Biology

The Hypocreales includes the more than 500 species of arthropod pathogenic (AP) fungi, more than any other order of Kingdom Fungi (Sung et al 2007). While any one species is generally restricted to a single host species or a set of closely related host species, hypocrealean AP fungi collectively attack species from 12 orders of Arthropoda (Kobayasi 1941, 1982). The distribution of these fungi is cosmopolitan, including all terrestrial regions except Antarctica, with the height of known species diversity occurring in subtropical and tropical regions, especially East and Southeast Asia.

Morphology. The majority of hypocrealean AP fungi produce a conspicuous fruiting body or stroma that erupts from the corpse of the arthropod host.  The stromata may be brightly colored orange to red or darkly pigmented brown to black, according to species.  The stromata often produce a stipe that serves to elevate the fertile or spore producing region away from the host, which is often buried in soil, wood or leaf litter.  The fertile region of the stromata is often terminal and club-like or head-like appearance. Spores derived from sexual reproduction are produced in specialized cells, called asci, that are housed in flasked-shaped structures called perithecia.  Ascospores and asci are microscopic and while individual perithecia are typically less than 0.5 mm in diameter, they collectively give the stroma the club-like appearance.  Keep in mind, however, that hypocrealean AP fungi are a large and morphologically diverse group with exceptions to just about any generalization.

Left: Three stromata of Cordyceps pruinosa on a limacodid pupa. Right: Several stromata of Ophiocordyceps melolonthae on larvae of Hercules beetles.

Stroma

Life cycle. Like many Ascomycota, hypocrealean AP fungi may be pleomorphic. That is, their life cycle may comprise more than one spore producing stage. By definition, these life cycles can contain 0-1 meiotic (teleomorphic) stage and 0-many mitotic (anamorphic) stages. As allowed by Article 59 of the International Botanical Code, both the teleomorph and anamorph may receive Latin binomials. In some cases the anamorph has been described with a species epithet, while in others it is simply designated to an anamorph or form genus (Hodge 2003). Some examples of pleomorphic life cycles among the hypocrealean AP fungi include (Zare & Gams 2001; Liang et al. 1991; Hodge et al 1996; Liu et al 2001):

 

Teleomorph

Anamorph

Cordyceps militaris

Lecanicillium

Metacordyceps taii

Metarhizium anisopliae

Elaphocordyceps subsessilis

Tolypocladium inflatum

Ophiocordyceps sinensis

Hirsutella sinensis

 

Habitat. Arthropod pathogenic fungi of the Hypocreales are typically found in regions of the world with a hot and humid climate, but numerous notable exceptions exist (e.g., O. sinensis a grassland species endemic to the Tibetan plateau). They are predominately forest species, and may be found in tropical, temperate deciduous or coniferous habitats. The highest occurrence of fruiting structures (stromata) is near stream corridors with major niche preferences including leaf litter, moss, soil, wood and the underside of leaves. Stromata production is most prolific during the rainy season in any given region, and as such, there is still much to learn about how they survive between fruitings. Many species are capable of growing saprobically on simple media and some can be commonly isolated from soil (e.g., Metarhizium spp.). Additionally several strains of M. anisopliae have shown rhizosphere competence, with populations growing on the roots of plants 100 times greater than those in bulk soil, an arrangement able to attract host insects and initiate infection (Kepler and Bruck 2006). Similarly, Beauveria bassiana is capable of infection of insect hosts during endophytic growth in corn.

Host affiliation. Hypocrealean AP fungi are similar to other groups of pathogenic organisms in that individual species attack only one species, or at most a group of closely related species. As with other pathogenic organisms (such as the Ichneuonoid wasps), the necessity of specialization on a particular host has resulted in a staggering level of species diversity. Collectively these fungi attack hosts from 12 orders of Arthropoda including Aranae, Acari, Blattaria, Coleoptera, Diptera, Hemiptera, Hymenoptera, Isoptera, Lepidoptera, Odonota, Orthoptera, and Phasmida (Kobayasi 1941, 1982; Mains 1957, 1958; Samson et al 1988; Spatafora et al 2007). The most frequently encountered species infect immature stages (larvae and pupae) of Coleoptera (e.g., Ophiocordyceps stylophora) and Lepidoptera (e.g., Cordyceps militaris) and adults of Hemiptera (e.g., O. nutans) and Hymenoptera (e.g., O. sphecocephala).

