A light and electron microscopic study of the flagellate-to-ameba conversion in the MyxomyceteStemonitis pallida

Summary The flagellate-to-ameba conversion process of the MyxomyceteStemonitis pallida was investigated with Nomarski optics and electron microscopy. The flagellate has two flagella, a long and a short one. When the water film containing the flagellates becomes very thin, they retract their flagella, usually the short one first and then the long one. The short flagellum is retracted by only one method, in which the sheath membrane of the flagellum fuses with the cell membrane, consequently causing the axoneme to be absorbed into the cytoplasm. Retraction of the long flagellum can be divided into four types. In all cases, fusion of the sheath membrane and the cell membrane takes place. The retracted axoneme of the long flagellum sometimes beats convulsively for about 10 minutes after retraction, and after 10–15 minutes it became indistinguishable as it was detached from the blepharoplast.Analysis of thin sections shows that the retracted axonemes disintegrate in the following squence: B-tubules, A-tubules, spokes, central microtubules. In almost all cells the degradation begins immediately after retraction and is completed within 90 minutes. Only on rare occasions, structures which seem to have been derived from retracted axonemes are observed in the ameba about 90 minutes after conversion. The basal bodies and cytoplasmic microtubules are a little more stable than the retracted axonemes. Some basal bodies of the short flagellum, whose C-tubules are affected, are present in the amebae more than 90 minutes after conversion. Cytoplasmic microtubules decrease in number and become shorter in the amebae after about 24 hours, when newly formed regions filled with flocculent material appear.