Sordaria

closely related to Neurospora and Podospora, is a member of the numerous class
Pyrenomycetes or flask-fungi. The natural habitat of the three species of Sordaria that have been the principal subjects in genetic studies is the dung of herbivorous animals. The species S. fimicola is common and worldwide in distribution. The species of Sordaria are similar morphologically, producing black perithecia containing asci with eight dark ascospores in a linear arrangement. These species share a number of characteristics that are advantageous for genetic studies. They all have a short life cycle, usually 7-12 days, and are easily grown in culture. Most species are self-fertile and each strain is isogenic. All kinds of mutants are easily
Hyphae branch and fuse during growth of the fungus to form a network of cells called a mycelium. During the sexual phase, cell fusion results in the formation of a single-celled, diploid zygote encased in an ascus (plural, asci). Within the ascus, each single-celled zygote undergoes meiosis and gives rise to four haploid cells. These haploid cells then undergo a single round of mitotic division and give rise to a total of eight haploid ascospores with thick cell walls in each ascus. Many of these rod-shaped asci, each with eight ascospores, are held together in a protective, flask-shaped perithecium.
In agar cultures, the perithecium, about 0.5 mm high, has its lower part immersed in the jelly with its neck projecting into the air (Figure 1). The perithecium opens to the outside by a narrow canal through which only one ascus can extend at a time.
The ascospores are actively discharged into the
Spore discharge is dependent on an adequate supply of water since the bursting of turgid cells is involved. The great majority of flaskfungi liberate spores under damp conditions.
Even though single spore cultures of S. fimicola are self-fertile, they may be crossed. For example, when two cultures are paired in a petri dish, each mycelium will give rise to perithecia and homokaryotic asci. However, in the region where the two mycelia meet, two haploid nuclei can fuse to form a heterozygous diploid. A suitable marker (e.g., an ascospore colour mutant) is usually employed to detect such crossed perithecia which may then be used in formal genetic studies. Not all such pairings are successful. There is some evidence suggesting that an heterogenic incompatibility mechanism is operating. Conveniently, from the perspective of geneticists, all the ascospores within an ascus remain ordered in the same position that their chromosomes were aligned in during the first round of meiosis (meiosis
I). This unique sequence of events allows a simple detection of the occurrence of crossing over