Figure 3. Conservation of the centriole and axoneme.

The centriole and the axoneme, i.e. the microtubule core of cilia and flagella, are conserved features of eukaryotes. These structures were probably present in the last eukaryotic common ancestor and are still found in most branches of the eukaryotic tree of life. Centrioles and axonemes were lost concomitantly during evolution of certain taxa, most notably angiosperms and higher fungi. Taxa in which all species have lost centrioles and axonemes are indicated by a red cross. Some taxa, such as amoebozoa, comprise species that form flagella (like Physarum) as well as species completely devoid of centrioles and axonemes (like Dictyostelium). The phylogenetic tree of life is adapted from the Tree of Life web project (http://tolweb.org/tree). The last eukaryotic common ancestor (inset) is schematically represented as a single-celled organism (the plasma membrane and the nucleus are in gray) bearing two motile flagella (in black). Schematic representations of cross-sections through the centriole and the axoneme are also shown.