| Genetic diversity which was determined by the analysis of 16S rDNA-RFLP and 16S rDNA-sequencing were carried out on Frankia strains isolated from the root nodules of different actinorhizal plants, including Myrica, Casuarina, Alnus and Coriaria. The main results of the research project were as follows.Combining the analysis of the conservative region of 16S rRNA gene of prokaryote, such as E. coli, Rhizobia and several Frankia strains, we designed several sets of primers to amplify the 16S rRNA gene of the Frankia strains tested. Through tentative experiments with these primers, we screened out primers UF/UR and Ec27f/Frl717r. But after subsequent RFLP experiments and sequencing analysis, we found that the primers UF/UR may be better in the amplification of the 16S rRNA gene of the strains tested. It could reflect the difference of the strains tested more appropriately than the primers Ec27f/Frl717r did.The nearly whole length of 16S rRNA gene was amplified respectively with primers UF/UR and Ec27f/Frl717r from the 13 Frankia strains tested and analyzed by using a set of restriction endonucleases Afal, Haelll, Hhal, Hinfl, Mspl and Sau3Al. Then the genetic distance and Shannon information index among these 13 strains were obtained by using the software Popgen2 on the basis of PCR-RFLP patterns. The phylogenetic tree was constructed by using the UPGMA algorithm method (Mega2 software package). The Shannon information index indicated that there was a higher genetic diversity among the Frankia strains and some strains had a big genetic distance with others. According to the phylogenetic tree, the thirteen strains were grouped into four distinct PCR-RFLP clusters, namely, Coriaria group, Myrica group, Myrica-Casuarina-Alnus group and CasuarinarMyrica group. In these four groups, Coriaria group showed a predominant difference with other three groups, this result was coincident with what HU Chuan-jiong et al. (1998) got after the nif -RFLP analysis of Coriaria strain 030. It is also accordant with the result that Nick et al. (1992) got with the anaylsis of partial sequence of some Coriaria isolates. In addition, we also found that an evident crossing-infection happened in three Frankia strains tested, which were respectively isolated from Myrica and Casuarina. However, there is not an apprarent connection between the clusters and the natural geographic range of host species.After analyzing the sequences of five strains selected, we also got the almost same results with the RFLP analyzing. And through blasting with other strains published in the GENEBANK, we got four clusters. FMr61, FCg07 and FCe42 have higher similarities and they miscellanyed with strains isolated from Alnus, they formed cluster I ; FMrl6 and 2215 had closer relationships the Frankia strains that infected the plants of Elaeagnus, Rhamnaceae and Hippophae, they formed cluster IV. In addition, the evident crossing-infection happened in the strains came from Myrica, Casuarina also could be found in the sequencial analysis. All these results we obtained from the sequencing and RFLP analysis were partly accorded with what Baker brought forward in 1987(the four host specific group, HSG). However, they also indicated the limit of this HSG. The miscellany appeared in strains of Myrica, Casuarina and Alnus were also partly coincident with what Normand et al. (1996) and LI Zhizhen (2002) obtained: the clusters devided by the isolates from the Myrica and Casuarinahave grest genetic diversities.Besides the analysis of the strains, we also tried to extract the DNA of Frankia from the nodules directly and analyse them also with the method of PCR-RFLP. Unfortunately, it may due to the immature of the technique and other reasons that we couldn' t have enough samples to do the research on this facet. However, we got some experiences from the unsuccessful experiments and it would be useful to the further work on this aspect.
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