LIFE CYCLES, SEXUALITY, AND SEXUAL iMECHANISIMS 



63 



the other. The major characteristics of these several strains and the 

 complex pattern of interstrain matings are diagrammatically rep- 

 resented in Fig. 5. The sexual characteristics of the various strains may 

 be further modified by mutations at loci other than the primary 

 sexual loci, A and B, and two of these, F' and st^, have been described 

 in detail (Wheeler and McGahen, 1952). Each of these mutants im- 

 poses self-sterility upon each of the normally self-fertile strains but 



SEXUALITY IN GLOMERELLA 



A'B' 



Introstraln Reaction 

 Circled- Selt-fertile 

 Not Circled- Self- sterile 



Interstrain Reaction 

 ^= Very Heavy 

 -= Heavy 



— Light 



— Weak, Uncertain 



Fig. 5. Intrastrain fertility and interstrain reactions in Glo?nerella cingulata. 



Three allelomorphs at each of two loci, A and B, define nine strains, each having 

 a distinct pattern of morphological and sexual characteristics. (Diagramed from 

 data of Wheeler and iVIcGahen, 1952.) 



does not interfere with interstrain reactions provided that the two 

 mates carry neither of the mutations in common. 



According to the definitions of homothallism and heterothallism 

 adopted here, Glovterella cingulata must be considered basically a 

 homothallic species since the self-sterile strains are in each case derived 

 through degenerative, mutative changes from self-fertile, wild-type 

 strains. The enhancement of sexual productivity in interstrain con- 

 trasts between self-fertile strains and the occurrence of self-sterile but 

 cross-fertile strains constitute a pattern that falls short of the totality 

 of self-sterility and obligatory cross-breeding of true heterothallism. 



