THE MOLECULAR PHYLOGENY AND SYSTEMATICS OF THE ACTINOMYCETES

Sequences of 16S ribosomal RNA have provided actinomycetologists with a phylogenetic tree that allows the investigation of the evolution of actinomy­ cetes and also provides a basis for classification. The origin of actinomycetes and, except for bifidobacteria, the order by which the main sublines evolved, cannot yet be determined with certainty. However, calibration of rRNA se­ quence divergence with palaeochemical data, and previously published sub­ stitution rates of endosymbiotic bacteria, suggest that the main radiation occurred less than 1 billion years ago. Within this radiation. several phyloge­ netically homogeneous, but sometimes phenotypically heterogeneous, clades appear to have diverged over a short evolutionary period. The resolution of the 16S rRNA molecule appears to be insufficient to clearly determine the branching patterns between clades in this area of the phylogenetic tree. The distribution of some morphological and chemotaxonomic traits such as types of peptidoglycan, menaquinone, phospholipids, cell wall sugars, and fatty acids facilitate the phenotypic delineation of genera within each clade. At higher taxonomic levels, e.g. at the family level, phenotypic similarities are unpre­ dictable and tend to be less conserved. With the exception of mycolic acids. most traits are polyphyletic-hence they are unreliable indicators per se of phylogenetic relationships. Nevertheless, combinations of phenotypic proper­ ties are invaluable for predicting whether a new organism is likely to be a member of an established or a novel taxon. Current knowledge about the phylogenetic structure of the actinomycetes provides not only a sound basis for future taxonomic work but also a framework for the rational exploration of their ecology and biotechnological potential.