Fig. 1.

Origins of the eukaryote nucleus based upon the AB/ABC hypothesis. The schematic presents a synthesis of the chronocyte (ABC hypothesis, white shaded area) and syntropic (AB hypothesis, grey shaded area) models. Both concur that the nucleus is an endosymbiont consequent of archaeal (A, red oval) and myxobacterial (B, orange oval) donors, however they differ in that the AB hypothesis proposes the “host” cell came from bacteria (B), whereas the ABC hypothesis concludes that the host cell was a chronocyte (C, green oval), wherein the archaea and myxobacteria were engulfed by the chronocyte. Symbiogenesis occurred (scribbled lines indicate genetic material) either prior and/or subsequent to the acquisition of the α-proteobacterium (α-P, ancestral mitochondrion) generating a hybrid archaeal-myxobacterial-α-proteobacterial nuclear material. While both hypotheses suggest different processes for the origins of the eukaryotic nucleus, they both involve endosymbiotic events, and conclude the nuclear genome is a genetic hybrid, consequent of symbiogenic events.