The Insect Pathogens

Abstract

Fungi are the most common disease-causing agents of insects; aside from playing a crucial role in natural ecosystems, insect-killing fungi are being used as alternatives to chemical insecticides and as resources for biotechnology and pharmaceuticals. Some common experimentally tractable genera, such as Metarhizium spp., exemplify genetic diversity and dispersal because they contain numerous intraspecific variants with distinct environmental and insect host ranges. The availability of tools for molecular genetics and multiple sequenced genomes has made these fungi ideal experimental models for answering basic questions on the genetic and genomic processes behind adaptive phenotypes. For example, comparative genomics of entomopathogenic fungi has shown they exhibit diverse reproductive modes that often determine rates and patterns of genome evolution and are linked as cause or effect with pathogenic strategies. Fungal-insect pathogens represent lifestyle adaptations that evolved numerous times, and there are significant differences in host range and pathogenic strategies between the major groups. However, typically, spores landing on the cuticle produce appressoria and infection pegs that breach the cuticle using mechanical pressure and cuticle-degrading enzymes. Once inside the insect body cavity, fungal pathogens face a potent and comprehensively studied immune defense by which the host attempts to eliminate or reduce an infection. The Fungal Kingdom stands alone in the range, extent, and complexity of their manipulation of arthropod behavior. In part, this is because most only sporulate on cadavers, so they must ensure the dying host positions itself to allow efficient transmission.

FIGURE 1

(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).

FIGURE 2

(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.

FIGURE 3

A phylogenetic tree representing relatedness of entomopathogenic fungal taxa. Important entomopathogenic groups are indicated in parentheses. (From reference , with permission.)

FIGURE 4

(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.

FIGURE 5

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.