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Abstract:
The harlequin ladybird Harmonia axyridis has emerged as a model species in the context of invasion
biology and possesses an expanded repertoire of antimicrobial peptides (AMPs). Here we measured
the expression of 22 AMP genes in adult beetles from native and introduced populations, and from a
biocontrol population, allowing us to compare populations differing in terms of invasive performance.
Strikingly, we observed population-specific expression profiles for particular AMPs. Following an
immune challenge, the genes for Coleoptericin1 (Col1), Coleoptericin-likeB and Defensin1 were induced
up to several thousand times more strongly in the invasive populations compared to the native and
biocontrol populations. To determine the role of Col1 in pathogen resistance, the corresponding gene
was silenced by RNA interference (RNAi), causing higher mortality in beetles subsequently infected
with the entomopathogen Pseudomonas entomophila. The RNAi-triggered susceptibility to this
pathogen was reversed by the injection of a synthetic Col1 peptide. We show that a native population
is more susceptible to P. entomomophila infection than an invasive population. This is the first study
demonstrating population-specific differences in the immune system of an invasive species and
suggests that rapid gene expression changes and a highly adaptive immune system could promote
pathogen resistance and thereby invasive performance.
