Extreme Polyploidy of Carsonella, an Organelle-Like Bacterium with a Drastically Reduced Genome

Abstract

Polyploidy is the state of having multiple copies of the genome within a nucleus or a cell, which has repeatedly evolved across the domains of life. Whereas most bacteria are monoploid, some bacterial species and endosymbiotic organelles that are derived from bacteria are stably polyploid. In the present study, using absolute quantitative PCR, we assessed the ploidy of Candidatus Carsonella ruddii (Gammaproteobacteria, Oceanospirillales), the obligate symbiont of the hackberry petiole gall psyllid, Pachypsylla venusta (Hemiptera, Psylloidea). The genome of this symbiont is one of the smallest known for cellular organisms, at 160 kb. The analysis revealed that Carsonella within a single bacteriocyte has ∼6 × 10⁴ copies of the genome, indicating that some Carsonella cells can contain thousands or even tens of thousands of genomic copies per cell. The basis of polyploidy of Carsonella is unknown, but it potentially plays a role in the repair of DNA damage through homologous recombination. IMPORTANCE Mitochondria and plastids are endosymbiotic organelles in eukaryotic cells and are derived from free-living bacteria. They have many highly reduced genomes from which numerous genes have been transferred to the host nucleus. Similar, but more recently established, symbiotic systems are observed in some insect lineages. Although the genomic sequence data of such bacterial symbionts are rapidly accumulating, little is known about their ploidy. The present study revealed that a bacterium with a drastically reduced genome is an extreme polyploid, which is reminiscent of the case of organelles.

Keywords: bacteriome-associated symbionts; insects; ploidy; small genome.

FIG 1

(A) Bacteriocytes stained with DAPI. Tubular Carsonella cells were squeezed out by applying gentle pressure on a coverslip. Extremely long Carsonella cells surrounding host nuclei show strong DAPI signals. Within Carsonella cells, numerous spots with higher signal intensities are observed. Bar, 10 μm. (B) Quantitative PCR analysis of Carsonella genomic copy number in the bacteriome of Pachypsylla venusta. Abundance values of the Carsonella 16S rRNA gene were normalized to the psyllid nuclear genes for RpL18 (left) and RpL32 (right). All data points (female, magenta; male, cyan) of six biological replicates of each sex are presented. Black dots and black bars represent means and standard deviations, respectively.