![]() ![]() Here we outline the origin of the nucleus, the membrane-bounded organelle that defines eukaryotes. This specific model of syntrophic chimeric fusion can be proved by sequence comparison of functional domains of motility proteins isolated from candidate taxa.Īll living beings are composed of cells and are unambiguously classifiable into one of two categories: prokaryote (bacteria) or eukaryote (nucleated organisms). The nucleus evolved in karyomastigont ancestors by detachment at least five times (archamoebae, calonymphids, chlorophyte green algae, ciliates, foraminifera). As predecessor of standard mitosis, the karyomastigont preceded free (unattached) nuclei. The karyomastigont organellar system, common in extant amitochondriate protists as well as in presumed mitochondriate ancestors, minimally consists of a single nucleus, a single kinetosome and their protein connector. This “earliest branching protist” that formed by permanent DNA recombination generated the nucleus as a component of the karyomastigont, an intracellular complex that assured genetic continuity of the former symbionts. Selection pressure for speed swimming and oxygen avoidance led to an ancient analogue of the extant cosmopolitan bacterial consortium “Thiodendron latens.” By eubacterial-archaebacterial genetic integration, the chimera, an amitochondriate heterotroph, evolved. ![]() The archaebacterium, a thermoacidophil resembling extant Thermoplasma, generated hydrogen sulfide to protect the eubacterium, a heterotrophic swimmer comparable to Spirochaeta or Hollandina that oxidized sulfide to sulfur. A chimeric cell evolved via symbiogenesis by syntrophic merger between an archaebacterium and a eubacterium. We present a testable model for the origin of the nucleus, the membrane-bounded organelle that defines eukaryotes. ![]()
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