Origins of the nucleate organisms II

Implications for evolution of nuclear structures of animals, plants, fungi and protoctists

The evolutionary variations of nuclear structure of animals, plants, fungi and protoctists were studied with electron microscopy by using techniques preferentially staining ribonucleoprotein (RNP) particles and chromatin. A remarkable similarity in the general morphological features of the RNP particles and chromatin arrangement is found in animals, plants and fungi. Important variations of these features were found in protoctists. These observations suggest that major evolutionary changes in the nuclear structure predate the acquisition of plastids by the ancestors of green plants. Once evolved, the nuclear structural pattern is conserved in plants and animals. Among protoctists studied, Kinetoplastida, Cryptomonadida and Volvocida have RNP particles and chromatin arrangement resembling those of plants and animals. These similarities may indicate a common ancestor. Important differences in the nuclear structure among Euglenida, Amebida, Cryptomonadida, Volvocida and Kinetoplastida support the view that Sarcomastigophora is a polyphyletic taxon. For the same reason Kinetoplastida and Euglenida must not be grouped in a monophyletic taxon. We propose that the variations of RNP particles may be related to the initial evolution of post-transcriptional processing.

Fine structure and taxonomic position of the giant amoeboid flagellate Pelomyxa palustris

Specimens of Pelomyxa palustris from five collecting sites had numerous nonmotile flagella. The structures are called flagella because of morphological similarities to flagella and because P. palustris has affinities with amoeboid flagellates. Flagella were photographed on living cells and studied by transmission and scanning electron microscopy. From 64 to 742 flagella per cell were estimated from scanning electron microscopy of ten cells 204 to 1269 micron in length. The nonmotile flagella arise from basal granules which were, in one strain, surrounded by radiating electron-dense microtubules. This strain also had excess axonemal microtubules. Abundant cytoplasmic microtubules were arranged in several different patterns. In about half of the P. palustris cells in which nuclei were studied, microtubules were either apposed to the nuclear membrane in a parallel alignment (with some also radiating) or radiating from the nuclear membrane (with none parallel). Bacteria associated with nuclei were of three characteristic types: Gram-negative rods, Gram-positive rods, and large rods. All nuclei within a given trophozoite had similar perinuclear features. Recent proposals for separation of Pelomyxa to its own phylum (based on its proposed primitive, unique nature) can not be justified. Pelomyxa is a complex, highly specialized organism adapted to live in a specific fresh-water environment. Mastigamoebid amoeboid flagellates of the genera Mastigamoeba, Mastigella, Mastigina, and possibly Dinamoeba are placed with Pelomyxa within the order Pelobiontida Page, 1976, emend., containing two families. Pelomyxidae Schulze, 1877, and Mastigamoebidae Goldschmidt, 1907.

Early evolution of microtubules and undulipodia

A critique of both autogeneous and symbiotic hypotheses for the origin of microtubules and cilia and eukaryotic flagella (undulipodia) is presented. It is proposed that spirochetes provided the ancient eukaryotic cell with microtubules twice; cytoplasmic microtubules originated from phagocytosed spirochetes whereas axopodial tubules of undulipodia were transformed from ectosymbiotic spirochetes. A role in transport for microtubules in spirochetes together with a detailed scenario by which free-living spirochetes attached as ectosymbionts and subsequently differentiated into undulipodia is outlined. A mechanism for the continuity of motility in the form of "training" of the novel microtubular axoneme by the ancient spirochete motility apparatus is proposed. Transitional states (missing links) are unlikely to have survived. Constraints regarding the nature of the host cell are discussed. A corresponding flowchart of the early evolution of eukaryotes is presented in which plastids and mitochondria are polyphyletic in their origins.