Living With Symbionts or Oragnelles

Red Aphids

Red Aphids

The cells of all multicellular plants and animals are eukaryotic cells. A feature of eukaryotic cells is the presence of mitochondria and other organelles. Organelles contain their own genes, but in diminished numbers compared to free living bacteria and prokaryotes. Current biological theory suggests that organelles are evolved from free living prokaryotes that were “captured” by larger eukaryotic cells. Over time, organelles lost many of their original genes that were no longer needed because the eukaryotic host cell provided them. There is evidence that prokaryote genes may transferred to the host and incorporated into the host genome. Such a process would involve incorporation of symbiont DNA fragments into the host geneome, acquiring the appropriate signals to express the gene export the gene product to the organelle.

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
DOI: 10.1016/j.cub.2014.06.038

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
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