MICROTUBULE CENTERS AND THE INTERPHASE MICROTUBULE CYTOSKELETON IN AMEBAS OF THE CELLULAR SLIME-MOLDS (MYCETOZOANS) ACYTOSTELIUM-LEPTOSOMUMAND PROTOSTELIUM-MYCOPHAGA

We investigated the microtubule (MT) cytoskeleton and microtubule cent ers (MTC) in undifferentiated amoebae by indirect immunofluorescence w ith six monoclonal antitubulin antibodies, and by transmission electro n microscopy and immunogold ultracytochemistry. Interphase amoebae of both species contain a distinct cytoplasmic complex of MTs, which is m ore elaborate in Protostelium mycophaga. In Acytostelium leptosomum am oebae a single MTC is attached to each interphase nucleus at its point ed end, as in the other dictyostelid cellular slime molds Dictyosteliu m discoideum and Polysphondylium violaceum. Ultrastructurally, MTCs of A. leptosomum also resemble those of these two species: They consist of an electron-opaque core shaped like a stout rod, which is embedded, together with nodules, in a fuzzy matrix. The nodules are the points of origin of the MTs. In most amoebae of P. mycophaga there are two MT Cs on opposite sides of and close to the nucleus, but many amoebae als o contain a variable number of MTCs that are remote from the nucleus. Nucleus-associated and ''remote'' MTCs are structurally identical. The y consist of a ring-shaped core with inner and outer diameters of ca. 130 nm and 340 nm. A plug sits in the ring, and satellites are connect ed to the core by fine fibrils. The satellites are the points of origi n of MTs. New MTCs are apparently formed during mitosis, the parent MT C probably serving as a template for the genesis of a new ring. The re sults support the notion that phylogenetically related organisms have similarly constructed MTCs and that these are dissimilar in less close ly related organisms. (C) 1994 Wiley-Liss, Inc.