Bio 325 Genetics Lab: Tetrad analysis in Sordaria fimicola
Sordaria fimicola is an ascomycete fungus (as is Neurospora crassa). Ordinarily these organisms are haploid, and grow to form filaments (hyphae) that form a tangled mat called a mycelium. However, these haploid organisms can engage in sexual reproduction, resulting in a transient diploid condition. That happens when hyphae from different organisms fuse, which introduces haploid nuclei from each into the same cytoplasm. These nuclei fuse to produce a diploid nucleus, which then immediately undergoes meiosis to produce 4 haploid nuclei. Each haploid nucleus undergoes one mitotic division, resulting in eight nuclei that become encased in individual ascospores (each of which can germinate into a new haploid organism). These meiotic and mitotic divisions occur in a cell that gradually elongates to accommodate the increasing number of nuclei, such that all nuclei are lined up in single file. This situation means that the order of the nuclei in each cell (ascus) reflects the lateral arrangement of homologous pairs formed during meiosis. In Sordaria, there are several asci contained inside specialized reproductive structures called perithecia.
Now, as I’m sure you remember fondly, alleles that are not involved in crossover events segregate during meiosis I, whereas alleles that are involved in crossovers segregate during meiosis II. Think about it. So, if you were to observe the genotypes of the resulting ascospores (or the organisms derived from them) you could determine if segregation occurred during the first meiotic division (no crossover) or the second meiotic division (crossover). Segregation during meiosis I should produce a 4:4 arrangement of ascospore genotypes. Segregation during meiosis II should produce a 2:2:2:2 or a 2:4:2 arrangement.
Yeah, so? I hear you saying. Well, the so is that you can map chromosomal loci relative to the centromere by determining the frequency of...