BENZER 279 



mally with respect to mutants located in Figure 4 by horizontal lines extend- 

 outside that span. They are indicated ing over the span. 



segment A 



'^ segment B .^ 



1^ 7 



r 



47 



r r r r 



104 101 103 105 



r I 



106 1 



r 



102 



/> '\% 



H 1 III HI I H I 



-ttf 



a b c d 



Fig. 4. Preliminary locations of various r II mutants, based upon spot tests. 



Spot tests on numerous other mu- 

 tants have shown that mutants of 

 varied reversion rates, transmission co- 

 efficients, and rates of "partial rever- 

 sion" occur at scattered positions in 

 both segments. 



Mapping of "Microclusters.''— The 

 spot test enables us to pick out "micro- 

 clusters," i.e., groups of very closely 

 neighboring mutations. Four such 

 groups selected for further study are 

 indicated in Figure 4, and the results 

 of mapping them are given in Figure 5. 

 While some intervals show reasonably 

 good additivity properties, there are 

 some mutants which give violently 

 anomalous results. Thus in microclus- 

 ter a, r47 gives no wild recombinants 

 (i.e., less than 1 in 10'^) with any of the 

 other three mutants, but two pairs of 

 the three do show recombination. 

 These results can be understood if it 

 is assumed that each mutation extends 

 over a certain length of the chromo- 

 some, and production of wild type re- 

 quires recombination within the space 

 between those lengths. According to 

 this interpretation, the mutations 

 would cover the lengths indicated by 

 the bars in Figure 5. These anomalies 

 resemble those observed in the spot 

 tests, only they are more limited in 

 span. 



This observation raises the question 

 of whether there exist true "point" 

 mutations (i.e., involving an alteration 

 of only one nucleotide pair) or 



whether all mutations involve more or 

 less long pieces of the chromosome. It 

 must be remembered that the mutants 

 used in these experiments were selected 

 for extreme stability against rever- 

 sion. This procedure would be ex- 

 pected to enrich the proportion of 

 mutants containing gross chromosomal 

 alterations. So far as is known, the 

 anomalous cases observed could 

 equally well be imagined to be due 

 to double (i.e., two near-by "point") 

 mutations, inversions, or deletions of 

 the wild-type chromosome. In con- 

 tinuing these experiments, it would 

 seem well advised to employ only mu- 

 tants for which some reversion is ob- 

 served. 



Discussion.— The set of rll mutants 

 defines a bounded region of a linkage 

 group in which mutations may occur 

 at various locations, all the mutations 

 leading to qualitatively similar pheno- 

 typic eff'ects. The rll region would 

 seem, therefore, to be functionally 

 connected, so that mutations arising 

 anywhere within the region affect the 

 same phenotype. This effect is ex- 

 pressed, in case strain B is the host, by 

 failure to produce lysis inhibition; in 

 case S is the host, by no consequence; 

 and in case K is the host, by inability 

 to multiply normally. The failure of 

 an rll mutant to mature in K can be 

 overcome by the presence of a wild- 

 type phage in the same cell. This could 

 be understood if the function of the 



