Characterization and phylogenetic analysis of the complete chloroplast genome sequences of six species of Wikstroemia (Thymelaeaceae) revealed paraphyletic relationship to the monotypic genus Stellera.

Abstract

Wikstroemia (Thymelaeaceae) is a diverse genus, spanning both Asia and Australia and also recorded from the Hawaiian Islands. However, due to its medicinal properties and resource utilization in pulp production, genetic studies on these important species have been overshadowed. In this study, the chloroplast genome sequences of six species of Wikstroemia were sequenced and analyzed. The chloroplast genomes of the six species ranged between 172,610 bp (W. micrantha) and 173,697 bp (W. alternifolia), and exhibited a typical genome structure consisting of a pair of inverted repeat (IR) regions separated by a large single-copy (LSC) region and a small single-copy (SSC) region. The six chloroplast genomes were similar with a predicted 139 genes that consisted of 92 or 93 protein-coding genes, 38 tRNA genes, and 8 rRNA genes. The overall GC contents were identical (36.7%). Genome comparative analyses were conducted with the inclusion of two additional published species of Wikstroemia in which the sequence divergence and expansion of IRs in the chloroplast genomes were determined. The phylogenetic analysis inferred that Wikstroemia in its current circumscription is paraphyletic to Stellera chamaejasme, while the ITS-based tree analyses could not properly resolve the phylogenetic relationship between Stellera and Wikstroemia. This finding kindled interest in the proposal to synonymize Stellera with Wikstroemia, which was previously brought up, but rejected due to taxonomic conflicts. Nevertheless, this study provides valuable genomic information to aid in the taxonomic implications and phylogenomic reconstruction of Thymelaeaceae.