Introduction to the Taxonomy of Actinobacteria
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Introduction to the Taxonomy of Actinobacteria ERKO STACKEBRANDT AND PETER SCHUMANN
Introduction During the past 25 years, comparative analysis of sequences of homologous and genetically stable semantides has demonstrated that several classification systems based on morphology and physiology do not reflect the natural relationships among actinomycetes and related organisms. Though today’s taxonomists can better recognize the phylogenetic branching pattern of lineages than previous generations of systematists could, this ability obscures the view that previous classification systems also were established in the belief that they reflected conceptual and methodological progress. In this respect, no difference exists in the history of the actinomycetes classification and that of other groups of bacteria. In hindsight, it is their tempo and mode of evolution that make actinomycetes and their relatives difficult to classify outside the context of phylogeny: though the majority of taxa evolved rather late in Earth’s history, individual characteristics (traditionally used in early classification) are highly diverse and only rarely reflect phylogenetic relationships. This chapter highlights the progress in the elucidation of the natural relationships among actinomycetes leading to the inclusion of phylogenetic data into a gene sequence-based higher classification system. Although this taxon, with the introduction of the Approved Lists of Bacterial Names (Skerman et al., 1980) and guidelines for the circumscription of species (Wayne et al., 1987), was demonstrated to be a rather coherent entity, significant changes occurred at higher taxonomic ranks. This is true not only for the genus level, for which traditional descriptors play the same key role as in the past, but also more obviously for the ranks between genera and class. The biology of most actinomycete genera has been covered in the previous edition of The Prokaryotes (Balows et al., 1992); some of them have been reclassified or dissected since then. Many additional genera were added in the last decade to the actinobacteria proper, increasing the phylogenetic and epigenetic diversity of the
class, even though the medical, ecological and/or biotechnological importance of these new taxa have not yet been evaluated thoroughly. Table 1 compiles the validly and some invalidly described genera and organizes them into families until January 2000, following the scheme of Stackebrandt et al. (1997). The chapter also covers the taxonomy of new, mainly monogeneric families, as well as several genera for which, as compared to the 2nd edition of The Prokaryotes (Balows et al., 1992), no substantial increase in information has been recorded. These taxa may subsequently receive renewed attention that would justify their recognition in future updates. For the time being, only the most important taxonomic features of these taxa will be summarized.
A Note on Early Classification Systems of Actinomycetes Probably no othe
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