Current Systematics of Rhizobia

Abstract

In the investigation of rhizobia, research on diversity forms a basis for studies of other kinds, since diversity studies offer characterised strains that serve as resource for further studies of taxonomy, genetics, biochemistry, evolution, ecology, application and so on. In general, biodiversity includes diversity at three levels: genetic, species and ecosystem. Among these three levels, diversity of species is closely related to the methods or criteria for species definition. In the past century, criteria for definition of bacterial species have changed depending on the development of biological and computer sciences, which also affected the taxonomy of rhizobia. The definition of rhizobial species was at one time based on the host specificity of rhizobial strains (1932–1982) (Fred et al. 1932); on numerical taxonomy and DNA-DNA or DNA-RNA relatedness (1980s–1990s) (Chen et al. 1988; Dreyfus et al. 1988; Jordan 1982); on phylogeny of the 16S rRNA gene combined with numerical taxonomy and DNA-DNA relatedness (Chen et al. 1995; Young et al. 1991); on polyphasic characterisation and multilocus sequence analysis (de Lajudie et al. 1994; Martens et al. 2007); and most recently on genome analysis (Román-Ponce et al. 2016; Wang et al. 2016a). With the addition and shifting of methods, the system of rhizobial taxonomy has been greatly improved, while the species definition is more related to their evolutionary relationships.