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 mum. 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 ( approximately 1%) 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.

Fig. 1.

Epulopiscium sp. type B life cycle. Offspring production follows a circadian cycle. (a) Early in the day, a mother cell possesses small, internal offspring. (b and c) Offspring size increases throughout the day (b) until they fill the mother-cell cytoplasm (c). (d) Finally, “mature” offspring cells emerge from the mother-cell envelope. Note that before emergence, these cells begin to develop the next generation of offspring. (e and f) Images of DAPI-stained cells representing the populations of small (e) and large (f) Epulopiscium cells used in these studies. (Scale bar: 50 μm.)

Fig. 2.

The relationship between gene copy number and cytoplasmic volume in Epulopiscium sp. type B cells. The pink circles represent data from individuals of the small-cell population (n = 26), and the blue diamonds represent data from the large-cell population (n = 31).