Dopamine pathway characterization during the reproductive mode switch in the pea aphid

Aphids are major pests of most of the crops worldwide. Such a success is largely explained by the remarkable plasticity of their reproductive mode. They reproduce efficiently by viviparous parthenogenesis during spring and summer generating important damage on crops. At the end of the summer, viviparous parthenogenetic females perceive the photoperiod shortening and transduce this signal to their embryos that change their reproductive fate to produce sexual individuals: oviparous females and males. After mating, those females lay cold-resistant eggs. Earlier studies showed that some transcripts coding for key components of dopamine pathway were regulated between long days and short days conditions suggesting that dopamine might be involved in the transduction of seasonal cues prior to reproductive mode switch. In this study, we aimed at going deeper into the characterization of the expression dynamics of this pathway but also in the analysis of its functional role in this context in the pea aphid Acyrthosiphon pisum . We first analysed the level of expression of ten genes of this pathway in embryos and larval heads of aphids reared under long days (asexual producers) or short days (sexual producers) conditions. We then performed in situ hybridization experiments to localize in embryos the ddc and pale transcripts that are coding for two key enzymes in dopamine synthesis. Finally, Using CRISPR-Cas9 mutagenesis in eggs produced after the mating of sexual individuals, we targeted the ddc gene. We could observe strong melanization defaults in ddc mutated eggs, which confidently mimicked the Drosophila ddc phenotype. Nevertheless, such a lethal phenotype did not allow us to validate the involvement of dopamine as a signaling pathway necessary to trigger the reproductive mode switch in embryos.