T. M. SONNEBORN 309 



and 2. Variety 1 with seven mating types shows the P. aurelia 

 group A system of mating type determination. A pair of alleles 

 limiting mating type potentialities has been found. This variety 

 has an immature period and is heterozygous in nature. It shows 

 hybrid vigor and inbreeding degeneration, but normal fertile 

 inbred lines can be selected eventually. During meiosis in crosses 

 of wild strains evidences of chromosomal aberrations are appar- 

 ent. The possible relation of these and the nonviability after 

 conjugation to age-induced chromosomal aberrations has not 

 been studied. Much selfing occurs during maturity in variety 1 

 when cultivated with excess food, but this is greatly reduced by 

 periodic starvation. In nature it therefore is probably not very 

 common. Taking all these features into account, variety 1 appears 

 to have an intermediate breeding system. It has many outbreed- 

 ing stigmata: immaturity, multiple mating types, natural hetero- 

 zygosity, and inbreeding degeneration. But it also has some 

 inbreeding features: distribution is concentrated in a narrow 

 range of latitude; synclones almost always include more than 

 one mating type; normal fertile inbred lines are obtainable; self- 

 ing occurs; and structural chromosomal differences between 

 natural strains lead to abnormality and death when they are 

 crossed. Some of these inbreeding features are to a degree com- 

 pensated by outbreeding features: synclonal nonuniformity of 

 mating type by immaturity and selfing by the stabilizing effect 

 of starvation. Others, such as chromosomal aberrations may be 

 peculiarities of aging. On the whole, therefore, variety 1 seems 

 to be primarily an outbreeder, but less extremely so than P. 

 bursaria. It has latent mechanisms for inbreeding, which it would 

 appear could easily give rise to inbreeding descendants. 



Variety 2 is better adapted to outbreeding and shows more 

 stigmata of that system of breeding. It has an immature period, 

 the largest number of mating types (11), the widest latitudinal 

 range, and synclonal uniformity of mating type due to the P. 

 bursaria form of the P. aurelia group B mechanism of mating 

 type determination. Little is known about the breeding systems 

 in the other varieties. Crosses between different populations 

 within variety 5 and within variety 9 yielded no survivors; the 



