Sexual reproduction in Neurospora involves mating types in which haploid microconidia from one type fertilize the macroconidia of the other mating type. This produces a diploid zygote which enters a meiotic phase followed by a single mitotic division. The result is 8 haploid spores contained in an ascus sac... hence their name "ascospores". The ascus sac maintains the ascospores in a linear arrangement. Thus the division products of post fertilization meiosis and mitosis are linearly arranged and crossover events within the tetrad of the first meiotic division can be traced. On the right is a diploid zygote. It arose as the result of a union between a microconidium from a gray spore plant and a macroconidium from a black spore plant. The black gene is dominant so the zygote displays the black color. Now click on the sequence of buttons starting with the 1st Meiotic Division and run down through the 1st Mitotic Division. Note that the ascospores containing the gray gene are segregated on the right side of the ascus sac while those containing the black gene appear on the left side of the sac. This arrangement arises because of the linear separation of the meiotic products in the ascus sac. Now flip to the next card to see the arrangement of spores in the ascus sac when a crossover event occurs between the innermost chromatids of the tetrad.
