328 GENETICS OF FUNGI 



in contact, hvphal fusions occur, and the mycelia become, in con- 

 sequence, dicarvotic (two-nucleate). The nuclei become associ- 

 ated in conjugate pairs of (//) + (7;) chromosomes. Then, as the 

 dicarvotic mycelium continues to grow, conjugate nuclear divi- 

 sions occur, but with each conjugate division a clamp connection 

 separates the two pairs of daughter nuclei. Finally a conjugate 

 pair is delimited in each basidium. Here they fuse, whereupon 

 meiosis occurs, and each resulting haploid nucleus migrates into a 

 developing basidiospore. 



Up to this point observations are quite in accord. Kniep (1919, 

 1922) found in Schizophyllum commune and Aleurodiscus poly- 

 gonius that sometimes two of the tetrad of basidiospores were of 

 one sex and two of the other sex, although each species is normally 

 quadrisexual, that is, quadripolar or quadripotential. In explana- 

 tion he proposed that, when the abnormal situation obtained, dis- 

 junction of sex occurred in the first division. The quadrisexual 

 situation he explained by assuming that sex is determined by two 

 pairs of allelomorphic factors, which segregate independently of 

 each other during the second division. 



In Coprimis rostmpiamis Newton (1926) found only two kinds 

 of spores in each basidium, two (A) spores and two (a) spores, in 

 which case sex is determined by one set of factors. In C. lagopus 

 she (1926) found, however, as had Kniep, that sex is determined 

 by two pairs of linked factors, so that the nucleus of the primary 

 basidium has the constitution AaBb. The basidiospores then can 

 be (1 ) AB, Ab, aB, and ab\ or (2) two AB and two ab\ or (3) two 

 Ab and two aB. Similar results have been obtained by others with 

 Hypholoma fascicular -e and Colly bia velutipes. Newton analyzed 

 42 tetrads, 25 of which were of the first of the 3 types, 9 of the 

 second, and 8 of the third. 



Brunswik (1924) analyzed 93 tetrads of Coprimis fimetarius 

 (lagopus) with these findings: 37 gave all four types of spores AB, 

 Ab, aB and ab\ 29 gave the two types AB and ab; 27 gave the two 

 types Ab and aB. 



These data, together with those of other observers [Buller 

 (1931)], show that sexuality is both bipolar and tetrapolar among 

 Basidiomvcetes. The mechanism of these patterns of behavior, 

 as Dodge ( 1940) indicates, is readily explainable if we assume that 

 the genotypes of the parental nuclei in the matings made by New- 

 ton and by Brunswik were either AB X ab or Ab X aB. This 



