Lifecycle of Neurospora, a haploid bread mold
Given two haploid
vegetative mycelia of opposite mating types (A and a), physical
contact allows haploid nuclei to fuse to form a diploid Aa nucleus. The
nucleus undergoes two typical meiotic divisions to form four haploid
gametes: A and
a products
of the second division remain in linear order in the ascus. A further
mitotic division produces eight ascospore. Released of the ascospores
and germination completes the cycle.
For purposes of genetic
experimentation, the significant features are (1) the vegetative mold
is haploid, such that all metabolic pathways are controlled by a single
allele, and (2) the separation of the products of second meiosis at
either end of the ascus means that each quartet expresses different
alleles at each locus.)