Preview. The incredible expanding ancestor of eukaryotes

Abstract

Comparing the genome sequences of free-living organisms in the five eukaryotic supergroups enables predictions to be made about the genome of the last common ancestor of eukaryotes. The genome sequence of the amoeboflagellate Naegleria gruberi reported by Fritz-Laylin et al. (2010) reveals the surprising complexity of this unicellular organism and, by inference, of the last common eukaryotic ancestor.

Figure 1. Eukaryotic Evolution

The five eukaryotic supergroups—Excavates, Rhizaria, Unikonts, Chromalveolates, and Plantae—are shown to diverge directly from the last common ancestor (black circle) because the relationship between individual supergroups is uncertain (Keeling, 2007). Analysis of the genome sequence of the free-living amoeboflagellate Naegleria gruberi in the Excavate supergroup reveals that this organism has 4133 genes that are shared by at least one other eukaryote supergroup (Fritz-Laylin et al., 2010). These 4133 genes are inferred to be ancestral genes that were present in the last common ancestor of eukaryotes, suggesting that the this common ancestor was surprisingly complex. The numbers of putative ancestral genes present in selected major clades are indicated in blue. The Rhizaria supergroup is included for completeness, despite the current absence of sequenced genomes. The names of groups that include mostly parasites are italicized. Branch lengths are arbitrary.