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., undulip odia (= eukaryotic flagella*), and axostyles] occurs in all early-branching lineages of eukaryotes (mastigamebids, diplomonads, oxymonads, retortamona ds and trichomonads). Dispersed throughout the cytoplasm, the 50 or more nu clei 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 gr oups of four typical of trichomonad mastigonts, but, since they lack nuclei , are akaryomastigonts. Each four-kinetosome akaryomastigont is connected t o a set of axostylar microtubules, which extends toward the cell posterior. The axostyles, that together form a central bundle, like the shaft of an u mbrella, position all the akaryomastigonts at the cell periphery. When grou ps of 20-50 motile akaryomastigonts become independently organized, two, th ree 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 th e karyomastigont, i.e., a stable nucleus-kinetid connection. Snyderella evo lved from more basal calonymphids by severance of the nuclear connection, w hich led to the lack of karyomastigonts. The akaryomastigonts were retained and reproduced with each cell division.