Extreme polyploidy in a large bacterium

Abstract

Cells rely on diffusion to move metabolites and biomolecules. Diffusion is highly efficient but only over short distances. Although eukaryotic cells have broken free of diffusion-dictated constraints on cell size, most bacteria and archaea are forced to remain small. Exceptions to this rule are found among the bacterial symbionts of surgeonfish; Epulopiscium spp. are cigar-shaped cells that reach lengths in excess of 600 m. A large Epulopiscium contains thousands of times more DNA than a bacterium such as Escherichia coli, but the composition of this DNA is not well understood. Here, we present evidence that Epulopiscium contains tens of thousands of copies of its genome. Using quantitative, single-cell PCR assays targeting single-copy genes, we have determined that copy number is positively correlated with Epulopiscium cell size. Although other bacteria are known to possess multiple genomes, polyploidy of the magnitude observed in Epulopiscium is unprecedented. The arrangement of genomes around the cell periphery may permit regional responses to local stimuli, thus allowing Epulopiscium to maintain its unusually large size. Surveys of the sequences of single-copy genes (dnaA, recA, and ftsZ) revealed genetic homogeneity within a cell consistent with only a small amount (Ϸ1%) of the parental DNA being transferred to the next generation. The results also suggest that the abundance of genome copies in Epulopiscium may allow for an unstable genetic feature, a long mononucleotide tract, in an essential gene. With the evolution of extreme polyploidy and large cell size, Epulopiscium has acquired some of the advantages of eukaryotic cells.dnaA ͉ Epulopiscium ͉ large bacteria ͉ mononucleotide repeat ͉ polyploid I t is well appreciated that many eukaryotes are orders of magnitude larger than all known members of the Bacterial and Archaeal domains. Eukaryotes have broken free of constraints on cell size by the development of sophisticated nutrient uptake systems, subcellular compartmentalization, and the use of a cytoskeleton and motor proteins to transport vesicles. With the further advance of multicellularity, cell and tissue specialization have allowed eukaryotes to attain tremendous dimensions (1, 2). Until recently (3, 4), bacterial (and archaeal) cells were considered simple, displaying little subcellular organization. Although we now know that bacterial cells are also highly organized, possessing motor and cytoskeletal proteins, and even extensive intracellular membranes in some instances (5), these cells are believed to rely on diffusion to access nutrients and other metabolically important chemicals. Diffusion coefficients of small molecules may impose time constraints on metabolite flux (6) that require bacterial cells to maintain very high surface-tovolume ratios. As a result, no part of the cytoplasm is very far from the external environment, and so exchange is rapid enough to sustain metabolic processes. Most large bacteria fit this paradigm and maintain a very thin cytoplasm; they are extremely long and sle...

Membrane protein expression triggers chromosomal locus repositioning in bacteria

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Massive multiplication of genome and ribosomes in dormant cells (akinetes) of Aphanizomenon ovalisporum (Cyanobacteria)

“…Such extreme levels of polyploidy are rare in prokaryotes with a single, disputed, description of polyploidy in the large bacterium Epulopiscium (Bresler et al, 1998;Robinow and Angert, 1998). PCR quantification of the Epulopiscium species type B genome suggested that this bacterium is highly polyploid with an individual cell containing 410 000 copies of its genome (Bresler and Fishelson, 2003;Mendell et al, 2008;Liu, 2009). If cyanobacterial akinetes represent starvation or aging responses, similar to those that induce stationary phase, we would expect akinetes to show a decrease in polyploidy, such as that reported for H. volcanii (Breuert et al, 2006).…”

Endoreplication and polyploidy: insights into development and disease

“…Although polyploidy is well appreciated in plants , many different animal tissues, including the skin, gut, placenta, liver, brain and blood have polyploid cells (Table 1) (Laird et al, 1980;Corash et al, 1989;Fox et al, 2010; Hedgecock and White, 1985;Melaragno et al, 1993;Sherman, 1972;Unhavaithaya and Orr-Weaver, 2012;Zanet et al, 2010). Interestingly, endoreplication is not limited to multi-cellular organisms, with examples described in ciliated protozoa (Yin et al, 2010) and even bacteria (Mendell et al, 2008).Two primary forms of endoreplication have been described: endocycling and endomitosis (Fig. 1).…”