Surface Kinetosomes and Disconnected Nuclei of a Calonymphid: Ultrastructure and Evolutionary Significance of Snyderella Tabogae

The karyomastigont cytoskeletal pattern in which a nucleus is connected to motility organelles [1-5 kinetosomes and associated axonemes, i.e., undulipodia (= eukaryotic flagella*), and axostyles] occurs in all early-branching lineages of eukaryotes (mastigamebids, diplomonads, oxymonads, retortamonads and trichomonads). Dispersed throughout the cytoplasm, the 50 or more nuclei of Snyderella (Calonymphidae: Trichomonadida) are not attached to the mastigont organelles. Except at the extreme posterior undulipodia cover the surface of the cell. All of the hundreds of undulipodia are arranged in groups of four typical of trichomonad mastigonts, but, since they lack nuclei, are akaryomastigonts. Each four-kinetosome akaryomastigont is connected to a set of axostylar microtubules, which extends toward the cell posterior. The axostyles, that together form a central bundle, like the shaft of an umbrella, position all the akaryomastigonts at the cell periphery. When groups of 20-50 motile akaryomastigonts become independently organized, two, three or more competing anterior conical regions appear. Karyokineses of the multiple nuclei in Snyderella are synchronized as in other calonymphids. We suggest that the ancestral character of eukaryotic cell organization is the karyomastigont, i.e., a stable nucleus-kinetid connection. Snyderella evolved from more basal calonymphids by severance of the nuclear connection, which led to the lack of karyomastigonts. The akaryomastigonts were retained and reproduced with each cell division