70 CELL HEREDITY 



This fungus consists of microscopic hyphal threads which, during 

 asexual growth, branch, fuse, and spread to form a mycehal mat. Some 

 hvphae form asexual spores which can germinate to give rise to a new 

 mycehum. There are two mating types, mt^ and mt~ (sometimes called 

 A and a); when they fuse, so-called sexual spores are formed in a special 

 organ called the fruiting body (Figure 3.2). These spores are the equiv- 

 alent of the gametes of Chlamijdomonas and, upon germination, they 

 give rise to a mycelium. Neurospora is usually orange pink in color, but 

 albino mutants occur. When the two are crossed, there is a 1:1 segre- 

 gation of pink and white types among the sexual spores, similar to the 

 segregation patterns of Chlamydomonas. How are these 1:1 ratios 

 obtained? 



The answer to this question lies in the behavior of the chromosomes. 

 During the formation of gametes in most organisms the chromosomes 

 segregate in two special divisions known as meiosis. Photographs of 

 meiosis in the corn plant, Zea mays, are shown in Plates V, VI, and VII. 

 Figure 3.3 shows a diagram of meiosis in Neurospora. The maternal and 

 paternal nuclei each contain one set of chromosomes and are called hap- 

 loid. When they fuse, the zygote nucleus so formed has both sets and is 

 called diploid. The haploid number of chromosomes in Neurospora is 

 seven; each is different in structure from the others. It is not known 

 exactly when the chromosomes duplicate before meiosis in Neurospora, 

 but during prophase in the diploid zygote nucleus each maternal 

 chromosome pairs with its paternal homologue. The result is seven 

 bivalents whose centromeres have not divided although the chromosomes 

 have. (The centromere is that structure in the chromosome concerned 

 with anaphase movement.) Thus each bivalent is composed of four 

 strands or chromatids but only two centromeres, one for each parental 

 homologue. The bivalents arrange themselves on the spindle at meta- 

 phase as tightly coiled structures; the centromeres, each with their two 

 associated chromatids, pass to opposite poles at anaphase. In this way 

 the homologous pairs of chromosomes are segregated. In the succeeding 

 interphase the chromosomes do not duplicate again, but in the second 

 meiotic division the centromeres divide, and the two chromatids which 

 were already doubled in the prophase of the first meiotic division are 

 separated. The result is four daughter nuclei, each haploid. If the 

 chromosomes carry genes, it is clear that the orderly process of meiosis 

 would segregate them in a 1:1 ratio, according to Mendel's first law. In 

 Neurospora the nuclei produced by meiosis are arranged in the ascus 

 sac in the order of their formation. Therefore, the four genes for pink 

 color are in the nuclei of the spores at one end, and their alleles for 

 albinism are at the other end. 



When the spores germinate, the nuclei they contain divide by an 



