Chong, R. A., & Moran, N. A. (2018). Evolutionary loss and replacement of Buchnera, the obligate endosymbiont of aphids. The ISME journal, 12(3), 898-908.
Chong and Moran (2018) provide evidence in "Evolutionary loss and replacement of Buchnera, the obligate endosymbiont of aphids" that unlike the vast majority of all other aphids, Geopemphigus aphids collected on pistache trees in western Texas and in Mexico do not harbor Buchnera spp. endosymbiotic bacteria, but a different prokaryote closely related to the Bacteroidetes endosymbiotic bacteria.
The 16S rRNA sequencing data collected from harvested aphids (in tree galls) suggested that the aphids studied did not have Buchnera within them, but a Bacteroidetes-related bacteria instead. Additionally, the genomic data harvested allowed analysis of the functional capacity Bacteroidetes endosymbiont as determined by homology-based annotations and metabolic pathway analysis, revealing that the Bacteroidetes endosymbionts appear to have adapted to the symbiosis machinery put in place by ancestral Buchnera, given the overlap of functional role between ancestral and current endosymbiont genomic features. On the host side, genes within the Geopemphigus host genome are related to peptidoglycan recycling or other cell wall degradation enzymes, potentially playing a role in establishing and maintaining a symbiotic relationship.
Combined with other literature referenced in this study which found similar endosymbiont replacement events over millions of years of evolutionary time, the process of endosymbiont loss and replacement is likely linked to genome degradation over time as effective population sizes are diminished and certain selective pressures are reduced by localization within the host.
Does Methionine use by the endosymbiont depend upon host export or bacterial import? Who controls this dynamic? If the host is driving this bargain, then there is the possibility that Methionine regulation is one way to prevent an endosymbiont from escaping or becoming pathogenic.
Lack of gluconeogenesis is sensible as a result of the vast amounts of glucose which are provided to the bacteria likely pushes the gluconeogenesis/glycolysis equilibrium to use the glucose to make energy (glycolysis), rather than to produce even more glucose.
What kinds of temperature shielding does an aphid host offer its bacterial endosymbionts?