T. M. SONNEBORN 269 



show that they are similarly induced in P. hursaria. The impor- 

 tant question therefore arises as to the extent to which age may 

 be a factor in the observed exconjugant mortality and in the 

 observed chromosomal aberrations. Until this is cleared up, any 

 conclusions as to the breeding system based on material in this 

 section, except the inbreeding studies on variety 1, would be 

 premature. The alleles for mating type potentiality discovered 

 within a single population are good evidence that some degree 

 of heterozygosity occurs within a population. The observation of 

 inbreeding degeneration followed by selection of fully normal 

 lines also probably indicates genie heterozygosity, though part of 

 these results might be consequences of age-induced chromosomal 

 aberrations in the starting material. 



When we now look back over the various features that have 

 been discovered in T. pyriformis and ask ourselves whether this 

 organism is an inbreeder or an outbreeder, the situation seems 

 confused. The organism clearly has some outbreeding features, 

 and it shows evidence that they work. Among them are the 

 usual ones: a period of immaturity, multiple mating types (in 

 most varieties), a very long life cycle, and low viability after 

 inbreeding. The finding of heterozygosity for genes within a 

 single local population is good evidence that outbreeding does 

 in fact take place. On the other hand, inbreeding features also 

 seem to be present. Among them, the most outstanding are (1) 

 the occurrence of selfing clones in considerable proportions and 

 (2) the method of mating type inheritance in variety 1, which 

 assures the presence in most clones of animals capable of mating 

 with each other. 



However, these various aspects of the biology of the organism, 

 as discovered in the laboratory, must be considered in the light 

 of conditions in nature. Three factors tend to reduce the impor- 

 tance of the inbreeding features. First, the likely recurrence of 

 starvation in nature would tend to reduce, if not prevent, the 

 development of selfing clones during early maturity. Second, 

 although mating type inheritance provides potential mates among 

 close relatives in variety 1, the immature period permits time to 

 spread and so reduces the chance of mating with closely related 



