T. M. SONNEBORN 267 



same. However, the degree to which gene flow occurs or is possi- 

 ble between different populations is not yet well worked out in 

 T. piriformis. Some further facts bearing on this will be brought 

 out in the following section. 



Breeding Systems. Some information is available with respect 

 to viability and variability of exconjugants after crosses between 

 representatives of a single local population of variety 1 and, with 

 respect to viability alone, after crosses between representatives 

 of different local populations of varieties 5, 6, and 9. Convincing 

 evidence that natural strains of variety 1 are highly heterozygous 

 has been obtained by Nanney, Caughey, and Tef ankjian ( 1955 ) 

 and by Nanney ( 1956a and unpublished ) . Among the sexually- 

 produced descendants of two individuals of complementary mat- 

 ing types that had been isolated from the same natural source, 

 two alleles for mating type potentialities segregated. The anal- 

 ysis likewise yielded evidence of polygenic differences controlling 

 the production of the semi-amicronucleate condition, other de- 

 generative conditions, and low viability in general. Two of the 

 segregant families were further inbred separately. In spite of 

 selection, viability of exconjugants in each inbred family oc- 

 curred in lower and lower frequencies in successive inbred 

 generations; but then selection resulted in improvement, and 

 by the sixth inbred generation it was possible to select a strain 

 which gave 100% viability at conjugation. Usually, crosses be- 

 tween the different inbred lines, at the stage when they were 

 giving poor survival and much abnormality, yielded more normal 

 sets of exconjugants. 



These results of Nanney show that variety 1 of T. pyriformis 

 differs from variety 1 of P. bursaria in the effects of inbreeding. 

 In the latter it seems to lead quickly to extinction, but in T. pyri- 

 formis completely normal and viable inbreeding lines can even- 

 tually be selected. 



Crosses between representatives of different local populations 

 were made in varieties 5 and 6 by Gruchy ( 1955 ) and in variety 

 9 by Ray (1955). In the single parental combination tested for 

 each variety, all the exconjugants of varieties 5 and 9 died, but 

 viable exconjugants were obtained from the cross in variety 6. 



