(a) Schematic diagram of archaeal phylogeny showing the evolutionary relationships between archaeal lineages for which completely sequenced genomes are available (updated from ). The positions of Ignicoccus hospitalis and Nanoarchaeum equitans are indicated by arrows. The position of N. equitans has been controversial [2,8], but the position suggested here has been recently supported by a specific synapomorphy (a derived character state shared by two or more groups)  and signature genes . (b) Electron micrograph showing a cell of N. equitans attached to a cell of I. hospitalis. The scale bar is 100 nm. Courtesy of Dr Rachel Reinhard, Regensburg University. OM: outer membrane.
Forterre et al. Journal of Biology 2009 8:7 doi:10.1186/jbiol110