PHYLOGENETIC ANALYSIS OF METABACTERIUM-POLYSPORA - CLUES TO THE EVOLUTIONARY ORIGIN OF DAUGHTER CELL PRODUCTION IN EPULOPISCIUM SPECIES, THE LARGEST BACTERIA

It is rare that there are molecular clues to the evolutionary origin o f developmental traits. We have encountered an evolutionary juxtaposit ion that may explain the origin of the unique replicative morphology o f Epulopiscium spp., the largest known bacteria, which reproduce by th e internal production of multiple live offspring. We report here a 16S rRNA-based phylogenetic analysis of Metabacterium polyspora, a multip le-endospore-forming, uncultivated inhabitant of guinea pig cecum. Cel ls of M. polyspora were harvested from cecum contents by sedimentation in a Ficoll gradient and lysed. The bacterial 16S rRNA genes of this lysate were amplified by PCR. Sequence analysis of the cloned PCR prod ucts revealed two dominant, closely related 16S rRNA types. In situ hy bridization of cecum contents with fluorescently labeled oligonucleoti des, diagnostic of these two sequences, showed that they represent dis tinct strains of M. polyspora. Phylogenetic analyses of the sequences shelved that M. polyspora is closely related to Epulopiscium spp. On t he basis of this result and other correlations, we propose that the pr ocess of sporulation was modified in a predecessor of Epulopiscium spp , to produce live offspring instead of quiescent endospores.