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Category: Microbial Genetics and Molecular Biology; Environmental Microbiology
The Insect Pathogens, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555819583/9781555819576_Chap45-1.gif /docserver/preview/fulltext/10.1128/9781555819583/9781555819576_Chap45-2.gifAbstract:
Estimates of the number of arthropod species vary between 1,170,000 and 10 million, accounting for over 80% of all known living animal species. One arthropod subgroup, insects, is the most species-rich member of all ecological guilds in land and freshwater environments ( 1 ). As arthropods were emerging as the dominant animals they are today, fungi were also colonizing the land. Over the past 400 million years fungi and insects have coevolved a wide array of intimate interactions ( 2 , 3 ). These interactions include mutualistic endosymbiosis ( 4 ); fungi as obligate food sources, such as those found in fungus-gardening ants ( 5 ); sexually and behaviorally transmitted parasites, such as Laboulbeniales ( 6 ); and the most common disease-causing agents of insects ( 7 ). Entomopathogenicity has evolved independently and repeatedly in all the major phyla of the Kingdom Fungi ( 3 ). The heterogeneity of entomopathogenic fungi probably derives from both they and their hosts having short generation times, i.e., rapidly driving new diversity with each generation, and from their occupation of a wide range of habitats, with near ubiquity in the soil and on plants. Interactions among fungi, hosts, and the environment are therefore diverse and dynamic, which complicates comparisons between different fungi infecting different insects since their interactions may be necessarily disparate. Historically, this quandary was dealt with by intensively studying the host pathogen interactions of a couple of experimentally tractable fungal species, and then extrapolating these results to distantly related species. Consequently, most of what we know about the biochemical and molecular basis of interactions between fungi and insects has been determined with the experimentally tractable hypocrealean ascomycete genera Metarhizium (family Clavicipitaceae) and Beauveria (family Cordycipitaceae). Metarhizium, in particular, has also emerged as an excellent model to explore a broad array of questions in ecology and evolution, host preference and host switching, and the mechanisms of speciation.
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(A) Scanning electron micrograph of M. robertsii growing on caterpillar (Manduca sexta) cuticle; appressoria (Ap) were most frequently produced on zones of weakness such as hair sockets. (B) Diagrammatic representation of cuticle penetration by M. robertsii using an appressorium along a seta (brown), glandular duct (beige), and trichogen cell (purple) followed by budding off of yeast-like blastospores in the hemolymph. (C, D, and E) Shown are M. robertsii-infected fly wings incubated with specific histochemical substrates to demonstrate aminopeptidase, subtilisin protease, and esterase activity, respectively, on appressoria and appressorial plates as described by St. Leger et al. ( 154 ).
(A) Scanning electron micrograph of M. robertsii growing on caterpillar (Manduca sexta) cuticle; appressoria (Ap) were most frequently produced on zones of weakness such as hair sockets. (B) Diagrammatic representation of cuticle penetration by M. robertsii using an appressorium along a seta (brown), glandular duct (beige), and trichogen cell (purple) followed by budding off of yeast-like blastospores in the hemolymph. (C, D, and E) Shown are M. robertsii-infected fly wings incubated with specific histochemical substrates to demonstrate aminopeptidase, subtilisin protease, and esterase activity, respectively, on appressoria and appressorial plates as described by St. Leger et al. ( 154 ).
(A) Malaria vector mosquito Anopheles gambiae killed by a transgenic strain of Metarhizium expressing GFP and the spider toxin, Hybrid. (B) Fully matured fruiting bodies of Cordyceps militaris emerging from a silk worm pupa. (C) A fruiting body of Cordyceps cicadae forming on the subterranean larvae of its specific host Cicada flammata. Images B and C courtesy of Chengshu Wang.
(A) Malaria vector mosquito Anopheles gambiae killed by a transgenic strain of Metarhizium expressing GFP and the spider toxin, Hybrid. (B) Fully matured fruiting bodies of Cordyceps militaris emerging from a silk worm pupa. (C) A fruiting body of Cordyceps cicadae forming on the subterranean larvae of its specific host Cicada flammata. Images B and C courtesy of Chengshu Wang.
A phylogenetic tree representing relatedness of entomopathogenic fungal taxa. Important entomopathogenic groups are indicated in parentheses. (From reference 15 , with permission.)
A phylogenetic tree representing relatedness of entomopathogenic fungal taxa. Important entomopathogenic groups are indicated in parentheses. (From reference 15 , with permission.)
(A) A Brazilian carpenter ant Camponotus rufipes biting a leaf with Ophiocordyceps camponoti-rufipedis just beginning its growth out of the ant’s body. (B) Ophiocordyceps unilateralis fruiting body emerging from the head of the Thai carpenter ant, Camponotus leonardi. (C) Spore-producing bodies of Ophiocordyceps camponoti-balzani on the Brazilian carpenter ant Camponotus balzani. Images courtesy of David Hughes.
(A) A Brazilian carpenter ant Camponotus rufipes biting a leaf with Ophiocordyceps camponoti-rufipedis just beginning its growth out of the ant’s body. (B) Ophiocordyceps unilateralis fruiting body emerging from the head of the Thai carpenter ant, Camponotus leonardi. (C) Spore-producing bodies of Ophiocordyceps camponoti-balzani on the Brazilian carpenter ant Camponotus balzani. Images courtesy of David Hughes.
Live cicada with its abdomen replaced by sporulating Massospora cicadina: the insect host disseminates the fungus during this stage of the disease. Image courtesy of Mike Raupp.
Live cicada with its abdomen replaced by sporulating Massospora cicadina: the insect host disseminates the fungus during this stage of the disease. Image courtesy of Mike Raupp.