MM CVCIA S, SFXUALI rV, AND SIXUAL AlECHANISiMS 



55 



clear distinction is impossible at the present time unfortunately does 

 not excuse him from making the attempt. Sexual factors are those 

 genetic determiners which characteristically result in morphologically 

 distinguishable S and 9 plants, or A and 2 sexual organs or gametes 

 or both. The criteria for the designation of S and 9 organs or cells 

 or both are largely borrowed from mammalian reproductive processes 

 and include relative size, inclusion of reserve food materials, motility, 



B/IS/C PATTERNS OF SEXUALITY IN FUNGI 



'MEIOSIS 



Spore Types: 

 Sex 



Compatibility 



Somotic 



Copulation 



Sexual Organs 

 or Cells 



HOMO- 

 THALUCI 



NUCLEAR 

 <=^=FUSION 



(±) 



d9 



(-) (+) 



cf9 



{-) (+) 



(-) {+) 



HETEROTHALLIC 



^^^^^^9 



6^d^ 



-9*9 



m 



I I 



A a 



A A 



9c5'd"9 



12 



9d'd'9 

 I I 11 



A A a a 



9^(59 



A 



21 



ABAbabaB 



Fig. 3. The genetic devices that underlie the seven distinct patterns of 



sexuaHty in fungi. 



All individuals of a species of tvpe O are alike and functionallv hermaphroditic; 

 individuals of species of types I to VI arc divided, by sexual or incompatibilitv 

 differences, into two or more distinct mating strains among which cross-breeding 

 is obligatory. Multiple allelic series at the incompatibility loci commonly occur in 

 types \" and M, and the number of distinct mating strains of a single species of 

 the latter type may be of the order of ten thousand. 



and particularly the direction of nuclear migration (or transport) in 

 fertilization. Incompatibility factors, by contrast, are those extra- 

 sexual genetic determiners of mating capacity which operate either 

 in addition to or in the absence of sexual factors. The difficulty of a 

 sharp, universally applicable distinction between the two arises pri- 

 marily from the occasionally known occurrence of heterothallic spe- 

 cies in which demonstrable sexual differences exist in the absence of 

 morphological differentiation. The best known example is the com- 



