IIFF CYCLES, SEXUALITY, AND SEXUAL MECHANISMS 61 



various individuals at the single incompatibility locus, and mating 

 occurs readily between any two haploid strains which carry different 

 alleles (Brunswik, 1924; BuUer, 1924; Vandendries, 1923; White- 

 house, 1949). Thus in lieu of the single pair, A and a, these forms 

 each comprise an extended series of mating types which may best be 

 designated by A^,. A~, A\ A'', • • • , A". This pattern of mating-type 

 determination has been termed bipolar sexuality. 



hicovipatibility Factors at Tido Loci. The final pattern of 

 obligatory^ interstrain mating behavior to be described is one found 

 only among the Basidiomycetes, exclusive of the rusts and related 

 groups. It involves mating-type determination by incompatibility 

 factors at t^'o loci; for example, the diploid condition may be desig- 

 nated A^A~B'B~, and independent assortment of these factors at 

 meiosis yields progeny of four mating types, A^B\ A^B^, A^B\ and 

 A^B^. Aiating occurs only in those combinations having different 

 alleles at both loci, for example, A'B' x A'B' and A'B' x A'B' 

 (Brunswik, 1924; Hanna, 1925; Kniep, 1920; Mounce, 1922, 1926). 

 This pattern of segregation was first described by Kniep about 1920 

 and was termed tetrapolar sexuality. 



In tetrapolar species, as in the bipolar forms discussed earlier, 

 the total number of mating types in the population is increased tre- 

 mendously by the occurrence of multiple alleles at both loci (Kniep, 

 1922). The number of equivalent alleles at each locus, however, ap- 

 pears to be consistently different in two large groups of Basidiomy- 

 cetes, the Gasteromycetes, which includes the "puffballs," and so 

 on, and the Hymenomycetes, which includes the "mushrooms," 

 "bracket fungi," and the like. In the former group about 10 alleles 

 at each locus has been indicated as the extent of the series (Fries, 1940, 

 1943), whereas in the latter group as many as 27 alleles at each locus 

 have been demonstrated (Brunswik, 1924; Fries and Janasson, 1941; 

 Kniep, 1922) and the minimal total number of alleles at each locus in 

 the population has been estimated to be of the order of 100 (White- 

 house, 1949). In all cases the alleles at each locus appear to be physi- 

 ologically equivalent. The device of multiple incompatibility factors 

 allows for almost complete outbreeding while maintaining inbreeding 

 at 50 per cent in bipolar forms and at 25 per cent in tetrapolar forms. 



Kniep, in his original work on tetrapolarity, observed that "muta- 

 tions" occurred at the A and B loci in frequencies of about 2 per cent 

 and slightly less than 1 per cent, respectively (Kniep, 1923). These 



