The symbiotic relationship with aphids began between 160 million and 280 million years ago, and has persisted through maternal transmission and cospeciation. Aphids have developed a bilobed bacteriome containing sixty to eighty bacteriocyte cells in which the life cycle of "Buchnera" associated with aphids is confined to. A mature aphid may carry an estimated 5.6 × 106 "Buchnera" cells. "Buchnera" has lost regulatory factors, allowing continuous overproduction of tryptophan and other amino acids. Each bacteriocyte contains multiple vesicles, symbiosomes derived from the cell membrane.