Figure 1. Asexual (inner) and sexual (outer) cycles of N. crassa. The sexual spores, or conidia, function in the asexual cycle as vegetative propagules, and in the sexual cycle as fertilizing agents. All matings require the participation of both mating types, A and a, but either mating type may function as the male or female parent. In the sexual cycle, the diameter of the protoperithecium (bottom) is about one-fifth or less of the diameter of the mature perithecium (top). The trichogyne shown protruding from the protoperithecium is shown being fertilized by a conidium of the opposite mating type. (Adapted from Davis and de Serres, 1970. Copyright © 1970 Academic Press.)

Figure 2. The meiotic process in N. crassa, starting with haploid gametic cells AB and ab. The fusion that takes place in the ascus initial (a and b, right) is followed by two meiotic (c and d, right) and a final mitotic (e, right) division. At metaphase of the first division, the two pairs of homologous chromosomes (bivalents) may line up such that A will segregate with B (case 1), or with b (case 2), leading to PD or NPD tetrads, respectively. In case 3, a crossover between gene B/b and the centromere of the long chromosome leads to delayed segregation of the B/b allele pair, such that a tetratype (T) tetrad is formed. Each meiotic product divides once before spore walls form around the eight nuclei in the linear ascus (f, right). (Adapted from Davis and de Serres, 1970. Copyright © 1970 Academic Press.)