The ultrastructural features of axoneme organization within the cytoplasm and exflagellation were investigated in detail in microgametes of a malaria parasite, Plasmodium berghei, by electron and fluorescence microscopy. The kinetosomes (basal bodies) of the microgamete were characterized by an electron dense mass in which singlet microtubules (MTs) were embedded. Around the kinetosomes, several singlet and doublet MTs were recognized in transverse sections. Incomplete doublets with growing B-tubule were also observed. As precursors of the axoneme, arrays of over three doublets showed a tendency to encircle the central pair MTs. Some of the doublet MTs were already equipped with inner and outer dynein arms. In the microgamete, which lacks an intraflagellar transport (IFT) system, self-assembly of microtubular and associated components appeared to proceed stepwise from singlet MTs through arrays of one to nine doublet MTs, surrounding the central pair, to form the complete axoneme in a quite short time. At exflagellation, some extra doublets were occasionally included between the axoneme and the flagellar membrane. At high magnification, the outer dynein arm of the Plasmodium microgamete had a pistol-like shape representing a three-headed dynein molecule like that of other Alveolata.
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Vol. 38 • No. 2