Figure 1.

(a) Schematic diagram of archaeal phylogeny showing the evolutionary relationships between archaeal lineages for which completely sequenced genomes are available (updated from [12]). 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) [13] and signature genes [14]. (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.