520 JOSHUA LEDERBERG 



Crosses were made between Y-40 and Y-53 (B-M-T + L + B 1 + Lac+Vi T XB + - 

 M + T~L~Bi~Lac~Vi 8 ) on ^-containing minimal agar medium. As already 

 noted, about 90 percent of the colonies from such a cross are B + M + T + L + Bi~. 

 The theoretical complementary class would be B~M~T~LrBi + . Because of its 

 nutritional deficiencies, it could not be expected to proliferate on the minimal 

 medium even had it been produced after meiosis. The possibility remains, how- 

 ever, that a few cells of this constitution might still be present among the io 8 

 or so Bi~ cells of the predominant type in a colony. By plating such colonies 

 into medium lacking B\ but containing biotin, methionine, threonine and 

 leucine, the B{~ cells would be suppressed, while the postulated multiple mu- 

 tant type could form colonies and be recovered. 



The experiment just described was carried out, testing 52 colonies for their 

 content of other cell types. In general, a thiaminless colony could be shown to 

 contain from 10-100 cells capable of forming colonies on the B, M, T, L 

 medium. However, in each case investigated these have been shown to be in- 

 distinguishable from the Y-40 parental B~M~ type, and must be presumed tc 

 arise from a surprisingly low degree of contamination of the colony with these 

 cells from the heavily seeded plate. A few colonies were found which could be 

 characterized as reversions from B\~ to B\ + . These experiments are then, in- 

 conclusive with respect to the occurrence of complementary genotypes in the 

 same colony. With appropriate stocks, not as yet available, it should eventu- 

 ally be possible to manipulate the situation so that the complementary type 

 could be recovered selectively, excluding both parents and the predominant 

 recombination class. 



A search for supplementary types was conducted with the same crosses, 

 except that colonies appearing on Bi agar were streaked out directly on EMB- 

 lactose agar to determine whether any of them were heterogeneous for Lac. 

 In some cases, a number of isolated colonies from each EMB-test plate were 

 then also tested for homogeneity with respect to Tj-resistance. About 90 

 colonies were so tested; only one colony was found containing both Lac + and 

 Lac~ cells. It is impossible to be certain that, with this low frequency, the 

 single colony which was picked was not actually derived from two distinct 

 zygotes. These experiments cannot be considered as bearing critically on the 

 question of the occurrence of two- or four-strand crossing over because of the 

 absence of information concerning (a) the viability of more than one meiotic 

 product and (b) chiasma interference. The results do, however, justify the 

 technique of picking the prototroph colonies directly, and testing them without 

 further purification for the collection of segregation data. 



A Comparison of Sexual Recombination and Transformation 



The occurrence of recombination types has been interpreted by us (Leder- 

 berg and Tatum 1946c, Tatum and Lederberg 1947) as a consequence of cell 

 fusion, "karyogamy" and meiosis with crossing over. This is, however, not the 

 only allowable interpretation of the general phenomenon of the occurrence of 

 new character combinations. By analogy with the systems which have been 

 described in pneumococci (Avery, MacLeod and McCarty 1944) and other 



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