T. M. SONNEBORN 255 



system as formulated in the hypothesis. Many such tests come 

 at once to mind. Meanwhile, this hypothesis is formally capable 

 both of accounting for most, if not all, of the known kinds of 

 facts and of leading to testable predictions. 



One aspect of the breeding results still remains as puzzling in 

 the extreme. This is the contrast between the high viability after 

 selfing and outcrossing and the low viability after crosses between 

 sib clones. I cannot suggest a satisfactory explanation, but atten- 

 tion might be called to one point: selfing occurs between two 

 types which, on my hypothesis, are necessarily as much alike in 

 cytoplasmic constitution as two different mating types can be: 

 the most concentrated cytoplasmic differentiator in each is the 

 same as the second most concentrated differentiator in the other. 

 How this could influence the viability of the resulting synclones 

 is far from clear, but it would be of interest to compare mortality 

 in sib crosses and outcrosses between clones that have this sort 

 of cytoplasmic relation and clones that are more diverse in cyto- 

 plasmic constitution. 



Evolution, Speciation and the Breeding System. The evolu- 

 tionary relation between the systems of mating type determina- 

 tion in P. bursaria and P. aurelia are close, and the differences 

 are simple. In principle, the system in P. bursaria is the most 

 complex, the one in the group A varieties of P. aurelia is the 

 simplest, and the one in the group B varieties is intermediate. 

 The single unique feature in P. bursaria is the regularity of mas- 

 sive cytoplasmic exchange between mates. In the group B varie- 

 ties of P. aurelia this feature occurs only in a small minority of the 

 conjugating pairs. In group A, the cytoplasmic role in mating 

 type determination is inapparent because of the lack of evidence 

 of flux equilibrium and the overriding influence of temperature 

 and of a genotypic action which is the same in both mating types. 

 All other features are found developed to varying degrees in 

 both species: caryonidal differences, nuclear differentiation, cyto- 

 plasmic differentiators and the genes affecting their concentra- 

 tion, dual and multiple mating type systems, and mating type 

 differentiation sometimes associated with and sometimes inde- 

 pendent of a preceding fertilization. 



