Schematic representation of the levels of selection acting on the fungal symbionts of a termite colony (A) and the life cycle of species of Termitomyces (B). A. Natural selection may act at the level of colonies (I), fungus gardens within colonies (II), fungal individual (mycelia) within fungus gardens and nuclei within mycelia (IV). B. Species of Termitomyces have a sexual (upper circle) and an asexual life cycle (lower circle). Sexual reproduction occurs via the production of sexual fruiting bodies (mushrooms), which produce haploid spores. Workers from newly established colonies start a fungus garden from sexual spores. Sexual spores germinate and give rise to mycelia with a single haploid nucleus (homokaryons). Multiple homokaryons can fuse and form a heterokaryon with genetically different nuclei in a mycelium. Within colonies, Termitomyces is propagated asexually via the production of asexual spores on nodules, which are inoculated on freshly collected plant material. There are two stages during which selection at the level of the nuclei can occur (indicated as IVa and IVb). First, in the sexual cycle, more than two homokaryons may fuse, so that nuclei are in competition to become part of the heterokaryon that will become dominant during subsequent growth (IVa). Second, during the formation of asexual spores within the colony, a nucleus with a replication or segregation advantage may increase in frequency with time (IVb).