Intracellular genomic variability driven by cellular compartmentalization in the giant bacterium Achromatium spp.

Abstract

Bacteria of the genus Achromatium harbor hundreds of chromosomes that were previously suggested to be genetically diverse. By sequencing multiple regions from individual cells, we demonstrate that chromosomes within a single cell differ in nucleotide and amino acid sequence, reaching levels of divergence well below accepted bacterial species boundaries and below the range associated with homologous recombination. Sequencing of dividing cells further revealed that daughter cells inherit distinct chromosome populations, a mode of inheritance previously associated with sexual reproduction in eukaryotes. Combining subcellular sequencing with high-resolution microscopy, we reconstruct the three-dimensional cellular architecture of Achromatium and show that extensive intracellular heterogeneity arises from the spatial segregation of chromosome populations by the cell's internal structure, which limits genome-wide recombination. Our findings establish cellular architecture as a determinant of genome evolution in giant polyploid bacteria and identify spatial genome segregation as a mechanism enabling the maintenance and inheritance of divergent chromosome populations in bacteria.