Question

In Neurospora, 8 ascospores are formed instead of 4. This indicates.
A. One meiosis
B. Two meiosis
C. Two mitosis
D. Meiosis followed by mitosis

Answer 1

Hint: Neurospora species are haploids; they spend most of the life cycles in the haploid state. However, the different species of Neurospora show one of three life cycles called, homothallic, heterothallic or pseudo homothallic.

Complete answer:
In the asexual part of the heterolithic life cycle of neurospora, growth and germination of a haploid asexual spore (conidium) results in mass of branched threads (hyphae) that constitute a colony. The hyphae have no cross walls hence a colony is a single cell containing many numbers of haploid nuclei. A colony buds off million more conidia from aerial hyphae, the uninucleate microconidia and the multinucleate macroconidia, and these disperse and the repeat the asexual cycle if they land on the suitable substrate.
In the sexual part of the heterolithic life cycle of neurospora, when colonies of different mating types (simple forms of sexes) come in contact, their nuclei and cell walls fuse which results in many transient diploid nuclei inside the fruiting bodies called perithecia.
Each diploid nucleus in them undergoes meiosis. The four haploid products of a meiosis stay together in a sac known as an ascus. Typically, a single ascus contains eight ascospores. Two meiotic divisions make the original diploid zygote nucleus into four haploid ones. And each of the four products of meiosis further undergoes a mitotic division that results in an octad of eight ascospores within each of the ascus.

Hence, the correct answer is option (D).

Note: Neurospora, especially N. crassa is commonly used in genetics as a model organism. This is because of its characteristics like: easy to culture, quickly reproducing and can survive on minimal media (glucose, inorganic salts, water and biotin in agar)