title: Complete chloroplast genome and phylogenetic analysis of the Richardia brasiliensis (Spermacoceae, Rubiaceae)
design: We determined the complete chloroplast genome sequence of Richardia brasiliensis (Rubiaceae, Rubioideae, tribe Spermacoceae) and performed phylogenetic analyses to clarify its relationships with closely related taxa. The chloroplast genome was 154,573 bp in length with an overall GC content of 36.7%, and exhibited a typical quadripartite structure comprising a large single-copy region of 84,388 bp, a small single-copy region of 17,411 bp, and two inverted repeat regions of 26,387 bp each. It contained 129 genes, including 82 protein-coding genes (CDSs), 37 transfer RNA genes, and 8 ribosomal RNA genes; among them, six CDSs (ndhB, rpl2, rpl23, rps7, rps12, and ycf2), seven tRNA genes (trnA-UGC, trnI-CAU, trnI-GAU, trnL-CAA, trnN-GUU, and trnR-ACG), and four rRNA genes (rrn4.5, rrn5, rrn16, and rrn23) were duplicated in the IR regions. For phylogenetic inference, 77 shared CDSs were extracted and concatenated from complete chloroplast genomes of 50 taxa (47 Rubioideae including R. brasiliensis and three Cinchonoideae as outgroups). The results indicated that R. brasiliensis is most closely related to Oldenlandia corymbosa within Spermacoceae. This newly generated chloroplast genome provides baseline genomic resources for Spermacoceae and will facilitate future phylogenetic and comparative plastome studies aimed at resolving complex intergeneric relationships within the tribe.