Invertebrate fungi refer to a group of fungi that occur on invertebrate hosts, including the entomopathogenic fungi that invade the bodies of invertebrate animals and use them for nutrition and finally cause death of their hosts. The first observation of an insect disease probably dates back to a muscardine silkworms infected by Beauveria bassiana in Japan or a dead lepidopteran larva infected by Cordyceps sinesis in China (Gray 1858). Since early times, people started to make use of these fungi-infecting insects, eating their fruiting bodies, curing wounds, sore throats, toothache, abscesses and extracting a pigment for tattooing (Samson et al. 1988). However, the relationships between fungi and insects were confused, for example, people considered Cordyceps sinesis as a plant-animals which could mutate into a vegetable, and from vegetable to animals (Cooke 1892). With an increasing records of insect disease being discovered in natural areas, such as a dead wasp infected with Cordyceps sphecocephala and a dead weevil infected with Cordyceps sp. (Steinhaus 1956) were collected from Cuba and South America (Cooke 1892), respectively. Pioneer scientist’s curiosity was triggered as to how the pathogenic fungi become connected with their hosts. Different hypothesis were proposed to explain the fungus-insect associations, such as the germ theory of disease (Steinhaus 1956) and the theory of ingesting fungal spores during insects feeding (Gray 1858, Cooke 1892). Roberts and Humber (1984) pointed out that most entomopathogenic fungi invade insects by penetrating their cuticle and less infections were due to ingestion. The later part 19th century witnessed the activity towards the taxonomy of entomopathogenic fungi with most significant contributions from mycologists Thaxter (1888) on the Entomophthorales and Kobayasi (1941), Petch (1931, 1937), Mains (1948), Samson & Evans (1973) on monographs of cordyceps.
Entomopathogenic fungi are essential players in the maintenance of ecosystem health through forming intimate interactions with other organisms (Vega & Blackwell 2005, Braga-Neto et al. 2008). An advanced progress have been made to taxonomy and phylogeny of invertebrate-associated fungi with different lifestyles, such as entomopathogenic fungi on arthropod hosts (e. g. Cordyceps, Ophiocordyceps and Simplicillium) (Xiao et al. 2018, 2019, Wei et al. 2019, Hyde et al. 2020a) and entoparasitic fungi on micro-invertebrates (e. g. Drechmeria, Harposporium, Rotiferophthora) (Barron 1890, Barron 1991, Spatafora et al. 2015). Apart from the aforementioned invertebrate-associated fungi, some fungi prefer to dwell in the tissue or certain organ of the invertebrate hosts, such as Lagenidium callinectes (Oomycota) on muscle tissues of Penaeus monodon (shrimp) (Selvi et al. 2019, Lau et al. 2020), Metarhizium libelullae in blood of Libelulla quadrimaculata (dragonfly) (Sorokin 1883), Microsporidia in guts of bees and chytrids in ova of mosquitoes (Araújo et al. 2016). The entomopathogenic taxa not only occupy diversiform ecological niches on their hosts, but are also widely distributed in various habitats, crossing terrestrial to aquatic environment. Notably, fungal communities are distinct in different environment. It is likely that the basal fungi within phyla Microsporidia and Zygomycota as well as fungi-like organisms within Oomycota were typically found on aquatic invertebrate hosts, while terrestrial arthropod hosts were susceptible to entomopathogenic fungi in phyla Ascomycota and Entomophthoromycota. (Höhnk &Vallin 1953, Pohlad & Bernard 1978, Sung et al. 2007, Araújo et al. 2016, Mora et al. 2017, Mandava 2019)
The foundational framework of entomopathogenic fungi have been revised overtime. Cooke (1892) categorized the entomopathogenic fungi into four groups: Cordyceps and allied Isaria, Entomophthorales, Laboulbeniales and opportunistic fungi. Samson et al. (1988) suggested to reduce Laboulbeniales and opportunistic fungi and they concluded that entomopathogenic fungi comprise five groups: Ascomycota, Chytridiomycota, Deuteromycota, Oomycota and Zygomycota. Araújo et al. (2016) noticed that entomopathogenic fungi were found in additional two phyla: Basidiomycota and Microsporidia, meanwhile the authors excluded an invalid phylum (Deuteromycota) from entomopathogenic fungal system. Recently, the taxonomic classification system of entomopathogenic fungi was reviewed by Mora et al. (2017) based on phylogenetic analyses, in which Laboulbeniomycetes was mentioned as an interesting group of ectoparasitic pathogens on insects.
During the 21st century (1818–2000), identification of entomopathogenic fungi relied on morphological observation. Currently, molecule-based phylogenetic analyses have become a powerful tool to resolve natural classifications of fungi (Sung et al. 2007). Hypocreales was the most diverse group of entomopathogens in Ascomycota (Mora et al. 2017). In the latest decade, the family notes, generic compositions and species modifications of Clavicipitaceae, Cordycipitaceae and Ophiocordycipitaceae in the order Hypocreales were revised based on the combination of morphological observation and phylogenetic analyses of LSU, SSU, ITS, TEF, RPB1 and RPB2 sequence (Kepler et al. 2013, Matočec et al. 2014, Quandt et al. 2014, Spatafora et al. 2015 Maharachchikumbura et al. 2016, Shrestha et al. 2017, Wijayawardene et al. 2018, Hyde et al. 2020b).
Entomopathogenic fungi are attracting more and more attention due to their high potential in biocontrol and the discovery of new bioactive compounds (Hyde et al. 2019). Correctly identification and nomenclature of entomophathogenic fungi is essential for downstream industry and promoting relevant scientific fields. Since 2005, the Center of Excellence in Fungal Research in Fah Luang University, Thailand has been investigating the diversity of entomopathogenic fungi (Aung et al. 2008, Hyde et al. 2018b). During this period, many entomopathogenic fungal sepecies were collected, described, documented and published with reliable taxonomic methods (Ban et al. 2015, Spatafora et al. 2015, Hyde et al. 2016, Li et al. 2016, Hyde et al. 2017, Araújo et al. 2018, Hyde et al. 2018a, b, Khonsanit et al. 2019, Hyde et al. 2020a, b). Northern Thailand has been found to present abundant novel fungal species and entomopathogenic species is no exception (Hyde et al. 2018b). These unknown entomopathogens still deserve sampling and further study.
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