HEREDITY IN SOMATIC CELLS 363 



duces uninucleate, asexual pores. When these germinate, hyphae and 

 eventually a mycelial mat are formed, as in Neurospora. The hyphae of 

 two genetically different strains may fuse together and contribute their 

 nuclei to a common mycelium. Since the cells making up this mycelium 

 have many nuclei of both genotypes in a common cytoplasm, the new 

 strain is a heterokaryon. A heterokaryon of Aspergillus nidulans forms 

 asexual spores which are not heterokaryotic. Each contains a single 

 nucleus of one or the other type, with the result that strains resembling 

 the parents can be re-established. A stable heterokaryon can be formed 

 by mixing two strains with alternate nutritional requirements. In this 

 event, the two types of nuclei in a common cytoplasm complement one 

 another, allow the synthesis of the required substances, and enable 

 growth on minimal medium. 



A heterokaryon between, say, a yellow para-amino benzoic acidless 

 ( y paba) and a white adenineless {w ad) strain possesses nuclei differing 

 both in the chemical site of pigment production and the growth factor 

 required. It will form a colony on minimal medium but, when spores 

 develop on the colony surface, they will not contain the wild-type pig- 

 ment because they are uninucleate, and spore color is an autonomous 

 character. But once in a while the colony will show a sector with green 

 wild-type spore pigmentation (Figure 12.5). Individual uninucleate 

 spores isolated from such a sector will grow on minimal medium and 

 develop into uniformly green colonies. These spores turn out to be 

 diploid and heterozygous for the parental chromosomes. Complementa- 

 tion took place between the w^ and t/"*" genes for pigment production 

 and allowed identification and selection of the diploid tissue. Alterna- 

 tively, diploids may be selected by plating the asexual spores from such a 

 heterokaryon onto minimal medium where only those with diploid nuclei 

 will form colonies with, of course, green pigmentation. Such selective 

 procedures allow the frequency of diploids of spontaneous origin to be 

 estimated at I0~®; when a heterokaryon is exposed to ultraviolet light, 

 this frequency may be, for unknown reasons, raised to 10"". Although 

 chromosome counts are difficult in Aspergillus, the diploid nature of 

 these spores may be ascertained in a number of ways: their volume and 

 DNA content are twice those of haploid spores, and the parental genes 

 can be recovered after meiosis. Aspergillus, unlike Neurospora, does not 

 have two mating types but any strain can form fertile fruiting bodies by 

 itself. Within these structures any of the nuclei present may fuse to 

 form a zygote nucleus which promptly undergoes meiosis. In the fruit- 

 ing body of the diploid strain when two nuclei fuse, a tetraploid zygote 

 is formed; this, at meiosis, gives rise to some haploid progeny, among 

 which combinations of the parental genes are represented. 



