Fungus commonly known as sac fungi or ascomycetes, constitute the largest taxonomic group within the Eumycota. These fungi form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This phylum includes morels, a few mushrooms and truffles, unicellular yeasts (e.g., of the genera "Saccharomyces", "Kluyveromyces", "Pichia", and "Candida"), and many filamentous fungi living as saprotrophs, parasites, and mutualistic symbionts (e.g. lichens). Prominent and important genera of filamentous ascomycetes include "Aspergillus", "Penicillium", "Fusarium", and "Claviceps". Many ascomycete species have only been observed undergoing asexual reproduction (called anamorphic species), but analysis of molecular data has often been
Parasites in fiction but not others. Parasites, represented as extraterrestrial aliens or unnatural beings, are seen in science fiction as distasteful, in contrast to (mutualistic) symbiosis, and sometimes horrible. Practical uses can be made of them, but humans who do so may be destroyed by them. For example, Mira Grant's 2013 novel "Parasite" envisages a world where people's immune systems are maintained by genetically engineered tapeworms. They form readily understood characters, since, as Gary Westfahl explains, parasites need to exploit their hosts to survive and reproduce. The social anthropologist Marika Moisseeff argues that Hollywood science fiction favours insects as villain characters because of