When a new and apparently primitive organism is discovered, how can we decide if it truly reflects an ancient organism that evolved slowly over many millions of years or if it arrived at its present compact state by jettisoning pieces of its genome, having developed a relationship with another living being (that is, living as a parasite or symbiote)? Phylogenetic analysis of the ribosomal RNA from Nanoarchaeum equitans indicated that its ancestors might have been amongst the first archaeons, dating back to just after the split between Archaea and Eukaryota. This symbiont is only about 400 nm in diameter and lives in physical contact with the 2-
m archaeon Ignicoccus.
Waters et al. have sequenced and analyzed the Nanoarchaeum genome (~490 kilobases). It contains many if not all of the components for handling genetic information (replication, transcription, translation, and repair) but lacks many of the biosynthetic enzymes to make building blocks such as amino acids and nucleotides; these are presumably acquired from its partner, either via membrane transporters, which are encoded in its genome, or by vesicular transfer from Ignicoccus, which would explain the need for physical contact. Unlike other cases of evolution by reduction, only about 5% of the genome does not code for proteins or RNAs, and few pseudogenes are present. Furthermore, the hot and anerobic environment (a submarine vent north of Iceland) in which Nanoarchaeum lives is similar to early life conditions. Taken together, this evidence is consistent with the proposal that this unusual organism is indeed primitive. -- GJC
Proc. Natl. Acad. Sci. U.S.A. 100, 12984 (2003)
