What Does Strain Persistence Really Mean?

Abstract

Most legumes used in Australian production systems have been introduced from other continents along with their micro-symbionts. In Australia, only 10 strains have been used in commercial inoculants for subterranean clover since its introduction in the 1950’s and only one strain, WU95, has been used since 1975. Despite this, field populations of subclover rhizobia are typically quite diverse. PCR fingerprinting of 59 isolates from 2 seasons samples at a single site in Western Australia yielded over 30 different profiles. Less than 7% of these isolates tested positive as WU95 by use of ELISA and WU95-specific polyclonal antisera. However, when a subset of isolates was examined by use of RFLP and chromosome-specific (dct and edd) and sym plasmidspecific (nif and nod) gene probes, the WU95 chromosomal type and sym plasmid type were detected with higher frequencies. RFLP analysis yielded less overall diversity than suggested by PCR fingerprinting. Significant recombination within the population was evident. These findings have important implications for how we assess strain persistence and just what such persistence really means. Should we aim to maintain the chromosomal type, the sym plasmid type or the two in their original configuration to achieve our inoculation objectives?