Figure 1.
Schematic overview of the establishment of a new termite colony in species with horizontal transmission via sexual spores and the intra-nest transmission of Termitomyces via asexual spores. Most species of macrotermitine termites and their fungal symbionts have horizontal symbiont transmission and reproduce and disperse separately. (A) Basidiospores originating from Termitomyces mushrooms are available in the environment after they have been wind-dispersed. (B) Two winged alates become the founders of a new termite colony when they pair and become the king and the queen. The king and the queen produce new termite workers, which form a primordial comb. This comb needs to be inoculated with a Termitomyces fungus for the termite colony to survive. (C) On one of their first foraging trips, termite workers bring basidiospores of Termitomyces into their nest to inoculate the fungal comb. (D) The basidiospores germinate and form homokaryotic mycelia. (E) When two compatible homokaryons of the same species meet, they can fuse to form heterokaryotic mycelium in the contact zone, with two nuclei in each cell. (F) The heterokaryons grow and produce nodules consisting of asexual spores. During the establishment of the colony, it is possible and likely that more than one heterokaryon is present within the termite nest. (G) Positive frequency-dependent selection on the different heterokaryons results in the elimination of fungal genetic variation leading to a fungal monoculture. (H) Meanwhile, the termites bring new substrate into the nest and build new fungal combs. While the termite colony grows, the termites (I) propagate small amounts of asexual spores which are transferred within the nest to new fungal combs. This intranest transmission is a repeating event which continues as long as the termite colony is active. With bottlenecks imposed by the propagation of asexual spores and positive frequency-dependent selection on the most dominant fungal genotype, the fungal mycelium is kept a monoculture, with no fungal genetic variation in the termite hill. (J) Seasonally, the termites form winged alates that disperse to establish new colonies. (K) Some time after the winged alates have left the colony Termitomyces starts forming mushrooms. The fruiting bodies of Termitomyces appear on top of the termite hill and disperse large amounts of sexual basidiospores into the environment.