124 



CHAPTER 9 



The red bread mold Neurospora ma) 

 provide critical evidence as to the time of 

 crossing over. Recall that in the sexual 

 process, so-called "fruiting" bodies arc 

 formed composed of cells containing two 

 haploid nuclei, each of which was derived 

 originall) from a different parent (Figure 

 9 8). Two such haploid nuclei fuse to 

 form a diploid nucleus containing seven 

 pairs of chromosomes, and the cell elon- 

 gates to form a sac. The diploid nucleus 

 immediately undergoes meiosis, as shown in 

 the figure, so that the four haploid products 

 are arranged in tandem; that is. the two 

 top nuclei come from one first-division nu- 

 cleus, the bottom two from the other first- 



Q b 



X 



DIPLOID NUCLEUS 



DIPLONEMA 



D AFTER 



FIRST DIVISION 



FOUR 



MEIOTIC 



PRODUCTS 



JO I TWO HAPLOID NUCLEI 



DIPLOID NUCLEUS 



DIPLOID NUCLEUS 



FIRST 



MEIOTIC DIVISION. 



SECOND 



MEIOTIC DIVISION 



MITOTIC DIVISION AND 

 SPORE FORMATION 



EIGHT SPORES 



figure 9-9. Chiasma and crossing over in 



Neurospora. 



figure 9-8. Meiosis in Neurospora. 



division nucleus. Since each haploid nu- 

 cleus subsequently divides mitotically once, 

 each meiotic product is present in duplicate 

 within the ascus. Each haploid ascospore 

 can be removed from the ascus, grown in- 

 dividually, and its genotype determined di- 

 rectly. In this organism, then, all of the 

 meiotic products derived from a single dip- 

 loid nucleus can be obtained and identified. 

 Using the symbols in Figure 9-6, let us 



