Study On Molecular Systematics Of Nostoc (Cyanophyta) And Relative Genera

Nostoc and the relative genera are important groups of cyanobacteria,widely distributed in various habitats.The taxonomy is primarily based on morphological features of filament,nutrition cell,heterocyst and akinete etc.They are prokaryote with simple structure and high plasticity , their morphology are vulnerable to influence by environment.So the taxonomy based on morphological characteristics is not reliable,and it is liable to cause confusion and controversy.With the development of molecular systematics,many nucleotide and protein sequences provide the most direct evidences for evolution and classification study . So , these sequences have been widely used cyanobacteria molecular systematics research.In this study,16S rRNA gene which is called"evolutionary timer"and tRNA^Leu(UAA) intron gene with faster variation rate were cloned and sequenced.Phylogeny of Nostoc and the relative gerena were studied based on the two sequences and other relative sequences from GenBank database.The main results were summarized as follows:[1] According to the phylogenetic results based on 16S rRNA gene and tRNA^Leu(UAA) intron gene sequences show that Nostoc and Anabaena cluster in two different clades.So,Nostoc and Anabaena are two separate genera.[2] The phylogenetic results of Nostoc are basically consistent in phylogenetic trees based on 16S rRNA gene and tRNA^Leu(UAA) intron gene sequences.The majority species of Nostoc cluster in a single clade.This result shows that they are monophyletic. [3] However,the analytical results show that a few species( Nostoc paludosum FACHB88,Nostoc spongiaeforme FACHB130,Nostoc piscinale CENA21,Nostoc sp.FACHB973,Nostoc sp.YK-01 and Nostoc sp.HK-01) are excluded from the monophyletic clade.These species and some Anabaena species cluster in a clade,and this clade located in between a monophyletic clade of Nostoc and clade of Anabaena,with clear boundaries.The genetic distance between them is more than that of that of within the genera.Our results do not support the existing taxonomic status.This clade may be a transitionary group beween Nostoc and Anabaena,or it is a new group,it is worth further studying.[4] The phylogenetic results show that Anabaena azollae FACHB190 is also cluster in the above-mentioned clade,and it cluster in neither Nostoc nor Anabaena.So,the results support that Anabaena azollae FACHB190 is a species independent of the two genera.[5] The research find that Aphanizomenon and Anabaena are mixed,and the genetic distance between them is small.So,the result indicates Aphanizomenon should be incorporated into Anabaena.[6] The results show that filamentous cyanobacteria with heterocyst cluster in a single clade,and they are monophyletic.[7] The phylogenetic results based on of 16S rRNA gene and tRNA^Leu(UAA) intron gene combined sequences show that N. commune,N. flagelliforme and N. sphaeroids are three separate species of Nostoc,which are three ecomomic sepcies of Nostoc in China.But,N. commune and N. flagelliforme have closer relationship.In this study,the phylogeny of Nostoc and relative genera were researched,and the current taxonomy of them were evaluated.Some controversial species were discussed.In addition,the results show taxonomic status of some genera and species need to be revised.