172 PROTOZOA 



Do the Strains of a Variety Form a Potentially Common Gene 

 Pool? The distribution of each variety over a greater or lesser 

 area raises the question of whether it is composed of subdivisions 

 between which genes cannot flow or whether each variety, no 

 matter how widely distributed, constitutes a potentially common 

 gene pool. Laboratory analysis shows that normal vigorous re- 

 combinants can always be obtained in the F2 generation after 

 crossing any two normal strains, i.e., samples of different local 

 populations, of the same variety. This happens regardless of 

 whether the two strains come from different continents or from 

 neighboring ponds or streams. It has indeed become standard 

 laboratory practice to make use of the genie diversity of different 

 strains by introducing certain genes of one strain into the genetic 

 background of another strain through a series of selected back- 

 crosses. There thus can be no doubt that the genes of one variety 

 form a potentially common pool. 



That is the major fact, but there are also two other related 

 facts of considerable significance in this connection. First, when 

 strains of the same variety are crossed, at least in some varieties, 

 the F2 and the first backcrosses invariably yield some nonviable 

 clones. The proportion of nonviable clones depends upon which 

 strains have been crossed. In variety 4, the frequency of non- 

 viable clones obtained in the autogamous F2 generation after 

 crossing certain strains may be as low as 15%; it may be as high 

 as 98% after crossing certain other strains; and various combina- 

 tions of strains yield percentages that cover the range between 

 these extremes (Sonneborn, unpublished; Dippell and Hanson, 

 unpublished). The same strain crosses regularly give approxi- 

 mately the same results. The magnitude of the mortality seems 

 to vary independently of the geographical proximity or remote- 

 ness of the source of strains; but this needs to be investigated 

 more fully. 



Certain strain crosses within a variety thus yield about the 

 same percentage of F2 mortality as intervarietal crosses. One 

 wonders therefore whether gene flow is possible between such 

 extreme pairs of strains. Margolin ( 1956a, 1)) lias obtained nor- 

 mal, \ iable recombinations between strains 32 and 172 of variety 



