Abstract
The harlequin ladybird, Harmonia axyridis, has emerged as a model species for invasion biology, reflecting its remarkable capacity to outcompete native ladybird species when introduced into new habitats. This ability may be associated with its prominent resistance to pathogens and intraguild predation. We recently showed that the constitutive antibacterial activity present in the haemolymph of H. axyridis beetles can be attributed to the chemical defence compound harmonine. Here, we demonstrate that H. axyridis differs from other insects, including the native ladybird Coccinella septempunctata, by reducing rather than increasing the antimicrobial activity of its haemolymph following the injection of bacteria. However, both species produce new or more abundant proteins in the haemolymph, indicating that bacterial challenge induces innate immune responses associated with the synthesis of immunity-related proteins. Our results suggest that H. axyridis beetles can switch from constitutive chemical defence to inducible innate immune responses, supporting hypothesis that inducible antimicrobial peptides protect host beetles against pathogens that survive constitutive defences. These alternative antimicrobial defence mechanisms may reflect a trade-off resulting from fitness-related costs associated with the simultaneous synthesis of harmonine and antimicrobial peptides/proteins.
Highlights
The harlequin ladybird, multicoloured ladybird or Asian ladybird beetle (Harmonia axyridis) is a model species for invasion biology, because it can successfully outperform native ladybird species when introduced into new habitats [1]
We recently showed that the constitutive antibacterial activity present in the haemolymph of H. axyridis beetles can be attributed to the chemical defence compound harmonine
We recently reported that the constitutive antimicrobial activity present in the haemolymph of this species is mediated by the chemical defence compound harmonine, known as (17R,9Z)-1,17-diaminooctadec-9-ene, which displays broad-spectrum activity against even human pathogens, including the agents responsible for tuberculosis and malaria [4]
Summary
The harlequin ladybird, multicoloured ladybird or Asian ladybird beetle (Harmonia axyridis) is a model species for invasion biology, because it can successfully outperform native ladybird species when introduced into new habitats [1]. We observed the opposite phenomenon in H. axyridis, i.e. constitutive antibacterial activity was inhibited following the injection of bacteria These contrasting results led us to postulate that the injection of bacteria into H. axyridis causes a switch from the constitutive production of harmonine to the inducible synthesis of immunity-related proteins such as antimicrobial peptides. To address this hypothesis, we quantified antimicrobial activity and harmonine levels in untreated H. axyridis eggs, larvae and beetles, and beetles injected with a bacterial suspension or with a control buffer, and we investigated the haemolymph protein content of treated and untreated H. axyridis and C. septempunctata beetles by polyacrylamide gel electrophoresis. The main signal of the extract ion chromatogram (m/z 1⁄4 283,3 + 0.2 Da) was integrated and correlated with the signal of synthetic harmonine
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