Neurospora crassa
Asci, notably those of "Neurospora crassa", have been used in laboratories for studying the process of meiosis, because the four cells produced by meiosis line up in regular order. By modifying genes coding for spore color and nutritional requirements, the biologist can study crossing over and other phenomena. The formation of asci and their use in genetic analysis are described in detail in "Neurospora crassa".