62 SEX IN MICROORGANISMS 



changes in the incompatibiUty alleles, repeatedly observed by various 

 workers, however, seemed to occur only in the germling mycelia 

 recently derived from basidiospores, never in wxll-established mycelia. 

 It has recently been shown beyond any reasonable doubt, by Papazian 

 (1951), that the A factor consists of a number of pseudo-alleles at 

 closely linked but distinct loci, that these act together as a physiolog- 

 ical unit, and that occasional recombination at meiosis yields new 

 factors. These data are compatible with a composite factor comprising 

 4 to 10 distinct loci, the upper limits of which range (> 6) 

 could account for the estimated factors in natural populations without 

 necessary recourse to multiple allelomorphic series at any locus 

 (Raper, 1953). 



Practically all species of fungi w^iich have sexual cycles may be 

 definitely assigned to homothaUism or heterothallism. There are, 

 however, a few species that appear to occupy positions which are 

 intermediate between these two opposed conditions. The yeasts, men- 

 tioned above, which are basically heterothallic but which produce 

 frequent, low-viability mutants of the incompatibility factors may be 

 considered to bridge, to some extent, the gap between true hetero- 

 thallic and homothallic conditions. Mather (1940) has suggested the 

 term partial heterothallism for cases of this type. 



In a few cases, furthermore, indeterminate patterns of sexuality 

 appear to be more closely allied with homothaUism. Outstanding 

 among such forms is the Ascomycete Glovterella cingjilata, a sexually 

 ambiguous species without peer. Edgerton (1914) described a strong 

 sexual interaction in this species between weakly self-fertile strains. 

 Subsequent and intensive work with G. cingiilata (Andes, 1941; 

 Edgerton, 1945; McGahen and Wheeler, 1951; Wheeler, 1950; 

 Wheeler and McGahen, 1952) has revealed an extremely complicated 

 pattern of sexuality which results from the interaction of numerous 

 genetic factors, some exhibiting high mutation rates. As currently in- 

 terpreted (IMieeler and McGahen, 1952), two loci, A and B, are con- 

 sidered primarily responsible for the basic sexual characteristics, with 

 some twenty other loci modifying the sexual reaction. Two mutant 

 states are known at each of the two primary sexual loci in addition 

 to the t^\ o wild-type alleles. Thus all combinations between the 

 three alleles at the two loci, ^+, A\ and A^ and B+, B\ and B\ deter- 

 mine nine distinct strains, each having a characteristic pattern of self- 

 sterility or self-fertility on the one hand and interstrain matings on 



