parent, because the marker susceptibility /resistance to 

 colicin V is not linked to any of the selected markers given 

 by the F+ parent. Less recombinants will be susceptible to 

 colicin K, thereby distinguishing the F~ parent, because the 

 K marker is linked to the TL markers which must be in- 

 herited from the F+ parent, owing to the selective technique 

 used. Still less recombinants will be susceptible to colicin E, 

 because the E marker is still more closely linked to the 

 selected B^^ marker of the F+ parent (Fredericq and Betz- 

 Bareau, 1952). If, now, selection is made on a minimal 

 medium supplemented with thiamin (where B^"^ is no 

 longer selected), the number of recombinants which receive 

 resistance to colicin E from the F+ parent will fall consider- 

 ably, but in cases where they do receive resistance to colicin 

 E they will also display the B ^-independence of the F+ parent 

 (Jenkin and Rowley, 1955). 



Resistance to a given colicin may sometimes be determined 

 at more than one locus. An auxotrophic mutant, received 

 from de Haan (1954), and derived from Esch. coli B after 

 repeated ultraviolet irradiation, was found to be spon- 

 taneously resistant to colicin E. Crosses of this B mutant 

 with a K 12 derivative also resistant to colicin E give a pro- 

 portion of fully susceptible recombinants, probably by 

 recombinations of \- X -\ loci. A possible cumulative 



