aphid bacteriocyte
Aphid genome expression reveals host-symbiont cooperation in the production of amino acids
The evolution of intimate symbiosis requires the coordination of gene expression and content between the distinct partner genomes; this coordination allows the fusion of capabilities of each organism into a single, integrated metabolism. In aphids, the ten essential amino acids (EAA) are scarce in the phloem sap diet and are supplied by the obligate bacterial endosymbiont (Buchnera), which lives inside specialized cells called bacteriocytes. Although Buchnera's genome encodes most genes for EAA biosynthesis, several genes in EAA pathways are missing as are most genes for production of non-essential amino acids (NEAA). Additionally, it is unresolved whether the supply of nitrogen for amino acid biosynthesis is supplemented by recycling of waste ammonia. We compared pea aphid gene expression between bacteriocytes and other body tissues using RNA sequencing and pathway analysis, and exploiting the genome sequences available for both partners. We found that 26 genes underlying amino acid biosynthesis were up-regulated in bacteriocytes. Seven of these up-regulated genes fill the gaps of Buchnera's EAA pathways. In addition, genes underlying five NEAA pathways lost from Buchnera are up-regulated in bacteriocytes. Finally, our results reveal that two genes, glutamine synthetase (GS) and glutamate synthase (GltS), which potentially work together in the GOGAT cycle to assimilate ammonia into glutamate, are up-regulated in bacteriocytes. Thus, host gene expression and symbiont capabilities are closely integrated within bacteriocytes, which function as specialized organs of amino acid production. Furthermore, the GOGAT cycle may be a key source of nitrogen fueling the integrated amino acid metabolism of the aphid-Buchnera partnership.