VARIATION AND INHERITANCE 409 



spore haploid cultures from the same fruit body fell into two compatibility- 

 groups. Such a condition is described as bipolar. Diploid mycelium 

 was formed only when two haploid mycelia of different compatibility 

 groups were paired. If A and a represent the genes for compatibility, 

 the combination of Aa would be necessary for the formation of diploid 

 mycelium. AA and aa would be incompatible. 



^\^lile the single-spore cultures of a single fruit body of Exidiaglandu- 

 losa give rise to two compatibility groups A and a, a second fruit body 

 collected at some distance away may give rise to haploid mycelia which 

 apparently fall into the same groups {A and a). Yet we may find that 

 all the haploid mycelia of the first fruit bod}^ are compatible with all the 

 haploid mycelia of the second fruit body. In other words, the two groups 

 of the second fruit body are slightly different from the two groups of the 

 first fruit body. It is, therefore, likely that numerous compatibility 

 groups exist, even though only two occur in any one fruit body. Com- 

 patibility, in this case, is apparently determined by multiple alleles. The 

 existence of geographic races has been described for a number of fungi 

 by Buller (1941) and others. 



A somewhat different situation exists in Collybia velutipes and a number 

 of other species. It has been found that each spore on a basidium may 

 differ in its compatibility factors. Compatibility in this case is deter- 

 mined by two pairs of genes on different chromosomes. These groups 

 are usually designated as AB, Ah, aB, and ah. The combination of 

 AaBb is then necessary for compatibility and formation of diploid 

 mycelium. 



Compatibility in itself does not necessarily indicate that fertile fruit 

 bodies will be formed. For instance, some of the single-spore isolates 

 of Lenzites trahea were found to produce fertile fruit bodies, while other 

 cultures failed to do so (Barnett and Lilly, 1949). By pairing compatible 

 fruiting isolates and also the compatible nonfruiting isolates, it was possi- 

 ble to establish a correlation between the fertility of the diploid mycelia 

 with that of the haploid "parents." It seems probable, therefore, that 

 the ability to produce fertile fruit bodies has a genetic basis, in addition 

 to that of compatibility. 



In Schizophyllum commune the ability to produce normal fruit bodies is 

 dominant over the formation of abnormal, knot-like fruit bodies (Zatler, 

 in Buller, 1941). If G represents the factor for normal fruit bodies and 

 g the factor for knot-like fruit bodies, the results could be expressed as 

 follows: G crossed with G or G crossed with g gives normal fruit bodies, 

 while g crossed with g gives knot-like fruit bodies. Zatler also showed 

 that in Collybia velutipes inheritance of pigmentation of his cultures was 

 due to two genes located on different chromosomes. A combination of 

 the two dominant factors in the haploid mycelium resulted in a deep 



