Glyphosate Inhibits Melanization and Increases Susceptibility to Infection in Insects

Abstract

13Melanin is a black-brown pigment found throughout all kingdoms of life playing diverse 14 roles including: UV protection, thermoregulation, oxidant scavenging, arthropod immunity, and 15 microbial virulence. Given melanin's broad functions in the biosphere, particularly in insect 16 immune defenses, it is important to understand how environmental conditions affect 17 melanization. Glyphosate, the most widely used herbicide, inhibits melanin production. Here we 18 elucidate the mechanism underlying glyphosate's inhibition of melanization demonstrate the 19 herbicide's multifactorial effects on insects. Glyphosate acts as an antioxidant and disrupts the 20 oxidation-reduction balance of melanization. The drug reduced wax moth larvae survival after 21 infection, increased parasite burden in malaria-transmitting mosquitoes, and altered midgut 22 microbiome composition in adult mosquitoes. These findings suggest that glyphosate's 23 environmental accumulation could contribute to the so called insect apocalypse, characterized 24 by species declines, by rendering them more susceptible to microbial pathogens due to 25 melanization inhibition, immune impairment, and perturbations in microbiota composition. 26 27 29 30 2Melanin is produced through a series of oxidation and reduction reactions, which are 32 typically catalyzed by two distinct classes of enzymes: laccases (EC. 1.10.3.2) and phenol 33 oxidases, the latter is divided into tyrosinases (EC. 1.14.18.1) and catechol oxidases (EC. 34 1.10.3.1) (Whitten and Coates, 2017). Tyrosinases are copper metalloenzymes found 35 throughout nature in fungi, protists, arthropods, birds, and mammals, and are responsible for 36 two catalytic roles: 1) hydroxylation of monophenols into ortho-diphenols, followed by 2) two-37 electron oxidation of ortho-catechols into ortho-quinones (Ramsden and Riley, 2014). In melanin 38 biosynthesis, tyrosinase first converts 3,4-dihydroxyphenylalanine (L-DOPA) into dopaquinone 93 mushroom tyrosinase model, and validate GLYPH as an inhibitor of insect melanogenesis using 94 Galleria mellonella and Anopheles gambiae. We further characterize the effects of GLYPH on 95 G. mellonella susceptibility to C. neoformans infection, and A. gambiae susceptibility to 96 Plasmodium falciparum infection. Our results indicate that GLYPH can be a major immune 97 modulator of insects through its effect on melanization. 98 99 4 RESULTS 100 GLYPH Inhibits Dopaquinone Production 101To investigate how glyphosate (GLYPH) inhibited melanization, we evaluated the 102 formation of melanin intermediates in a step-wise manner using a commercially available fungal 103 tyrosinase. Although this tyrosinase differs from insect PO, the melanization reaction in these 104 systems follows the same Mason-Raper pathway (Mason, 1948;Raper, 1927). First, L-DOPA is 105 oxidized into dopaquinone (DQ) enzymatically or spontaneously (García-Borrón and Sánchez, 106 2011). Using L-DOPA with and without tyrosinase, we monitored the initial step of melanin 107 production in a controlled syste...

Melanization is an important antifungal defense mechanism in Galleria mellonella hosts

“…Previously, we found that C. neoformans is encapsulated inside immune system-produced melanins during infection of G. mellonella , providing evidence that the melanin-based immune response is activated against C. neoformans in G. mellonella 30 (Figure 1A). To evaluate whether insect melanin encapsulation kills C. neoformans , we assessed viability using a GFP-expressing strain of C. neoformans , which expresses GFP under an actin promotor.…”

“…Infection of G. mellonella larvae was performed as previously described 30 . Briefly, washed C. neoformans or C. albicans cultures, resuspended to 10 7 cells/ml were injected in the right rear proleg of larvae ranging from 175 to 225 mg.…”

“…In C. albicans, we are able to observe how some melanin-encapsulated yeast are able to break through the melanization, and form melaninevasive hyphae and pseudohyphae during infection. The direct association of insect melanization with antifungal defense further heighten concern that pesticides that inhibit the melanin reaction 30 could have untoward and unpredictable effects on insect populations. AC; Visualization -DFQS, MK; Supervision -DFQS, JMH, AC; Project Administration -DFQS, AC; Funding Acquisition -JMH, AC Supplementary Figure 3.…”

Detection of insect health with deep learning on near-infrared sensor data

“…The health of insects is threatened by chemicals (1,2), and microbial pathogens (3,4), especially insect-pathogenic fungi, which are important regulators of natural insect populations (5,6). In addition, warming temperatures and human mediated radiation and chemical exposure may further exaggerate the negative influence on insect health (7,8), and there is thus a dire need for monitoring and studying insect health to preserve insect biodiversity.…”

Herbicide Roundup shows toxic effects in nontarget organism Drosophila

“…The glyphosate effects vary in a species‐dependent manner, accordingly making it difficult to clarify similarities and distinctiveness resulting in herbicide exposure. It was already found that glyphosate exposure is associated with genotoxicity, oxidative damage, and imbalanced melanin synthesis leading to decreased melanin‐based immunity, health, and survival in insects (de Aguiar et al, 2016; Kaya et al, 2000; Qiao et al, 2016; Smith et al, 2021).…”