60 SEX IN MICROORGANISMS 



incompatibility types of course, yield the various classes of progeny 

 predicted on the basis of the random assortment of the determining 

 factors at the two loci and non-disjunctive doubling of the chromo- 

 somes carrying the sexual factors. 



It is expected that this complex type of sexuality, with or without 

 the non-disjunctive feature or other complications, will eventually be 

 found in other species. 



hicompatibilhy Factors at a Single Locus. This and the suc- 

 ceeding pattern of sexuality involved no sexual factors and no dif- 

 ferentiated sexual organs. Mating is commonly reciprocal, and in 

 multicellular organisms any cell of the thallus is potentially capable 

 of donating a fertilizing nucleus and of accepting a fertilizing nucleus 

 from the mate. The term somatic copulation — "Somatogamie" (Ren- 

 ner, 1916) — has been applied to this type of sexual fusion. 



Species displaying sexuality of this type can be subdivided into 

 two classes according to the number of alternate allelomorphs at the 

 incompatibility locus. 



In species of certain groups a single pair of alleles determines 

 mating type; all individuals of each species therefore belong in one or 

 the other of two mating categories, commonly designated A and a. 

 The heterothallic yeasts are the best known examples of this pattern 

 (Winge, 1935, 1944; Winge and Laustsen, 1939), and a recent review 

 of the sexuality of the heterothallic smuts indicates basic control of 

 mating behavior in this group by a one-locus, single-allelic-pair 

 mechanism (Whitehouse, 1951). 



Mutations at the incompatibility locus in certain yeasts (Linde- 

 gren and Lindegren, 1944; Winge, 1944) may be considered slight 

 deviations of this pattern and may possibly indicate the mode of 

 origin of the following pattern. Either A or a may occasionally 

 mutate to altered states which permit fusion and ascus production 

 within a single clone. The ascospores of such unions, however, have 

 low viability. This possibly reflects either a lack of equivalence be- 

 tween the mutated alleles and the originals or the expression of delete- 

 rious, semisterility factors in the homozygous condition (Catcheside, 

 1951). 



What would appear to be a much more highly evolved pattern of 

 single locus control of mating behavior is characteristic of many 

 Basidiomycetes, exclusive of the rusts and smuts. In these forms a 

 very large number of completely equivalent alleles may be found in 



