2. The life history of entomopathogenic Cordyceps species

 

    These fungi may occur on insects in any type of soil. The ascospores or conidia of these species invade living insects’ body through the cuticle or possibly through a spiracle during spring, summer or fall. Curiously, it is extremely rare for insect-pathogenic fungi to infect their hosts through the gut after the ingestion of spores. Spores form a germ tube that penetrates into the insect's body, and, if the fungus successfully evades the host's natural defense mechanism, it then subsists entirely upon the nutrients inside the host's body. The insects eventually die from the cumulative effects of the fungal mass growing inside the body. once hyphae fill the insect's body, Cordyceps species form a solid, tissue-like resting state (a sclerotium) inside the host's body. The sclerotia maintain the insect's body shape and develop into an external fruiting body during the next year. The hyphae growing out of the body from the sclerotium usually form leathery to hard, erect structures called stromata (singular: stroma) emerging from the mouth, chest, head, and abdomen. Cells that produce asexual conidia or the sexual structures (perithecia containing asci and ascospores) form on the mature stromata. It is these conidia or ascospores that are dispersed and infect other insects.

        There are three steps by which entomopathogenic fungi invade insects. The first step is for spores to stick to the host's surface and germinate. The ability of infective spores to reach a susceptible host depends on many factors including the density of available spore inoculum, the density of the potential host population and micro-environmental factors such as relative humidity. The second stage of infection is the actual penetration of the host exoskeleton by the fungus. This depends on the ability of the germ tube to penetrate through each of the chemically different layers of the cuticle, epidermis and underlying dermis. The key for success is usually a highly localized enzymatic digestion of and/or a mechanical penetration through each of these layers. If the fungus is able to evade each of the defensive mechanisms thrown against it by the host insect either during the penetration of the cuticle or upon reaching the hemocoel, the last step is for the fungus is to proliferate inside of the insect body. When this happens the fungi grow rapidly as either thread-like hyphae or as small, yeast-like structures called hyphal bodies whose growth and metabolic activities eventually lead to the insects' death. Many of these fungi produce antibiotics that are able to suppress the bacteria and other microbes present in the intestine of insects and that are usually released into the body as the gut and other tissues degenerate at the time of the host’s death. In a suitable environment, the fungus forms one or more erect fruiting bodies (stromata) outside of the insect. During the winter or other environmentally unfavorable periods, Cordyceps species usually survive in the metabolically inactive sclerotial state; in entomopathogenic fungi other than Cordyceps, most fungi will survive similar periods either in some sort of thick-walled spore state (e.g. as chlamydospores, zygospores, or oospores) or as a mummified and desiccated mycelial mass inside the host's body.

        The hosts of entomopathogenic fungi are usually scale insects, cicadas, beetles, moths, flies, ants and wasps (although bees in the strict sense are relatively little affected by fungal pathogens), dragonflies, and spiders. These insect hosts may become infected during different stages of development such as egg, larva, pupa and adult insect, but any given pathogenic fungus may not be able to infect all of these possible developmental stages.

        Most Cordyceps specimens are found with the host cadaver buried in soil, leaf litter, or in decaying wood and with the stromatic tips emerging into the air. Relatively few Cordyceps species are found on plainly visible host cadavers attached to branches, twigs or leaves. The size of the stromatic fruiting bodies usually ranges from a few mm to 10 cm or more in height. The range of colors - red, yellow, purple, green, black, white, orange or olive - and shapes of these stromata make them appear like beautifully colored little flowers arising from the insect bodies.

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