Herbaceous bamboos (Olyreae) are a separate lineage with idiosyncratic traits, e.g., unisexual flowers and annual or seasonal flowering lifestyle, in the grass family. To elucidate the evolution of herbaceous bamboos we produced two complete chloroplast (cp) genomes from two monotypic genera i.e., Froesiochloa and Rehia via the genome-skimming approach. The assembled F. boutelouoides and R. nervata cp genomes were 135,905 and 136,700 base-pair (bp), respectively. Further whole-genome comparative analyses revealed that the cp genes order was perfectly collinear, but the inverted repeats (IRs) borders, i.e., the junctions between IRs and single copy regions, were highly divergent in Olyreae. The IRs expansions/contractions occurred frequently in Olyreae, which have caused gene content and genome size variations, e.g., the copy number reduction of rps19 and trnH(GUG) genes in Froesiochloa. Subsequent nucleotide mutation analyses uncovered a greatly heterogeneous divergence pattern among different cpDNA regions in Olyreae cp genomes. On average, non-coding loci evolved at a rate of circa 1.9 times faster than coding loci, from which 20 rapidly evolving loci were determined as potential genetic markers for further studies on Olyreae. In addition, the phylogenomic analyses from 67 grass plastomes strongly supported the phylogenetic positions of Froesiochloa and Rehia in the Olyreae.
Keywords: chloroplast genome; divergent IR borders; herbaceous bamboos; mutation hotspots; phylogenomics.