Bread moulds were the first fungi shown to use sex hormones
Some pin moulds cannot reproduce sexually alone. Two strains, labelled plus and minus, must find each other, and they do it by chemistry: each releases compounds the other finishes into trisporic acid, a pheromone that makes their hyphae swell and grow toward one another until they fuse. It was the first sex hormone found in any fungus.
Zygomycota, the zygote fungi, was once treated as a single phylum of about 1,060 species, now divided between the Mucoromycota and Zoopagomycota because the old grouping did not reflect a single ancestry. Most live in soil or on decaying plants and animals; some parasitise plants, insects or small animals, and others partner with plant roots. Their threads often lack cross walls, forming them only where sex cells develop or to seal off dead sections.
Greek zygon, a yoke, lends the group its label, a nod to the joining of two hyphae that produces a tough, warty zygospore. Albert Blakeslee found that certain Mucorales were self-sterile and needed opposite strains. Hans Burgeff at Göttingen then showed small molecules drifting through air and soil triggered the encounter. Trisporic acid itself, a terpenoid built from beta-carotene, was identified in 1964 because it boosted carotene output in one mould, and it was later shown to spur the sex organs of another. Because the final compound forms only when both partners contribute, the system doubles as an identity check. A parasitic mould even exploits the same signals to fuse with its host and pass genes into it.
These fungi make their walls from chitosan instead of the chitin most fungi use, stripping chitin chains of a chemical group as they build. Asexual spores form in thin-walled sacs perched on stalks that grow upward and toward light, bursting when rain or a passing animal knocks them. Spores can lie dormant for hours or many years.
Light shapes their lives. Phycomyces stalks bend toward blue light, away from ultraviolet and even respond to red, making it a model for studying how fungi sense light. Light also switches on carotene production and promotes asexual spores, while it suppresses sexual development in Phycomyces.
Source: Zygomycota
