iGen 06-02smc

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 ascospores. Release of the ascospores and germination completes the life 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: the ascus can be dissected and the four identical spores at either end used in crosses.

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2014 by Steven M. Carr