318 GENETICS OF FUNGI 



one-celled structure called the zygote. The most important part 

 of each gamete is the nucleus. Gametes of fungi are not known to 

 possess any appreciable or functional cytoplasm, such as is known 

 to occur among seed plants. Undoubtedly gametes of fungi 

 possess functional cytoplasm, but as yet proof of cytoplasmic in- 

 heritance either is not forthcoming or is meager. 



Each gametic nucleus is constituted of chromatic materials, the 

 chromosomes. Each zygote contains In chromosomes and is 

 therefore diploid. As a rule, however, among the Basidiomycetes 

 and Ascomycetes, when the two gametic nuclei are brought into 

 juxtaposition, they do not fuse immediately but remain as a pair. 

 Then the two divide at one and the same time, the process being 

 called conjugate nuclear division and giving rise to two daughter 

 pairs. Hundreds or even thousands of successive conjugate nu- 

 clear divisions may follow, extending in time over a period of 

 weeks or months. Finally two such paired nuclei come to lie in 

 special cells (basidia in the Basidiomycetes, teliospores in the 

 Uredinales, chlamydospores in the Ustilaginales, young asci, asco- 

 gonia, or ascogenous hyphae in the Ascomycetes), where they 

 actually fuse. 



The fusion nucleus resulting contains 2/7 chromosomes and is 

 thus diploid. Shortly after fertilization the fusion nucleus divides 

 twice. The processes involved in these two divisions constitute 

 meiosis. In one of the divisions the number of chromosomes is 

 reduced to 77, and the other division is homotypic (equational). 

 Sex factors are segregated during meiosis. Each of the four nuclei 

 resulting from meiosis contains n chromosomes, the haploid num- 

 ber. In the Basidiomycetes each of the four nuclei migrates into 

 a developing basidiospore, which is a haploid cell. When these 

 basidiospores germinate, they produce haploid mycelia; if two 

 such mycelia or their equivalents of opposite sex fuse, cells again 

 containing a conjugate pair of nuclei arise. In the Ascomycetes 

 each of the four haploid nuclei again undergoes a mitotic division, 

 whereupon each of the eight haploid nuclei becomes invested with 

 a wall and is an ascospore. The germination of ascospores gives 

 rise to haploid mycelia, and the nuclei may again become paired 

 in preparation for fusion within the ascogonium, young asci, or 

 ascogenous hyphae, as the case may be. 



