| Summary: | Generalist parasitoids of aphids, such as the wasp <i>Aphidius ervi</i>, display significant differences in terms of host preference and host acceptance, depending on the host on which they developed (natal host), which is preferred over a non-natal host, a trait known as host fidelity. This trait allows females to quickly find hosts in heterogeneous environments, a process mediated by chemosensory/olfactory mechanisms, as parasitoids rely on olfaction and chemical cues during host selection. Thus, it is expected that proteins participating in chemosensory recognition, such as odorant-binding proteins (OBPs) and odorant receptors (ORs) would play a key role in host preference. In this study, we addressed the effect of parasitoid reciprocal host switching between two aphid hosts (<i>Sitobion avenae</i> and <i>Acyrthosiphon pisum</i>) on the expression patterns of chemosensory genes in the wasp <i>A. ervi.</i> First, by using a transcriptomic approach based on RNAseq of <i>A. ervi</i> females reared on <i>S. avenae</i> and <i>A. pisum</i>, we were able to annotate a total of 91 transcripts related to chemoperception. We also performed an in-silico expression analysis and found three OBPs and five ORs displaying different expression levels. Then, by using qRT-PCR amplification, we found significant differences in the expression levels of these eight genes when the parasitoids were reciprocally transplanted from <i>S. avenae</i> onto <i>A. pisum</i> and vice versa. This suggests that the expression levels of genes coding for odorant receptors and odorant-binding proteins would be regulated by the specific plant−aphid host complex where the parasitoids develop (maternal previous experience) and that chemosensory genes coding for olfactory mechanisms would play a crucial role on host preference and host acceptance, ultimately leading to the establishment of host fidelity in <i>A. ervi</i> parasitoids.
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