Abstract
Symbioses between animals and microbes are often described as mutualistic, but are subject to tradeoffs that may manifest as shifts in host and symbiont metabolism, cellular processes, or symbiont density. In pea aphids, the bacterial symbiont Buchnera is confined to specialized aphid cells called bacteriocytes, where it produces essential amino acids needed by hosts. This relationship is dynamic; Buchnera titer varies within individual aphids and among different clonal aphid lineages, and is affected by environmental and host genetic factors. We examined how host genotypic variation relates to host and symbiont function among seven aphid clones differing in Buchnera titer. We found that bacteriocyte gene expression varies among individual aphids and among aphid clones, and that Buchnera gene expression changes in response. By comparing hosts with low and high Buchnera titer, we found that aphids and Buchnera oppositely regulate genes underlying amino acid biosynthesis and cell growth. In high-titer hosts, both bacteriocytes and symbionts show elevated expression of genes underlying energy metabolism. Several eukaryotic cell signaling pathways are differentially expressed in bacteriocytes of low- versus high-titer hosts: Cell-growth pathways are up-regulated in low-titer genotypes, while membrane trafficking, lysosomal processes, and mechanistic target of rapamycin (mTOR) and cytokine pathways are up-regulated in high-titer genotypes. Specific Buchnera functions are up-regulated within different bacteriocyte environments, with genes underlying flagellar body secretion and flagellar assembly overexpressed in low- and high-titer hosts, respectively. Overall, our results reveal allowances and demands made by both host and symbiont engaged in a metabolic "tug-of-war."