Abstract
Theories for the origin of sex traditionally start with an asexual mitosing cell and add recombination, thereby deriving meiosis from mitosis. Though sex was clearly present in the eukaryote common ancestor, the order of events linking the origin of sex and the origin of mitosis is unknown. Here, we present an evolutionary inference for the origin of sex starting with a bacterial ancestor of mitochondria in the cytosol of its archaeal host. We posit that symbiotic association led to the origin of mitochondria and gene transfer to host's genome, generating a nucleus and a dedicated translational compartment, the eukaryotic cytosol, in which-by virtue of mitochondria-metabolic energy was not limiting. Spontaneous protein aggregation (monomer polymerization) and Adenosine Tri-phosphate (ATP)-dependent macromolecular movement in the cytosol thereby became selectable, giving rise to continuous microtubule-dependent chromosome separation (reduction division). We propose that eukaryotic chromosome division arose in a filamentous, syncytial, multinucleated ancestor, in which nuclei with insufficient chromosome numbers could complement each other through mRNA in the cytosol and generate new chromosome combinations through karyogamy. A syncytial (or coenocytic, a synonym) eukaryote ancestor, or Coeca, would account for the observation that the process of eukaryotic chromosome separation is more conserved than the process of eukaryotic cell division. The first progeny of such a syncytial ancestor were likely equivalent to meiospores, released into the environment by the host's vesicle secretion machinery. The natural ability of archaea (the host) to fuse and recombine brought forth reciprocal recombination among fusing (syngamy and karyogamy) progeny-sex-in an ancestrally meiotic cell cycle, from which the simpler haploid and diploid mitotic cell cycles arose. The origin of eukaryotes was the origin of vertical lineage inheritance, and sex was required to keep vertically evolving lineages viable by rescuing the incipient eukaryotic lineage from Muller's ratchet. The origin of mitochondria was, in this view, the decisive incident that precipitated symbiosis-specific cell biological problems, the solutions to which were the salient features that distinguish eukaryotes from prokaryotes: A nuclear membrane, energetically affordable ATP-dependent protein-protein interactions in the cytosol, and a cell cycle involving reduction division and reciprocal recombination (sex).