Aphids are major crop pests, most species attacking crops specialize on a narrow range of plant species from a single family. By contrast, Aphis gossypii is a highly polyphagous species, for which host races specializing on particular crops have been clearly described. Salivary components, which aphids inject into the phloem via their stylets, play a key role in establishing compatible interactions between plants and aphids, and are probably involved in specialization.
We used the extensive resources available for Myzus persicae and Acyrthosiphon pisum to identify putative salivary proteins expressed in Aphis gossypii, despite the lack of genomic resources for this species. In silico, we identified 51 putative salivary proteins; we focused on 17 genes with orthologs in at least one aphid species, assuming that some of the conserved genes expressed in salivary glands are involved in host specialization. We amplified and sequenced 10 coding sequences in full, from 17 clones of Aphis gossypii specialising on plants from Malvaceae, Cucurbitaceae or Solanaceae. We reconstructed the phylogenetic tree for these genes, on which we identified a clade corresponding to all clones specializing on cucurbits. Three of these genes were under positive selection.
Full adaptation to a particular host plant may require a combination of alleles at quantitative trait loci in aphids. The three genes we identified could potentially be part of a cocktail of effectors manipulating the immune system of cucurbits and therefore responsible for A. gossypii specialization on that plant family.