THE MOLECULAR PHYLOGENY AND SYSTEMATICS OF THE ACTINOMYCETES

Sequences of 165 ribosomal RNA have provided actinomycetologists with a phylogenetic tree that allows the investigation of the evolution of actinomycetes and also provides a basis for classification. The origin of actinomycetes and, except for bifidobacteria, the order by which t he main sublines evolved, cannot yet be determined with certainty. How ever, calibration of rRNA sequence divergence with palaeochemical data , and previously published substitution rates of endosymbiotic bacteri a, suggest that the main radiation occurred less than 1 billion years ago. Within this radiation, several phylogenetically homogeneous, but sometimes phenotypically heterogeneous, clades appear to have diverged over a short evolutionary period. The resolution of the 165 rRNA mole cule appears to be insufficient to clearly determine the branching pat terns between clades in this area of the phylogenetic tree. The distri bution of some morphological and chemotaxonomic traits such as types o f peptidoglycan, menaquinone, phospholipids, cell wall sugars, and fat ty acids facilitate the phenotypic delineation of genera within each c lade. At higher taxonomic levels, e.g. at the family level, phenotypic similarities are unpredictable and tend to be less conserved. With th e exception of mycolic acids, most traits are polyphyletic-hence they are unreliable indicators per se of phylogenetic relationships. Nevert heless, combinations of phenotypic properties are invaluable for predi cting whether a new organism is likely to be a member of an establishe d or a novel taxon. Current knowledge about the phylogenetic structure of the actinomycetes provides not only a sound basis for future taxon omic work but also a framework for the rational exploration of their e cology and biotechnological potential.