Aspergillus

Aspergillus ( ) is a genus consisting of several hundred mold species found in various climates worldwide. Aspergillus was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Micheli was reminded of the shape of an aspergillum (holy water sprinkler), from Latin spargere (to sprinkle), and named the genus accordingly. Aspergillum is an asexual spore-forming structure common to all Aspergillus species; around one-third of species are also known to have a sexual stage. While some species of Aspergillus are known to cause fungal infections, others are of commercial importance.

Aspergillus ( ) is a genus consisting of several hundred mold species found in various climates worldwide. Aspergillus was first catalogued in 1729 by the Italian priest and biologist Pier Antonio Micheli. Viewing the fungi under a microscope, Micheli was reminded of the shape of an aspergillum (holy water sprinkler), from Latin spargere (to sprinkle), and named the genus accordingly. Aspergillum is an asexual spore-forming structure common to all Aspergillus species; around one-third of species are also known to have a sexual stage. While some species of Aspergillus are known to cause fungal infections, others are of commercial importance.

Inner taxonomy

The expansive genus Aspergillosis is currently divided into six subgenera of which many are further split into a total of 27 sections. Subgenus Circumdati, divided in 10 sections. Linked to former teleomorphic genera Petromyces, Neopetromyces and Fennellia. Subgenus Nidulantes, divided in 9 sections. Linked to former teleomorphic genus Emericella. Subgenus Fumigati, divided in 4 sections. Linked to former teleomorphic genus Neocarpenteles, Neosartorya and Dichotomomyces. Subgenus Aspergillus, divided in 2 sections. Linked to former teleomorphic genus Eurotium. Subgenus and section Cremei. Linked to former teleomorphic genus Chaetosartorya and Cristaspora Subgenus and section Polypaecilum. Includes former genera Polypaecilum and Phialosimplex. The linkage to former teleomorphic genera derive from Samson et al. (2014). The current division is based on Houbracken et al. (2020), which also notes that the former teleomorphic genera are now understood as morphotypes: eurotium-type, neosartorya-type, emericella-type, petromyces-type, chaetosartorya-type, fennellia-type and neopetromyces-type.

Sexual reproduction

Of the 250 species of aspergilli, about 64% have no known sexual state. However, many of these species likely have an as yet unidentified sexual stage. Sexual reproduction occurs in two fundamentally different ways in fungi. These are outcrossing (in heterothallic fungi) in which two different individuals contribute nuclei, and self-fertilization or selfing (in homothallic fungi) in which both nuclei are derived from the same individual. In recent years, sexual cycles have been discovered in numerous species previously thought to be asexual. These discoveries reflect recent experimental focus on species of particular relevance to humans. A. fumigatus is the most common species to cause disease in immunodeficient humans. In 2009, A. fumigatus was shown to have a heterothallic, fully functional sexual cycle. Isolates of complementary mating types are required for sex to occur. A. flavus is the major producer of carcinogenic aflatoxins in crops worldwide. It is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals. In 2009, a sexual state of this heterothallic fungus was found to arise when strains of opposite mating types were cultured together under appropriate conditions. A. lentulus is an opportunistic human pathogen that causes invasive aspergillosis with high mortality rates. In 2013, A. lentulus was found to have a heterothallic functional sexual breeding system. A. terreus is commonly used in industry to produce important organic acids and enzymes, and was the initial source for the cholesterol-lowering drug lovastatin. In 2013, A. terreus was found to be capable of sexual reproduction when strains of opposite mating types were crossed under appropriate culture conditions. These findings with Aspergillus species are consistent with accumulating evidence, from studies of other eukaryotic species, that sex was likely present in the common ancestor of all eukaryotes. A. nidulans, a homothallic fungus, is capable of self-fertilization. Selfing involves activation of the same mating pathways characteristic of sex in outcrossing species, i.e. self-fertilization does not bypass required pathways for outcrossing sex, but instead requires activation of these pathways within a single individual. Among those Aspergillus species that exhibit a sexual cycle, the overwhelming majority in nature are homothallic (self-fertilizing). This observation suggests Aspergillus species can generally maintain sex though little genetic variability is produced by homothallic self-fertilization. A. fumigatus, a heterothallic (outcrossing) fungus that occurs in areas with widely different climates and environments, also displays little genetic variability either within geographic regions or on a global scale, again suggesting sex, in this case outcrossing sex, can be maintained even when little genetic variability is produced.

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