Genetic diversity of Fusarium oxysporum populations isolated from different soils in France.

The genetic diversity of soil-borne populations of Fusarium oxysporum was assessed using 350 isolates collected from six different French soils. All isolates were characterised by restriction fragment analysis of the PCR-amplified ribosomal intergenic spacer (IGS). Twenty-six IGS types were identified among the 350 isolates analysed. Five to nine different IGS types were detected in each soil. None of the IGS types was common to all of the soils. An analysis of the molecular variance based on IGS type relationships and frequency revealed that the genetic structure of the populations of F. oxysporum varied widely among the soils. Some populations were both highly diverse within the soils and differentiated between the soils. A possible relationship between the intrapopulation or interpopulation level of diversity and some external factors such as the soil type or the crop history was evaluated. A subsample representative of the diversity of the six populations was further characterised by analysing the genomic distribution of two transposable elements, impala and Fot1. One to 10 copies of the impala element were present in most of the isolates, irrespective of their soil of origin. The Fot1 element was only detected in 40% of the isolates originating from the three populations less diverse in terms of IGS types, but in 82.6% of the isolates originating from the three more diverse populations.

Populations of Nonpathogenic Fusarium oxysporum Associated with Roots of Four Plant Species Compared to Soilborne Populations.

ABSTRACT The effect of the plant on the diversity of soilborne populations of Fusarium oxysporum was assessed after successive cultures of flax, melon, tomato, and wheat in separate samples of the same soil. Forty soil-borne isolates of F. oxysporum and forty root-colonizing isolates of each plant species were sampled during the first (T0) and fourth (T1) cultures. The population structures were assessed by a genotypic method based on restriction fragment analysis of polymerase chain reaction-amplified ri-bosomal intergenic spacer (IGS) DNA. Sixteen IGS types were defined among the four hundred isolates analyzed. The distributions of soil isolates among IGS types were similar at both sampling times. The structure of F. oxysporum populations associated with the roots of flax or melon did not differ from the structure of soilborne populations. In contrast, the structure of F. oxysporum populations associated with roots of wheat or tomato differed from the structure of soilborne populations. High frequencies were found for IGS type 4 among wheat isolates at both T0 and T1 and for IGS type 11 among tomato isolates at T1. Moreover, a high level of genetic divergence was obtained between IGS types 4 and 11. Our results suggest that tomato and wheat have a selective effect on soilborne populations of F. oxysporum and that this effect seems to be plant specific.

Genomic heterogeneity among French Rhizobium strains isolated from Phaseolus vulgaris L.

Levels of DNA relatedness between strains isolated from root nodules of Phaseolus vulgaris and reference strains of different Rhizobium species were determined by performing DNA-DNA hybridization experiments (S1 nuclease method). The nine strains examined were members of three genomic groups previously delineated by a restriction fragment length polymorphism analysis among strains isolated from P. vulgaris at different sites in France. In agreement with the results of the restriction fragment length polymorphism analysis, three genomic species were found. We confirmed that one of these species corresponded to Rhizobium leguminosarum since the strain examined was 100% related to the type strain of this species. The other two species were new genomic species which were less than 21% related to reference strains belonging to other Rhizobium species, including Rhizobium etli and Rhizobium tropici, and were 18% related to each other. As determined by an analysis of partial 16S ribosomal DNA sequences, each of the genomic species was found to belong to a lineage independent from the lineages of previously described Rhizobium species. Nevertheless, they were included in the group formed by the fast-growing Rhizobium species. Both genomic species 1 and genomic species 2 contained a majority of strains which were capable of nodulating both P. vulgaris and Leucaena leucocephala, like R. tropici. However, they also contained strains with a nodulation phenotype restricted to P. vulgaris, like R. leguminosarum bv. phaseoli and R. etli bv. phaseoli. Our data are the first evidence that in Europe species other than R. leguminosarum nodulate P. vulgaris.