Chloroplast Genome Sequences and Comparative Analyses of Combretaceae Mangroves with Related Species

In the Combretaceae family, only two species ofLumnitzeraand one species ofLagunculariabelong to mangroves. Among them,Lumnitzera littorea(Jack) Voigt. is an endangered mangrove plant in China for the limited occurrence and seed abortion. In contrast,Lumnitzera racemosaWilld. is known as the most widespread mangrove plant in China.Laguncularia racemosaC. F. Gaertn., an exotic mangrove in China, has the fast growth and high adaptation ability. To better understand the phylogenetic positions of these mangroves in Combretaceae and in Myrtales and to provide information for studies on evolutionary adaptation for intertidal habitat, the complete chloroplast (cp) genomes ofLu. racemosaandLa. racemosawere sequenced. Furthermore, we present here the results from the assembly and annotation of the two cp genomes, which were further subjected to the comparative analysis withLu. littoreacp genomes we published before and other eleven closely related species within Myrtales. The chloroplast genomes of the three Combretaceae mangrove species:Lu. littorea,Lu. racemosa, andLa. racemosaare 159,687 bp, 159,473 bp, and 158,311 bp in size. All three cp genomes host 130 genes including 85 protein-coding genes, 37 tRNAs, and 4 rRNAs. A comparative analysis of those three genomes revealed the high similarity of genes in coding-regions and conserved gene order in the IR and LSC/SSC regions. The differences betweenLumnitzeraandLagunculariacp genomes are the locations ofrps19andrpl2genes in the IR/SC boundary regions. Investigating the effects of selection events on shared protein-coding genes showed a relaxed selection had acted on theycf2,ycf1, andmatKgenes of Combretaceae mangroves compared to the nonmangrove speciesEucalyptus aromaphloia. The phylogenetic analysis based on the whole chloroplast genome sequence with one outgroup species strongly supported three Combretaceae mangroves together with other two Combretaceae species formed a cluster in Combretaceae. This study is the first report on the comparative analysis of three Combretaceae mangrove chloroplast genomes, which will provide the significant information for understanding photosynthesis and evolution in Combretaceae mangrove plants.