The Chromatin Extrusion Phenomenon in Amoeba proteus Cell Cycle

Abstract

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.

Phylostratic Shift of Whole-Genome Duplications in Normal Mammalian Tissues towards Unicellularity Is Driven by Developmental Bivalent Genes and Reveals a Link to Cancer

“…Therefore, aggressive cancer associated with polyploidy and multinuclearity often bears the features of the early embryo [111,112], including such inalienable components of morphogenesis as cell motility and angiogenesis. Moreover, through polyploidy, the TP53 cancer mutants can acquire the phenotypical features of more ancient unicellular eukaryotes ("amoeboidisation") [24,27,33,108,113,114], in line with the atavistic theory of cancer [29,30,33,115,116], confirmed by gene phylostratigraphic analysis [38] and here.…”

“Mitotic Slippage” and Extranuclear DNA in Cancer Chemoresistance: A Focus on Telomeres

“…IM coupling to telomere maintenance by ALT (or even substituting it) could resolve the long-standing puzzle of the Muller's Ratchet [95] in the obligately agamic amoeba existing on Earth for eons. They undergo cyclic polyploidy accompanied by chromatin diminution and express the orthologs of genes employed in meiosis of sexual eukaryotes Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh, and Mlh [83,87,[96][97][98][99]. Archetti [100] recently presented calculations showing that asexual reproduction can replace sexual reproduction with inverted meiosis due to recombinative gene conversion, providing protection from the deleterious loss of heterozygosity and outweighing the cost of sex.…”

Amoeba proteus and ploidy cycles: from simple model to complex issues

“…By the end of the cell cycle, the amount of nuclear DNA may exceed the initial level by more than three times, according to previous fluorimetric measurements (Makhlin, 1987(Makhlin, , 1993Afon'kin, 1989). The depolyploidization occurs through the phenomenon of chromatin extrusion in the late interphase and in the prophase (Demin et al, 2019;. The "excessive" nuclear DNA is eliminated from the amoeba nucleus by ejection into the cell cytoplasm.…”

“…Nevertheless, the Amoeba species have a very special cell (=life) cycle (Fig. 2), during which a strategy of the so-called cyclic polyploidy, or ploidy cycle, is implemented (Demin et al, 2019).The latter implies an alternation of polyploidization and depolyploidization stages preceding reproduction in the life cycle (Kondrashov, 1994(Kondrashov, , 1997Parfrey et al, 2008;Lahr et al, 2011). In A. proteus cell cycle the presynthetic phase G 1 is absent and an intensive DNA hyperreplication (stage of unproportional polyploidization) may occur starting from the interphase until the next mitosis (Ord, 1968;Makhlin, 1987Makhlin, , 1993.…”

“…The "excessive" nuclear DNA is eliminated from the amoeba nucleus by ejection into the cell cytoplasm. The depolyploidization, including prophase reduction of the chromosome number (Demin et al, 2019), reverts the Amoeba nucleus to the euploid state and therefore prepares the cells for equal separation of chromosomes in the daughter nuclei.…”