A high level of chloroplast genome sequence variability in the Sawtooth Oak Quercus acutissima

The Sawtooth Oak, Quercus acutissima Carruth., is an economically and ecologically important tree species in the family Fagaceae with a wide distribution in China. Here, we examined its intraspecific chloroplast (cp) genome variability using available and a newly sequenced genome. The new cp genome comes from a Q. acutissima individual collected from Shenyang (Northeast China; "Q. acutissima Shenyang" in the following), and then is compared with two recently published cp genomes from Tongchuan (Northwest China) and Nanjing (East China). The cp genome of Q. acutissima Shenyang exhibits a slightly larger genome size than the other two individuals, although each encodes 86 protein-coding genes, 40 tRNA genes and eight rRNA genes. We also found the length difference for the IR/SC boundary region among the three cp genomes. Sequence comparison revealed a high intraspecific genetic divergence: the three cp genomes differ by 332 sequence patterns including 77 single nucleotide polymorphisms, and 255 indels (each gap considered) scattering across 67 regions. Phylogenetic analyses based on the cp genome recovered the split between the subgenus Cerris and the subgenus Quercus, but revealed that three Q. acutissima individuals did not cluster together, indicating that even complete cp genome data fail to reproduce species boundaries in Asian members of section Cerris. Our results show that more complete plastomes covering remote ranges needs to be sequenced to provide a solid backbone for future population-scale in-depth studies and phylogenetic analysis of section Cerris.