The maternal vGluT2 and embryonic mGluR3 signaling relay system controls offspring wing dimorphism in pea aphid

Transgenerational phenotypic plasticity (TPP) refers to the phenomenon that environmental conditions experienced by one generation can influence the phenotype of subsequent generations to adapt to the environment without modification of their DNA sequences. Aphid wing dimorphism is a textbook example of TPP by which a maternal aphid perceives the environmental cues to decide the wing morph of her offspring. However, the signaling mechanism from mother to daughter remains unclear. In this study, we showed that the population density and physical contact caused high proportion of winged offspring in the pea aphid Acyrthosiphon pisum . Its vesicular glutamate transporter 2 ( ApvGluT2 ) and metabotropic glutamate receptor 3 ( ApmGluR3 ) were identified by tissue-specific RNA-seq as differentially expressed genes in the head and embryo respectively between solitary and more densely housed maternal aphids. Elevated expression of brain ApvGluT2 and embryonic ApmGluR3 led to increases in the winged proportion. Knockdown of either gene inhibited phosphorylation of ApFoxO in embryos. Furthermore, EMSA showed that dephosphorylated ApFoxO directly bound to the promotor of hedgehog ( ApHh ), a morphogen gene for wing development, to repress its transcription in stage 20 embryos, causing a lower winged proportion. Our results demonstrated that brain vGluT2 and embryonic mGluR3 coordinately relayed the maternal physical contact signals and control wing development in offspring, showcasing a novel regulatory mechanism underlying physical contact-dependent, transgenerational wing dimorphism in aphids. Significance Transgenerational phenotypic plasticity is a widespread phenomenon that endows organisms and their progenies with abilities to maximize their fitness under different habitats. Aphids exemplify a successful evolutionary strategy through their transgenerational wing dimorphism. We show that transcripts of a vesicular glutamate transporter vGluT2 in maternal brain and a metabotropic glutamate receptor mGluR3 in embryo varied in a density-dependent manner and that increased expression of vGluT2 and mGluR3 were necessary triggers for signal transduction, leading to production of a high proportion of winged offspring. The vGluT2 - mGluR3 cascade increased the phosphorylation of embryonic FoxO, which released its suppression on hedgehog in stage 20 embryos. These findings have brought novel insight into the complicated parent-offspring communications during the wing morph transitions of aphids. ### Competing Interest Statement The authors have declared no competing interest.