Highly specific host-pathogen interactions influence Metarhizium brunneum blastospore virulence against Culex quinquefasciatus larvae

Virulence. 2018;9(1):1449-1467. doi: 10.1080/21505594.2018.1509665.

Abstract

Entomopathogenic fungi are potential biological control agents of mosquitoes. Our group observed that not all mosquitoes were equally susceptible to fungal infection and observed significant differences in virulence of different spore types. Conidiospores and blastospores were tested against Culex quinquefasciatus larvae. Blastospores are normally considered more virulent than conidia as they form germ tubes and penetrate the host integument more rapidly than conidia. However, when tested against Cx. quinquefasciatus, blastospores were less virulent than conidia. This host-fungus interaction was studied by optical, electron and atomic force microscopy (AFM). Furthermore, host immune responses and specific gene expression were investigated. Metarhizium brunneum (formerly M. anisopliae) ARSEF 4556 blastospores did not readily adhere to Culex larval integument and the main route of infection was through the gut. Adhesion forces between blastospores and Culex cuticle were significantly lower than for other insects. Larvae challenged with blastospores showed enhanced immune responses, with increased levels of phenoloxidase, glutathione-S-transferase, esterase, superoxide dismutase and lipid peroxidase activity. Interestingly, M. brunneum pathogenicity/stress-related genes were all down-regulated in blastospores exposed to Culex. Conversely, when conidia were exposed to Culex, the pathogenicity genes involved in adhesion or cuticle degradation were up-regulated. Delayed host mortality following blastospore infection of Culex was probably due to lower adhesion rates of blastospores to the cuticle and enhanced host immune responses deployed to counter infection. The results here show that subtle differences in host-pathogen interactions can be responsible for significant changes in virulence when comparing mosquito species, having important consequences for biological control strategies and the understanding of pathogenicity processes.

Keywords: Mosquito; adhesion; atomic force microscopy; blastospores; conidia; cuticle; immune responses; infection processes.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Culex / immunology
  • Culex / microbiology*
  • Esterases / metabolism
  • Host-Pathogen Interactions*
  • Integumentary System / microbiology
  • Larva / immunology
  • Larva / microbiology
  • Metarhizium / genetics
  • Metarhizium / pathogenicity*
  • Monophenol Monooxygenase / metabolism
  • Mycoses / immunology
  • Mycoses / microbiology*
  • Pest Control, Biological
  • Spores / pathogenicity
  • Spores, Fungal / pathogenicity
  • Superoxide Dismutase / metabolism
  • Virulence / genetics

Substances

  • Monophenol Monooxygenase
  • Superoxide Dismutase
  • Esterases

Grants and funding

This work was supported by a visiting professor fellowship awarded to RIS (CAPES Coordenação de Aperfeiçoamento de Pessoal de Nível Superior BEX: 1720/14-7) and research grants awarded to RIS (FAPERJ: Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro E-26/102.353/2013; E-26/202.066/2016). The TEM at UENF was purchased with a grant from FAPERJ (E-26/110.040/2013). RIS and FLO are CNPq research fellows. We thank the Saudi Arabian Cultural Bureau in London (UKSACB) and Tabuk University, KSA, for funding AMA. AML would like to thank the Ser Cymru Fellowship scheme part funded by the European Regional Development Fund. This work was supported by the Centre for Nanohealth, Swansea University, UK. TMB was supported by a grant funded jointly by the Biotechnology and Biological Sciences Research Council, the Department for Environment, Food and Rural affairs, the Economic and Social Research Council, the Forestry Commission, the Natural Environment Research Council and the Scottish Government, under the Tree Health and Plant Biosecurity Initiative.