The complete chloroplast DNA sequence of Eleutherococcus senticosus (Araliaceae); comparative evolutionary analyses with other three asterids.

Abstract

This study reports the complete chloroplast (cp) DNA sequence of Eleutherococcus senticosus (GenBank: JN 637765), an endangered endemic species. The genome is 156,768 bp in length, and contains a pair of inverted repeat (IR) regions of 25,930 bp each, a large single copy (LSC) region of 86,755 bp and a small single copy (SSC) region of 18,153 bp. The structural organization, gene and intron contents, gene order, AT content, codon usage, and transcription units of the E. senticosus chloroplast genome are similar to that of typical land plant cp DNA. We aligned and analyzed the sequences of 86 coding genes, 19 introns and 113 intergenic spacers (IGS) in three different taxonomic hierarchies; Eleutherococcus vs. Panax, Eleutherococcus vs. Daucus, and Eleutherococcus vs. Nicotiana. The distribution of indels, the number of polymorphic sites and nucleotide diversity indicate that positional constraint is more important than functional constraint for the evolution of cp genome sequences in Asterids. For example, the intron sequences in the LSC region exhibited base substitution rates 5-11-times higher than that of the IR regions, while the intron sequences in the SSC region evolved 7-14-times faster than those in the IR region. Furthermore, the Ka/Ks ratio of the gene coding sequences supports a stronger evolutionary constraint in the IR region than in the LSC or SSC regions. Therefore, our data suggest that selective sweeps by base collection mechanisms more frequently eliminate polymorphisms in the IR region than in other regions. Chloroplast genome regions that have high levels of base substitutions also show higher incidences of indels. Thirty-five simple sequence repeat (SSR) loci were identified in the Eleutherococcus chloroplast genome. Of these, 27 are homopolymers, while six are di-polymers and two are tri-polymers. In addition to the SSR loci, we also identified 18 medium size repeat units ranging from 22 to 79 bp, 11 of which are distributed in the IGS or intron regions. These medium size repeats may contribute to developing a cp genome-specific gene introduction vector because the region may use for specific recombination sites.