During the formation of zygospores, two compatible Cell Cycle inhibitor mating type hyphae fuse and form a zygote, which appears similar to the scales of a balance (in Greek, zygos, meaning a balance scale) (Fig. 1) (reviewed in ). The zygospores have a prolonged period of dormancy (a month to years) before germinating to produce meiospores. This long period of spore dormancy renders these species less facile genetic model systems.
The zygospores germinate to form a single aerial hypha with a sporangium at the apex, which is morphologically similar to the asexual sporangia. The sexual sporangium harbours the meiospores (reviewed in ). Mucorales fungi were first studied as a model for fungal sexual reproduction more than a century ago. For example, heterothallism was first described in a Rhizopus species, where hyphal fusion during mating only occurs between two different thalli (from Greek, thallos, meaning a twig); in contrast, formation of zygospores from a single thallus was referred to as homothallism, first defined for the zygomycete Syzygites megalocarpus. Both terms were then adapted to describe cross-fertility
(or opposite-sex mating) and self-fertility in fungi respectively. Indeed, the first report of sex in fungi was in the Mucoralean species S. megalocarpus in 1820, and early in the 1900s this fungus represented the first homothallic fungal species in the establishment of the terms homothallic and heterothallic.[10, 11] In heterothallic Mucoralean fungi,
two mating types are required to complete sexual reproduction. The mating types, plus (+) and minus (−), were assigned arbitrarily in R. nigricans and CP-690550 manufacturer the designation of mating type in other Mucoralean fungi was based on pairing with the tester (+)/(−) 4-Aminobutyrate aminotransferase strains of R. nigricans (reviewed in ). The two mating types are likely indistinguishable in morphology (isogametic).[7, 10] Burgeff characterised the first fungal mating pheromone as trisporic acid from Mucor mucedo. Unlike peptide pheromones found in ascomycetes and basidiomycetes, trisporic acid is a volatile organic C18 compound produced from β-carotene.[8, 13] Interestingly, it is thought that trisporic acid can trigger mating in all Mucoralean fungi and Mortierella.[9, 14, 15] Multiple enzymatic steps are required to produce trisporic acids and both mating types must be present in proximity to complete this synthetic process. In both mating types, β-carotene is cleaved into retinol to β-C18-ketone, which is then converted into 4-dihydrotrisporin. From this point, each mating type has a separate pathway to produce trisporic acid.[8, 16, 17] In the (+) mating type, an enzyme converts 4-dihydrotrisporin into 4-dihydromethyl trisporate, which then has to be transferred to the (−) mating type. The 4-dihydromethyl trisporate is then converted into methyltrisporate by 4-dihydromethyltrisporate dehydrogenase (TDH).