72 SEX IN MICROORGANISMS 



proceeding from primitive to more highly speciaHzed groups. The 

 haploid cycle predominates in the Phycomycetes, the haploid with 

 restricted dicaryon cycle in the Ascomycetes, and the haploid-dicar- 

 yotic and the dicaryotic cycles in the Basidiomycetes. The exceptions 

 to this generalization, however, are numerous, and, when considered 

 in respect to probable phylogenetic lines, they are more than a little 

 puzzling. Haploid-diploid and diploid cycles, those cycles which 

 would seem to be the most highly advanced of all, occur only in one 

 group of aquatic Phycomycetes and in a number of yeasts. 



The pattern of sexuality in heterothallic species shows a similar 

 progression. The role of sexual factors as the critical determinants 

 of mating behavior is for the most part limited to the more primitive 

 forms, particularly the aquatic Phycomycetes, although there are 

 several cases of strict sexual dimorphism among the Ascomycetes. A 

 single pair of incompatibility factors at a single locus possibly occurs 

 in the more complex Phycomycetes, the Mucorales, is very common 

 among the Ascomycetes, and is frequently encountered in two large 

 groups of Basidiomycetes, the rusts and smuts. The essentiality of 

 differentiated sexual organs would seem to follow similar broad phy- 

 logenetic lines: they are present and functional in practically all 

 Phycomycetes and most Ascomycetes, except the yeasts, and absent 

 in the Basidiomycetes, except the rusts. 



Multiple incompatibility allelism is known only among members 

 of the most highly evolved fungi, the Basidiomycetes, and is unques- 

 tionably the most efficient of all means to insure for those species 

 possessing it the maximal benefit to be derived from genetic recom- 

 bination. 



This might suggest a sort of coupling of the culmination of 

 incompatibility control of mating behavior with a high degree of 

 morphological development, particularly in the tetrapolar species, 

 were it not for the fact that species which are obviously closely 

 related to such tetrapolars are strictly homothallic and get along quite 

 nicely with no restrictions imposed by incompatibility factors. 



Sexual mechanisms and developmental histories are fairly con- 

 stant within groups at the level of orders. There is also a tendency, in 

 passing from primitive to highly evolved forms, to progress from 

 gametic copulation through the loss of gametic differentiation in one 

 sex or the other (gamete-gametangial copulation), to loss of gametic 

 differentiation in both sexes (gametangial copulation), to the loss of 



