Differential selection pressure on Buchnera aphidicola from gall aphids associated with their primary hosts plant Rhus and Pistacia

Obligate endosymbionts such as Buchnera aphidicola are expected to evolve under strong genetic drift due to small effective population sizes and repeated transmission bottlenecks, which may limit the efficacy of natural selection. However, whether ecological differences among aphid host plants can impose detectable selection on these symbionts remains unclear. Here, we investigated the evolutionary dynamics of Buchnera in gall-forming aphids associated with two distinct primary host plants, Rhus and Pistacia. Using comparative genomics of 18 species, we reconstructed phylogenetic relationships and analyzed genome-wide patterns of selection. We found that Buchnera from aphids associated with Pistacia exhibited stronger purifying selection, whereas lineages associated with Rhus showed a progressive relaxation of selective constraints. This consistent directional pattern is unlikely to be explained by stochastic processes alone, suggesting a role for host plant-associated selection. To further assess short-term physiological responses, we compared transcriptomes of Schlechtendalia chinensis across its primary host plant (Rhus chinensis) and secondary overwintering host (the moss Plagiomnium maximoviczii). Genes involved in amino acid biosynthesis were downregulated during overwintering, whereas stress-response genes were upregulated, indicating transient physiological adjustments to environmental conditions. Together, these results demonstrate that ecological variation among aphid host plants can shape the long-term evolution of an obligate symbiont despite the strong influence of genetic drift, while seasonal host shifts primarily affect short-term gene expression rather than long-term selective regimes.