306 PROTOZOA 



most. Selfing appears to take its place. All these features, plus 

 outbreeding Fl nonliability, point toward inbreeding. There 

 may be one or more outbreeding varieties, but the evidence on 

 this is still not clear. 



Paramecium bursaria. Five varieties of P. bursaria are well 

 known. One of these has two mating types, three have four 

 mating types, and one has eight mating types. The sexual isola- 

 tion of these varieties is complete except that one of the two 

 mating types of one variety conjugates with four of the eight 

 types of another variety. This exception is fruitless: all the con- 

 jugants die. Fruitful conjugation can occur between local popu- 

 lations of the same variety. Each variety thus constitutes a 

 potentially common gene pool which is completely cut off from 

 all others. Two of the varieties are confined to North America, 

 two to Europe; one is common to Asia and North America. 



Varieties differ in the following traits: number of mating types, 

 specificity of mating types, diurnal periodicities in sexual reactiv- 

 ity; speed of conjugation; length and thickness of chromosomes; 

 and geographical distribution. Identification of varieties is simpler 

 than in the other species, partly because not more than three va- 

 rieties are known from any land mass. These three could be 

 readily distinguished if all the mating types were available, even 

 without living mating type standards. Since the task of obtaining 

 all mating types of a variety could be very laborious and time-con- 

 suming, this is not a satisfactory basis for a species description. 



In variety 1, the only one very extensively studied, the life 

 features mark it clearly as a strong outbreeder. The immature 1 

 period is 3 to 5 months with continuously favorable conditions, 

 but even a short unfavorable period triples the duration of imma- 

 turity. In the laboratory, maturity lasts about three years, senility 

 about four to five years. In nature, all these stages of life arc 

 probably much longer than in the laboratory. Unlike the prc\ ions 

 species, P. bursaria can mate readily throughout senility; but 

 during this period the fertility, i.e., survival after conjugation, 

 progressively decreases. This variety thus has remarkably long 

 periods for dispersal before mating can occur and much longer 

 periods (at least 7 years) during which mating can occur, as 



