308 PROTOZOA 



Tetrahymena pyriformis. At least nine varieties of this species 

 have been found in a survey confined to North, Central, and 

 northern South America. There may well be more varieties in this 

 region, even among the materials already collected, because a 

 large proportion of the collections could not be classified for tech- 

 nical reasons ( selfers and amicronucleates ) and the basis of clas- 

 sification is solely the capacity to mate, genetic isolation not yet 

 having been systematically studied. All but two of the varieties 

 are confined to North America, four of them being concentrated 

 in the northern latitudes of the United States. Of the other two, 

 one (variety 2) ranges from Canada to northern South Amer- 

 ica, and the other (variety 9) is limited to the southernmost part 

 of this range. All varieties have five chromosomes in the haploid 

 set, and their distinctive morphology is the same in all varieties. 

 The number of known mating types is two in three varieties, 

 three in two varieties, and five, seven, eight, and eleven in one 

 variety each. These are minimal figures based chiefly on collec- 

 tions from nature; breeding analysis in the laboratory has re- 

 vealed some not yet found in nature and may reveal more. 



Varietal differences include the following: number of mating 

 types, system of mating type determination, geographical dis- 

 tribution, preference for running or still water, size, possibly 

 ciliary patterns, and length of immature period. Little is yet 

 known about possible differences in the characteristics and dura- 

 tion of the periods of maturity and senility. In some varieties, 

 senility is marked by loss of micronuclei and capacity to mate; 

 but the organisms may live at least 20 years in this condition. 

 Variety 1 may begin to reach this stage at ages 500 to 1500 fis- 

 sions. Senility may be marked by the onset of selfing with de- 

 creasing survival accompanying increasing parental age in some 

 varieties; but this is not yet certain. Among the varietal differ- 

 ences, at present only mating type specificity can be used to 

 identify all the varieties. This requires live standard cultures of 

 at least two mating types of each variety. Since identification by 

 routine taxonomic procedures is impossible, assigning specific 

 names to the varieties is not practicable. 



Knowledge of the breeding system is greatest for varieties I 



