Fig. 6

Model for holobiont metabolism regulated by the nutritional demand of the asexual aphid reproductive system. The non-essential amino acid (non-EAA) glutamine (Gln) is transported from hemolymph to the bacteriome and developing embryos by transporter ApGLNT1. Glutamine and glutamate (Glu) are interconverted in the cytoplasm of the maternal bacteriome via the aphid genome-encoded GOGAT cycle (shaded gray) [51]. Glutamine and glutamate serve as metabolic precursors for amino acid biosynthesis in the holobiont [1]. We extend Price et al’s [2] regulatory model of amino acid biosynthesis in the maternal bacteriome to include the nutrient demands of embryogenesis such that developing embryos within viviparous aphids act as sinks that consume essential and non-essential amino acids. Arginine (Arg) demand in the embryos will maintain amino acid biosynthesis in the maternal bacteriome. Thus, we propose the localization of an as yet unidentified arginine transporter, putative A. pisum Arginine Transporter 1, p-ApARGT1, on the outside membrane of the maternal follicular epithelium and the apical membrane of the embryonic epithelium. The membranes that ApGLNT1 has been shown to localize to are shown with a thick green line. These membranes include the bacteriome and follicular epithelial membranes. The symbiosomal membrane and Buchnera cell membrane are marked with a thin green line and thin blue line, respectively. Uncharacterized transporters are indicated by question marks