Genetic characterization of MexicanFrankiastrains nodulatingCasuarina equisetifolia

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Abstract
There is a need to increase the utilization of the Casuarina equisetifolia J.R. Forst. & G. Forst. - Frankia symbiosis and be sure of its effectiveness in Mexico. This may be facilitated by selecting appropriate bacterial strains for which ecological characteristics are known. We tested various typing methods to develop genetic markers for ecological studies. DNA, extracted from clonal cultures of native strains or from reference cultures of Casuarina-infective Frankia strains, was used as the template in polymerase chain reactions (PCR) with primers targeting different DNA regions. nifH and 16S rDNA probes from the reference strain Frankia Br were utilized to authenticate the isolates. Polymorphisms of the restricted fragments of the intergenetic spacer between the 16S-23S rDNAs were analyzed. Repetitive extragenic palindromic sequences (rep-PCR) (BOXA1R primer) were used to generate genomic fingerprints. All studied strains showed two copies of the ribosomal operon and a single copy of the nifH gene. PCR - restriction fragment length polymorphism patterns of the 16S-23S intergenetic spacer (IGS) were similar for all Frankia isolates; however, the rep-PCR technique was sensitive enough to distinguish between some of these Frankia strains. The Mexican cultured strains of Frankia nodulating C. equisetifolia appeared to be closely related to the isolated and nodular Frankia from trees growing outside Australia.Key words: Frankia, Casuarina, 16S rRNA, 16S-23S IGS, nifH, repetitive sequence polymerase chain reaction.
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