Aphid

Aphids harbor endosymbiotic Buchnera bacteria that synthesize amino acids the aphid cannot produce from its plant sap diet. Buchnera are transmitted vertically - aphids pass bacteria to offspring within eggs. Neither can survive without the other.

Buchnera genomes have evolved to optimize serving aphid needs, losing genes for functions unnecessary within the host while retaining genes for essential amino acid synthesis. Even in these highly aligned relationships, residual conflicts persist - bacterial mutations that slightly reduce aphid fitness but increase bacterial replication still occur, though selection against such mutations is stronger than in horizontally transmitted symbionts.