. "3 The Chimeric Eukaryote: Origin of the Nucleus from the Karyomastigont in Amitochondriate Protists." Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years after Stebbins. Washington, DC: The National Academies Press, 2000.
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Variation and Evolution in Plants and Microorganisms: TOWARD A NEW SYNTHESIS 50 YEARS AFTER STEBBINS
structures hypertrophied as typically they do in extant motility symbioses (Margulis, 1993). The archaebacterium-eubacterium swimmer attachment system became the karyomastigont. The proteinaceous karyomastigont that united partner DNA in a membrane-bounded, jointly produced package, assured stability to the chimera. All of the DNA of the former prokaryotes recombined inside the membrane to become nuclear DNA while the protein-based motility system of the eubacterium, from the moment of fusion until the present, segregated the chimeric DNA. During the lower Proterozoic eon (2,500–1,800 million years ago), many interactions inside the chimera generated protists in which mitosis and eventually meiotic sexuality evolved. The key concept here is that the karyomastigont, retained by amitochondriate protists and later by their mitochondriate descendants, is the morphological manifestation of the original archaebacterial-eubacterial fused genetic system. Free (unattached) nuclei evolved many times by disassociation from the rest of the karyomastigont. The karyomastigont, therefore, was the first microtubule-organizing center.
KARYOMASTIGONTS PRECEDED NUCLEI
The term “karyomastigont” was coined by Janicki (1915) to refer to a conspicuous organellar system he observed in certain protists: the mastigont (“cell whip, ” eukaryotic flagellum, or undulipodium, the [9 (2) + (2)] microtubular axoneme underlain by its [9 (3) + 0)] kinetosome) attached by a “nuclear connector” or “rhizoplast” to a nucleus. The need for a term came from Janicki's work on highly motile trichomonad symbionts in the intestines of termites where karyomastigonts dominate the cells. When kinetosomes, nuclear connector, and other components were present but the nucleus was absent from its predictable position, Janicki called the organelle system an “akaryomastigont.” In the Calonymphidae, one family of entirely multinucleate trichomonads, numerous karyomastigonts, and akaryomastigonts are simultaneously present in the same cell (e.g., Calonympha grassii) (Kirby and Margulis, 1994).
The karyomastigont, an ancestral feature of eukaryotes, is present in “early branching protists” (Dacks and Redfield, 1998; Delgado-Viscogliosi et al., 2000; Edgcomb et al., 1998). Archaeprotists, a large inclusive taxon (phylum of Kingdom Protoctista) (Margulis and Schwartz, 1998) are heterotrophic unicells that inhabit anoxic environments. All lack mitochondria. At least 28 families are placed in the phylum Archaeprotista. Examples include archaemoebae (Pelomyxa and Mastigamoeba), metamonads (Retortamonas), diplomonads (Giardia), oxymonads (Pyrsonympha), and the two orders of Parabasalia: Trichomonadida [Devescovina, Mixotricha, Monocercomonas, Trichomonas, and calonymphids (Coronympha, Snyderella)]