Fig. 1. Evolutionary origin and loss of endomembrane components across the major eukaryotic lineages. Current understanding of the relationships between major eukaryotic lineages suggests a rapid radiation early in evolution that gave rise to the six major groups. The stem of the tree represents the transition between prokaryotes and eukaryotes, and is a period of radical innovation. Red dots indicate presumed secondary loss of factors, because multiple sampled lineages above the internode lack the relevant gene. Blue dots indicate that most taxa above the symbol possess a given gene or gene complement, suggesting that the system/complex/factor arose at this point. The split dot represents the apparent secondary reduction of multiple trafficking components in the metamonad Giardia contrasted with the expansion of trafficking machinery in its sister lineage Trichomonas. Significantly, many of the major components are universal, indicating that the basic mechanisms for vesicle specificity and fusion arose very early, together with establishment of major landmarks of the endomembrane system (i.e. clathrin-dependent endocytosis, exocytosis and recycling pathways). In addition, there is evidence for the acquisition of lineage-specific components (e.g. caveolin by metazoa) and multiple secondary losses (e.g. the Rab4 recycling pathway). Sampling bias in choice of experimental taxa means that novel factors in most lineages have not been identified. Rhizaria are shown as a dotted line because, at present, there are no genome sequences for this group. Figure redrawn and modified from Field et al. (Field et al., 2006), with permission from Landes Bioscience and Springer Science + Business Media.