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Single-celled microorganisms are greatly diverse,including many eco-type strains of single species in niches.Intraspecies interactions between these close relatives are suggested to play pivotal roles in modeling microbial community,affecting their ecological functions and pushing speciation and diversification,which,however,remain mostly unclear.Myxobacteria are able to dwell in various environmental conditions.In single niches,myxobacteria are greatly diverse,including many intraspecies relatives.Myxobactefia are a group of bacteria with complex social behavior.The social behavior occurs within the same kind of myxobacterial cells.If two different species or strains of the same species were co-cultivated,they might exhibit not only competition but also cooperation.For example,co-cultivation ofcellulolytic Sorangium cellulosum strains decreased cell masses,but increased the production of secondary metabolites epothilones,which are consistent with their varied genome compositions.Sorangium genomesare varied in size,ranging from 9.81 Mbp to 14.78 Mbp in ten assayed strains.Sorangium strains have a considerably small core genome,but large imbalanced accessory genomes.Mostof the differences among the accessory genomes are associated to the genes for polysaccharide degradation enzymes,biosynthesis of secondary metabolites,and complex regulation systems.On the other hand,pairwise close inoculation of bacteriolytic Myxococcus swains formed clear boundaries to separate colonies,and mixed co-cultivation led to massacre of partners.Some intraspecies interactions have been reported,for example,cell-contact inhibitions,type Ⅵ secretion system dependent killing system,quorum sensing and other signal-dependent interactions.In order to investigate the involving mechanisms,we mutated M.xanthus DK1622 randomly and pairwise inoculated the mutants to screen self-identification(SI)mutants.From 3,392 mutations,we obtained nine independent SI mutants,with unknown functions of the mutated genes.These SI mutants formed polymorphic unmerged colony boundaries with wild type and each other but retained the capacity to recognize and conduct social behaviors themselves.When pairwise mixed for cultivation,two Myxococcus SI mutants were damaged and exploited by their partners,while others survived in mixture with mutually beneficial or repressive vitality and sporulation,suggesting different SI determinants probably render Myxococcus cells to conduct hierarchical interactions.The results shed a light of multiple self-identification systems for bacterial intraspecies interactions.