The common origin and degenerative evolution of flagella in .

Flagellar motility plays an important role in the environmental adaptation of bacteria and is found in more than 50% of known bacterial species. However, this important characteristic is sparsely distributed within members of the phylum , which constitutes one of the largest bacterial groups. It is unclear why this important fitness organelle is absent in most actinobacterial species and the origin of flagellar genes in other species. Here, we present detailed analyses of the evolution of flagellar genes in , in conjunction with the ecological distribution and cell biological features of major actinobacterial lineages, and the co-evolution of signal transduction systems. The results presented in addition to clarifying the puzzle of sporadic distribution of flagellar motility in , also provide important insights into the evolution of major lineages within this phylum.