Endospore formation is a specific property found within bacteria belonging to the Gram-type-positive low G+C mol% branch ( Firmicutes) of a phylogenetic tree based on 16S rRNA genes. Within the Gram-type-positive bacteria, endospore-formers and species without observed spore formation are widely intermingled. In the present study, a previously reported experimental method (PCR and Southern hybridization assays) and analysis of genome sequences from 52 bacteria and archaea representing sporulating, non-spore-forming, and asporogenic species were used to distinguish non-spore-forming (void of the majority of sporulation-specific genes) from asporogenic (contain the majority of sporulation-specific genes) bacteria. Several sporulating species lacked sequences similar to those of Bacillus subtilis sporulation genes. For some of the genes thought to be sporulation specific, sequences with weak similarity were identified in non-spore-forming bacteria outside of the Gram-type-positive phylogenetic branch and in archaea, rendering these genes unsuitable for the intended classification into sporulating, asporogenic, and non-spore-forming species. The obtained results raise questions regarding the evolution of sporulation among the Firmicutes.

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