Segregation of sister chromosomes during the shape change of developing <i>Myxococcus xanthus</i> cells

Chromosome organization is critical for the maintenance of genetic integrity. Most studies of bacterial nucleoids have focused on growing rod-shaped organisms. Studying nucleoid dynamics during <i>Myxococcus xanthus</i> development offers the unique opportunity to investigate the localization of two sister chromosomes as rod-shaped cells transition into round spores. During starvation-induced multicellular development, DNA replication is required for <i>M. xanthus</i> rods to transition into spores with two copies of the chromosome. Here, we report novel approaches using confocal fluorescence microscopy to observe the chromosome number and arrangement, and nucleoid localization in developing cells <i>in situ</i>. We discovered that sister chromosomes are present in some rods and transitioning cells (TCs) early in development. The arrangement of the two chromosomes in developing cells was novel compared to predivisional growing cells studied previously. We observed segregated nucleoids in ~40% of TCs and spores. The majority of TCs contained a crescent-shaped nucleoid along one side, perhaps due to ongoing chromosome segregation, whereas most spores appeared to have undergone nucleoid decondensation. During unicellular glycerol-induced sporulation of <i>M. xanthus</i>, we observed segregated nucleoids in only ~10%-20% of TCs and spores. In addition, early in starvation-induced development, we discovered a subpopulation of cells that may be spheroplasts destined for lysis, which is the fate of most cells under these conditions. Chromosome segregation in developing <i>M. xanthus</i> may be a bet-hedging strategy to increase survival under different conditions and/or an evolutionary remnant of ancestral events that included cell division to produce spores with one copy of the chromosome.IMPORTANCEThe cell cycle normally involves DNA replication, chromosome segregation, and cell division. During starvation-induced <i>Myxococcus xanthus</i> development, DNA replication is necessary for progression to spore formation, which occurs without cell division, resulting in spores with two copies of the chromosome. The organization of sister chromosomes during the morphological change of rod-shaped cells into round spores was unknown. We discovered that the two nucleoids often segregate during the transition from rods to spores. Mature spores contained decondensed nucleoids. Our observations raise important questions about the mechanism of chromosome segregation during <i>M. xanthus</i> development and the reason for its existence. We also discovered a subpopulation of developing cells with characteristics suggesting they are spheroplasts on the verge of cell death.