Studies On Phylogeny And Molecular Evolution Of Nostoc (Cyanophyta)

Nostoc is a genus of non-branching filamentous heterocytous cyanobacteria that is widely distributed in both terrestrial and aquatic ecosystems.The analyses based on 16S rRNA gene show that the reconstruction of phylogeny on Nostoc is sensitive to search algorithm. The statistic tests support the maximum-likelihood tree (ML) reflects the phylogenetic relation for the data set better than alternative topologies obtained by other search algorithms. The ML tree shows that the genus Nostoc is not monophyletic. According to the phylogeny, the definition of the genus and morphological taxonomic criteria were discussed.On the basis of the 16S rRNA secondary structure of Nostoc, the evolutionary mechanisms of 16S rRNA under water stress was investigated. The results suggest that 16S rRNA secondary structures of the desiccation-tolerant Nostoc strains are more stable than that of planktonic species belonging to Nostocaceae. The stabilizing mutations can be divided into two categories, those causing GC to replace other types of base pairs in stems and those causing extension of stems. Mapping the stabilizing mutations onto the phylogenetic tree of Nostoc indicates that most of them had evolved independently, during the adaptive radiation among Nostoc spp..The 16S rRNA gene, the most popular molecular marker in reconstructing phylogenetic relations among cyanobacteria, however, is not sufficient for studies at the subgeneric level. This research performed a case study in Nosotc to evaluate rpoC1, hetR, rbcLX and 16S rRNA- tRNAIle - tRNAAla-23S rRNA ITS as phylogenetic markers. The results indicated that the gene trees of these loci are not congruent with the species phylogeny based on 16S rRNA gene. The mechanisms contributing to the incongruence include randomized variation, recombination and...