T. M. SONNEBORN 241 



In brief, nine of the 16 varieties, composing group 1, are sex- 

 ually isolated from each other and all others, and apparently they 

 conform to "biological species" or syngens. The other 7 varieties, 

 composing group 2, are not so sharply isolated from each other. 

 The status of these varieties is still unsettled. It is possible that 

 each local population is to a marked degree isolated from all 

 others. The inbreeding with which this extreme evolutionary 

 diversification is correlated is supported by a number of other 

 adaptations for inbreeding in both groups of varieties, though 

 some varieties may be outbreeders. Because of the widespread 

 close inbreeding, the species problem in P. caadatum is even 

 more difficult than in P. aurelia. 



Paramecium bursaria 



Mating Types and Varieties. Six varieties of this organism 

 have been reported: varieties 1, 2, and 3 by Jennings (1939), 

 varieties 4 and 5 by Jennings and Opitz (1944), and variety 6 by 

 Chen (1946b). The putative variety 5 is known from only one 

 collection, and its existence is suspected only from the fact that 

 the clones attributed to it do not mate with any other mating 

 type standards. However, they also did not mate with each other. 

 It is therefore dubious whether they were immature or impotent 

 members of a known variety or whether they were of a distinct 

 variety but all of one mating type. Because of the uncertain status 

 of variety 5, it will be largely ignored in the present account. 



Of the five remaining varieties, only one, variety 4, has a sys- 

 tem of two mating types. The rest have systems of multiple mat- 

 ing types. Each of the varieties 1, 3, and 6 has four mating types, 

 and variety 2 has eight. The breeding relations are the same in 

 all: each mating type can conjugate with all the others in the 

 same variety, but cannot mate with animals of its own mating 

 type. 



One and only one combination of different varieties can react 

 sexually and conjugate; this is the combination of varieties 2 and 

 4. One of the two mating types of variety 4, type R, reacts and 

 conjugates with four of the eight mating types of variety 2, the 

 types E, K, L, and M. Chen (1946a) and Sonneborn (1947) 



