T. M. SONNEBORN 307 



befits an outbreeder. The selection of such long mating periods 

 suggests that the achievement of fruitful cross-mating is relatively 

 rare. Other adaptations to, and correlates of, outbreeding are the 

 systems of multiple mating types, the almost invariable synclonal 

 uniformity in mating type, the total absence of selfers of the 

 sort found in other species, the very much higher death rate 

 after inbreeding ( sib mating ) as compared with outbreeding, the 

 rapid rise in death rate through successive inbreedings. A puz- 

 zling exception is associated with the very rare production of a 

 complementary mating type in a clone. When this mates with the 

 original type in the clone, survival is as good as in outbreeding. 

 Whatever the basis of this exceptional inbreeding, the result is 

 highly adaptive, since, like autogamy and selfing in senility in 

 P. aurelia, it both rejuvenates and permits the retention of 

 heterozygosity. 



The synclonal uniformity in mating type is based upon the 

 P. aurelia group B system of mating type determination plus 

 the regular massive exchange of cytoplasm between mates. The 

 rare exceptions (2.6% of the synclones) which yield more than 

 one mating type per synclone reveal the basic B system. A hy- 

 pothesis is presented to account for all the remarkable kinds of 

 observations on the inheritance of mating type which have 

 hitherto remained unexplained. The system of mating type de- 

 termination in P. bursaria is the most complex of those discussed. 

 Loss of the mechanism assuring regular cytoplasmic exchange 

 between mates transforms it to the P. aurelia form of the group B 

 system. Loss of one or two other features (explained in the text) 

 converts the B into the A system. The P. bursaria system is a 

 strong adaptation to outbreeding by almost completely blocking 

 mating within a synclone. 



The highly developed complex of adaptations to outbreeding 

 in P. bursaria inhibits progressive divergence of local populations. 

 As a result of the wide distribution of potentially common gene 

 pools, speciation is poor in comparison with that of inbreeders. 

 In the United States there are probably only three varieties of 

 P. bursaria as compared with at least 14 (probably more) va- 

 rieties of P. aurelia. 



