The Chromatin Extrusion Phenomenon in Amoeba proteus Cell Cycle.

Amoeba proteus is possibly the best known of all unicellular eukaryotes. At the same time, several quintessential issues of its biology, including some aspects of the cell cycle, remain unsolved. Here, we show that this obligate agamic amoebae and related species have a special type of cyclic polyploidy. Their nucleus has an euploid status only for a small fraction of the cell cycle, during metaphase and telophase. The rest of the time it has an aneuploid status, which is a consequence of polyploidization. Extrusion of "excess" chromatin from the nucleus in late interphase and during prophase results in depolyploidization. Such a strategy of life cycle in unicellular eukaryotes is thought to be the main mechanism of "resetting" the Muller's ratchet and is a satisfactory alternative to the meiotic recombination for agamic protists.

Karyotypic instability of endoprophase and mitotic cells of Amoeba sp. strain Cont from the "proteus-type" group (Amoebozoa, Euamoebida, Amoebidae).

We performed karyotyping of Amoeba sp. strain Cont. Based on the results of a cytological analysis, we concluded that the chromosome number of Amoeba sp. strain Cont in mitosis was unstable. In all cases they appeared to be hypergaploid (the basic chromosome number is 30), with monosomy of all chromosomes except four shortest ones. The presence of "extrachromosomes" in the nucleus could prolong until the beginning of the anaphase. It was only then that they were ejected from the nucleus and the euploidy (haploidy) was restored. The stage of endoprophase nucleus was revealed in the cell cycle of Amoeba sp. strain Cont. This stage has not yet been found in other amoebae from the "proteus-type" group that had been previously studied (A. proteus strain B and A. borokensis). The maximum number of endoreplication rounds in the strain Cont amoebae nuclear cycle was 4 or 5. The regular extrusion of chromosomes from the nucleus into the cytoplasm occurred in each of the endoreplication rounds. Comparative cytological analysis of A. proteus strain B, A. borokensis and Amoeba sp. strain Cont karyotypes indicated that strain Cont, though rather close to the former two amoebae, is actually a distinct species.