Stages of a Neurospora cross

The life cycle of Neurospora crassa, the orange bread mold. Self-fertilization is not possible in this species: there are two mating types, determined by the alleles A and a of one gene, and either can act as “female.” An asexual spore from the opposite mating type fuses with a receptive hair, and a nucleus from the asexual spore travels down the hair to pair with a female nucleus in the knot of cells. The A and a pair then undergo synchronous mitoses, finally fusing to form diploid meiocytes.