The pea aphid, Acyrthosiphon pisum, contains a gene, RlpA4, that has orthologues in other aphids and in many prokaryotes. The RlpA4 gene is incorporated into the pea aphid chromosomes and is part of its genome. The pea aphid’s obligate symbiont, Buchnera aphidicola, lacks many genes including RlpA4. A group of scientists* have demonstrated that the protein encoded by RlpA4 is produced in some pea aphid cells (maternal bacteriocytes) that harbor the symbiont. The proteins are exported from the aphid cells and into the Buchnera aphidicola. Thus Buchnera aphidicola has some features of an organelle in which some essential genes of the organelle are part of the host geneome.
*Atsushi Nakabachi, Kinji Ishida, Yuichi Hongoh, Moriya Ohkuma, Shin-ya Miyagishima. Endosymbiosis: Protein Targeting Further Erodes the Organelle/Symbiont Distinction
Current Biology, Volume 24, Issue 14, 21 July 2014, Pages R654-R655