 

Left: Two stromata of Ophiocordyceps dipterigena emerging from the thorax and one stroma with Hirsutella anamorphs emerging from the anus of an adult fly. Right: A stroma of Ophiocordyceps nutans emerging from body of a stink bug.

 

Pathogenicity. The infection process has been studied in a limited number of species, but is generally thought to be similar across all known taxa. An infection initiates with a spore or spores adhering to the exoskeleton of the host. Spores germinate a short germ tube that terminates in an appresorium, a flattened disc-like structure. An infection peg develops on the ventral surface of the appressorium and penetrates through the exoskeleton via mechanical pressure and the production of lipases and proteases. Once inside the body cavity, the fungus grows and divides in a blastospore or yeast-like, hyphal body stage (Clarkson & Charnley 1996). The infection often results in aberrant behavior of the host. This may manifest itself in several ways, such as causing the host to climb before death, a phenomenon referred to as summit disease. At the time of death or shortly thereafter, the fungus grows in a filamentous stage and consumes all of internal organs and structures of the host, producing a tightly packed mass of mycelium termed an endosclerotium. Interestingly, in most species the exoskeleton is left almost entirely intact. When conditions are appropriate, the endosclerotium produces a stroma or several stromata that may either rupture through the exoskeleton at a random point or at a region characteristic of the species. The stroma(ta) then produces perithecia, asci and ascospores if it is meiotically reproductive, or conidiophores and conidia if it is reproducing by the production of mitotic spores. Some species are capable of producing both types of spores from the same stroma. Most spores are airborne and complete the cycle by infecting a new host.

Medicine and secondary metabolites. Perhaps the best known and commercially valuable species is Ophiocordyceps sinensis (=Cordyceps sinensis). It has been used as a traditional herbal medicine throughout Asia since at least 1757 A.D., but its use likely dates back much earlier. Historical uses include treatment of respiratory and renal conditions as well as formulation as a tonic to instill vigor. Recent work with O. sinensis has focused on possible tumor suppression properties (Nakamura 1999; Im 2003). Hypocrealean AP fungi are known to produce a myriad of biologically active secondary metabolites that are the products of peptide synthetases and nonribosomal peptide synthases, many of which are involved in arthropod pathogenicity (e.g., destruxins) (Isaka et al 2003). These secondary metabolites have attracted significant attention in modern medicine as potential sources of novel pharmaceuticals and drug treatments. For example, Cyclosporin A is a drug used to help prevent rejection of transplanted organs and was originally isolated from the fungus Tolypocladium inflatum (Wenger 1984). This fungus is now understood to be the anamorphic state of Elaphocordyceps subsessilis (=Cordyceps subsessilis) (Hodge et al 1996; Sung et al 2007).

Biological control. In addition to medical applications, several mitotic or anamorphic species have received attention as biological control agents of insect pests.  Candidate species have largely come from Cordycipitaceae or Clavicipitaceae.  Anamorphic forms of Ophiocordycipitaceae are often difficult to culture and slow growing, two characteristics important in large scale propogation schemes.  In the Clavicipitaceae, species of the genera Metarhizium, Nomuraea and Pochonia have all proven useful against a range of pests.  The biopesticide Green Muscle was developed from Metarhizium anisopliae var. acridum as an alternative to synthetic pesticides for the control of locusts in Africa. Although typically more expensive than conventional pesticides, large scale field trials have provided an acceptable level of control. Other isolates of this fungus are under development for control of a wide array of pests including mosquito vectors of malaria. Closely related to M. anisopliae, Nomuraea rileyi has also shown potential as a biological control organism. Able to cause large, natural epizootic events on Lepidopteran hosts, this fungus is now under development for the control of pests for such as Anticarsia gemmatalis, a significant pest of soybean.  Pochonia chlamydosporia has been used against nematode pests of potato.

In the Cordycipitaceae, several species of Beauveria have been developed, the most common of which is B. bassiana.  This is currently available as a formulated product in the United States.  This species has shown activity against a wide range of hosts, including beetle and moth larvae. Leccanicillium lecanii is effective against whiteflies and thrips in greenhouse settings.