Complete Chloroplast Genomes Of Sequoia Sempervirens And Sequoiadendron Giganteum And The Phylogeny Of Gymnosperms

Chloroplast is a functional plasmid predominantly responsible for plant photosynthesis.Because of the complex nuclear genomes and the slow evolution rate of mitochondrial genome,there is an increasing prevalence to analyze plant phylogeny using chloroplast genomes.With the development of the next generation sequencing technology,numerous chloroplast genomes have been completely sequenced in many species.However,there are still several controversies in the gymnosperms,especially for the phylogenetic postions of Gnetales and Ginkgo.In addition,the phylogenetic relationships among Metasequoia glyptostroboides,Sequoiadendron giganteum and Sequoia sempervirens are still ambiguous.The three species have the closest genetic relationships in the previous study,but which two of them have a much closer relationship has not been solved yet.1)In the present study,the phylogenetic tree of gymnosperms based on these chloroplast genomes was constructed based on a combined data using two newly sequenced chloroplast genomes of S.sempervirens and S.giganteum and 67 complete chloroplast genomes from other species(2 ferns as outgroup,4 anginosperms and 61 gymnosperms)downloaded from NCBI,in order to answer the questions mentioned above.Several conclusions could be made as follows.2)The chloroplast genome sizes of S.sempervirens and S.giganteum are 135,651 bp and 131,574 bp,respectively.Compared with chloroplast genome of M.glyptostroboides,they all have 118 genes including 82 protein-coding genes,4 rRNA genes and 32 tRNA genes,but S.sempervirens and S.giganteum had lost the IRA.3)After constructing phylogenetic tree,we found that the Gnetales was a sister species group to Pinaceae,supporting Gnepine hypothesis,while Ginkgo is also as sister group to a clade comprising conifers and Gnetophytes.S.sempervirens and S.giganteum have a much closer phylogenetic relationship than M.glypostroboides.Our results showed generally similar phylogenetic pattern to the gymnosperms phylogeny researches using a variety of chloroplast genes,but have a contrasting difference between those based on nuclear genes.