Odorant Binding Proteins in the vetch aphid Megoura viciae (Hemiptera: Aphididae) display distinct expression patterns

Insect odorant binding proteins (OBPs) are required for chemoreception and play a key role in transporting hydrophobic semiochemicals across the lymph in chemosensilla to the olfactory receptors (ORs). To investigate the putative roles of chemosensilla involved in the selection of the host plant, scanning electron microscopy (SEM) was performed on antennae, legs, mouthparts and cauda of the adult vetch aphid Megoura viciae (Buckton), an economically important broad bean pest. In order to identify chemoreception candidate transcripts, RNA-seq was performed on Megoura viciae antennae. The de novo transcriptome assembly resulted in 43,251 predicted transcripts. The assembled data were used in the annotation of candidate OBP genes, on the basis of a hand-curated database of insect OBP proteins, leading to the identification of ten candidate OBP genes. Additionally, the differential expression patterns of the identified OBPs, in different body parts and at different developmental stages, were measured by real-time qRT-PCR. MvicOBP1, MvicOBP3, MvicOBP6, MvicOBP7 and MvicOBP8, whose antibodies were already available, were selected for further experiments of whole-mount immunolocalization on different tissues of the aphid (antennae, mouthparts, cornicles and the cauda). Moreover, the behavioral response of M. viciae to the components of the alarm pheromone (E)-β-farnesene, (±)-α-pinene, β-pinene, (-)-α-pinene, (+)-limonene and a mixture composed by (E)-β-farnesene, (-)-α-pinene and β-pinene was investigated using a glass Y-tube olfactometer. The obtained outcomes offer a chance to discuss OBPs: the connection between tissue-specific OBPs, their differential expression and their putative role in Megoura viciae chemoreception. The understanding of the molecular mechanisms underlying olfactory perception in this aphid may lead to the development of new biological control strategies for this pest.