INCOMPATIBILITY 



allelomorphs. And, although closely related, the 60 cultivated 



varieties of Sweet Cherry examined must have over 20 allelomorphs. 



The S gene tells us many things of importance. Its large number 



of allelomorphs is without parallel. The specificity of their action 



04 0-6 0-8 



AVAILABILITY OF NON-SISTERS 



Fig. 61. — Hetcrothally is an outbreeding mechanism in fungi. The proportion of 

 sister haploids (i.e. from the same diploid parent) available for mating is plotted 

 against the proportion of non-sister haploids available for mating. Thus with two 

 loci each of three allelomorphs, ] of the sisters will be available but ^ of non-sisters. 

 A line projected from the origin through this point shows that the outbreeduig 

 bias, found as the ratio of availability of non-sisters to availability of sisters, is 

 ^ -^ l or 1-78. The maximum outbreeding bias with one locus is 2, with two 

 loci is 4, and with three loci is 8. Nothing short of intervention of diploid tissue, 

 as in incompatibility in flower plants, can give an outbreeding bias of oo. 



in the style is nearly always complete, since S^ pollen fails on a style 

 carrying S^ no matter what the other allelomorph may be. In some 

 cases this specificity extends to the strength of action, for some 

 allelomorphs are stronger than others whatever others are present, 

 though in other cases the allelomorphs can strengthen one another's 



247 



