Abstract
Metarhizium is an entomopathogenic fungus widely employed in the biological control of arthropods. Hemocytes present in the hemolymph of invertebrates are the cells involved in the immune response of arthropods. Despite this, knowledge about Rhipicephalus microplus hemocytes morphological aspects as well as their role in response to the fungal infection is scarce. The present study aimed to analyze the hemocytes of R. microplus females after Metarhizium robertsii infection, using light and electron microscopy approaches associated with the cytotoxicity evaluation. Five types of hemocytes (prohemocytes, spherulocytes, plasmatocytes, granulocytes, and oenocytoids) were described in the hemolymph of uninfected ticks, while only prohemocytes, granulocytes, and plasmatocytes were observed in fungus-infected tick females. Twenty-four hours after the fungal infection, only granulocytes and plasmatocytes were detected in the transmission electron microscopy analysis. Hemocytes from fungus-infected tick females showed several cytoplasmic vacuoles with different electron densities, and lipid droplets in close contact to low electron density vacuoles, as well as the formation of autophagosomes and subcellular material in different stages of degradation could also be observed. M. robertsii propagules were more toxic to tick hemocytes in the highest concentration tested (1.0 × 108 conidia mL−1). Interestingly, the lowest fungus concentration did not affect significantly the cell viability. Microanalysis showed that cells granules from fungus-infected and uninfected ticks had similar composition. This study addressed the first report of fungal cytotoxicity analyzing ultrastructural effects on hemocytes of R. microplus infected with entomopathogenic fungi. These results open new perspectives for the comprehension of ticks physiology and pathology, allowing the identification of new targets for the biological control.
Highlights
Ticks are obligate hematophagous ectoparasites relevant to public and veterinary health (De la Fuente et al, 2008; Kernif et al, 2016)
Ticks infected with 107 conidia mL−1 showed 21% ± 2.35 mortality 24 h after infection and 48% ± 2.35 mortality 48 h after infection, while ticks infected with 108 conidia mL−1 had 31,66% ± 2.0 and 83,3% ± 4.71 mortality 24 and 48 h after infection, respectively
Some entomopathogenic fungi are known to be highly virulent to ticks, as well as to different species of insects (Roberts and St Leger, 2004; Wang and St Leger, 2006; Leemon and Jonsson, 2008; Samish et al, 2014; Perinotto et al, 2017)
Summary
Ticks are obligate hematophagous ectoparasites relevant to public and veterinary health (De la Fuente et al, 2008; Kernif et al, 2016). Metarhizium anisopliae sensu lato (s.l.) is a complex of species of cosmopolitan entomopathogenic fungi which includes species that can infect specific hosts, as Metarhizium acridum with Orthoptera insects, or a wider range of insect groups, such as Metarhizium robertsii and M. anisopliae sensu stricto (s.s.). These fungi are considered endophytes and rhizosphere competent (Hu et al, 2014; Vega, 2018). All R. microplus life stages have been shown to be sensitive to entomopathogenic fungal infection (Alonso-Díaz et al, 2007; Leemon and Jonsson, 2008), either in vitro assays (Perinotto et al, 2017), associated with chemical acaricides (Bahiense et al, 2008; Webster et al, 2015), or in vivo tests using available commercial products (Camargo et al, 2016)
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