PROTOZOA 



siderable mortality, approaching 100% in certain crosses. Parallel 

 and corresponding mating types occur in different varieties, but 

 each mating type is uniquely specified by its pattern of sexual 

 reactions. In most cases there is no attempt of different varieties 

 to interbreed or the attempt is unsuccessful; sexual isolation is 

 complete. In the few combinations of varieties that can inter- 

 breed, the Fl or F2 is almost completely nonviable, and survivors 

 are always weak. The latter are incapable of yielding normal 

 vigorous progeny either by inbreeding or by backcrosses. Ge- 

 netic isolation is virtually complete. 



Only two varieties are known to have worldwide longitudinal 

 and latitudinal distributions; two have wide, but not so wide, 

 distributions. The rest are restricted to a single continent, some- 

 times to but a small part of it. The varieties are distributed in 

 temperature clines, many overlapping; not uncommonly more 

 than one variety is found in a small sample of a single body of 

 water. 



Differences among the Varieties of P. aurelia and the Problem 

 of Identification. Differences of the following sorts are found 

 among the varieties: mating type specificity; geographical dis- 

 tribution; maximal and minimal tolerated temperature (these not 

 always being completely correlated with geographic distribu- 

 tion); fission rates at the same temperature; body length; number 

 of micronuelei in early life; the occurrence of killer and resistant 

 strains; specificity of serotypes; system of determination and 

 inheritance of serotypes; system of mating type inheritance and 

 determination; the existence of and nature of diurnal periodicities 

 in sexual reactivity; the temperature optima for mating; other 

 conditions for mating; the occurrence of and duration of a period 

 of immaturity alter conjugation (immaturity never occurs after 

 autogamy ); the duration of the period of sexual maturity; the 

 frequency of occurrence of lines that self-conjugate during ma- 

 turity: the occurrence of autogamy or selfing during senility; 

 the period of survival after loss of micronuelei during senility. 



Each ol these many differences between varieties was de- 

 scribed in lull. No one of them, except mating type and serotype' 

 specificities, can be used to define or identify all the varieties, 



