HOMOTHALL1SM AND HETEROTHALLISM 323 



Another type of evidence has been provided by Dodge (1927) 

 from cytological study of N. tetrasperma. In this species the 

 spindle of the first nuclear division is longitudinal with respect 

 to the ascus, and the two daughter nuclei come to lie one above 

 the other in the ascus. At the second division two types of posi- 

 tion and orientation of the spindle may occur. The spindles may 

 lie approximately parallel, perhaps just slightly oblique to the 

 long axis, or else are aqain longitudinal. The spindles of the 

 third division are nearly transversely oriented, bringing non-sister 

 nuclei into symmetrical arrangement. On delimitation of the 

 spores two non-sister nuclei are included in each ascospore, 

 whether segregation takes place in the first, second, or third 

 division. 



A somewhat different explanation may account for the situation 

 in PI enrage cms er in a [Dowding (1931)]. She found that the 

 paired dwarf spores are always of opposite sex. This discovery, 

 together with the fact that normal spores are always bisexual, 

 indicates that the ( + ) and (-) nuclei are arranged alternately 

 at time of spore formation. This alternate arrangement might be 

 taken as prima facie evidence that sex segregation occurs at the 

 third division. Dowding indicates, however, that the ascus 

 is so wide that there is opportunity for the nuclei or even the 

 young spores to slip by each other, so that the final arrangement 

 f (_j_) a nd ( — ) nuclei could permit segregation of sex factors 

 at any of the divisions. 



Lindegren ( 1929) found that the ratio of first-division to second- 

 division segregation f sex in Nenrospora crassa is 8:15. Later 



CO L 



(1936, 1939) he determined that the gene for sex is linked with 

 other factors and, by determining crossing-over percentages, was 

 able, for the first time with fungi, to construct chromosome maps. 

 These data provide an explanation of the mechanism involved 

 and appear to prove that the chromosomes disjoin at the first divi- 

 sion and that the factors are segregated at the second division. 

 Lindegren also emphasizes that pure lines of fungi must be ob- 

 tained by inbreeding as stock for genetical studies. Such stocks 

 also serve best for experimentation on interspecific hybrids, one 

 of which was secured by Dodge (1928) by crossing the eight- 

 spored N. sitophila with the four-spored N. tetrasperma. 



Dimock (1939) hybridized strains of Hypomyces ipomoeae 

 obtained by isolating single ascospores. From these isolations 



