Figure 1.

Overview of scenarios for the origin of the eukaryotic cell. Schematic depiction of the classical three-domain tree of life (A) and a tree that supports fusion hypotheses in which the eukaryotic nuclear lineage evolved from within the archaeal radiation (B). Of the latter category, a number of hypotheses have been proposed that can be classified as amitochondriate fusion scenarios (i.e., the fusion event leads to a mitochondrion-lacking proto-eukaryotic lineage). The following scenarios have been outlined schematically: (1) The Serial Endosymbiosis Theory (Margulis et al. 2006), which involves a fusion between a Spirochete and a Thermoplasma-like archaeon; (2) “Syntrophy 1” representing the original syntrophic hypothesis proposed by Moreira and Lopez-Garcia (1998), involving a fusion between a syntrophic community comprising hydrogen producing deltaproteobacterial cells and hydrogen consuming methanogens; (3) “Pyrococcus + Gamma,” depicting the endokaryotic model proposed by Horiike et al. (2004) in which the eukaryotic lineage emerges via a Pyrococcus-related archaeal endosymbiont in a gammaproteobacterial host; (4) The eocyte model proposed by Lake (1988), which suggests that the eukaryotic nucleus evolved from a crenarchaeal lineage. Another class of fusion models involves scenarios in which the origin of the proto-eukaryotic lineage coincides with that of the mitochondrial origin (D), and include the following examples: (1) The Hydrogen hypothesis, involving the endosymbiosis of a hydrogen-producing alphaproteobacterium in a methanogen (Martin and Muller 1998); (2) Sulfur Syntrophy, in which eukaryotes evolved from a sulfur-dependent syntrophy between a Thermoplasma-like archaeaon and an alphaproteobacterium (Searcy and Hixon 1991; Pisani et al. 2007); (3) “Syntrophy 2,” which involves a refined version of the original Syntrophic hypothesis, which now also includes anaerobic methane oxidizing alphaproteobacterial cells from which the mitochondria supposedly emerged (Lopez-Garcia and Moreira 1999); (4) Phagocytosing Archaeon Theory (PhAT), which involves the engulfment of an alphaproteobacterium by a phagocytic archaeon belonging to the TACK superphylum (Martijn and Ettema 2013). Archaeal, bacterial, and (proto)eukaryotic cells are depicted in red, green and blue, respectively. A, Archaea; B, Bacteria; E, Eukarya; ND, not determined. (C,D, Inspired by Table 1 in Martin 2005.)