Complete Chloroplast Genome Determination of Ranunculus sceleratus from Republic of Korea (Ranunculaceae) and Comparative Chloroplast Genomes of the Members of the Ranunculus Genus

Ranunculus sceleratus (family: Ranunculaceae) is a medicinally and economically important plant; however, gaps in taxonomic and species identification limit its practical applicability. This study aimed to sequence the chloroplast genome of R. sceleratus from Republic of Korea. Chloroplast sequences were compared and analyzed among Ranunculus species. The chloroplast genome was assembled from Illumina HiSeq 2500 sequencing raw data. The genome was 156,329 bp and had a typical quadripartite structure comprising a small single-copy region, a large single-copy region, and two inverted repeats. Fifty-three simple sequence repeats were identified in the four quadrant structural regions. The region between the ndhC and trnV-UAC genes could be useful as a genetic marker to distinguish between R. sceleratus populations from Republic of Korea and China. The Ranunculus species formed a single lineage. To differentiate between Ranunculus species, we identified 16 hotspot regions and confirmed their potential using specific barcodes based on phylogenetic tree and BLAST-based analyses. The ndhE, ndhF, rpl23, atpF, rps4, and rpoA genes had a high posterior probability of codon sites in positive selection, while the amino acid site varied between Ranunculus species and other genera. Comparison of the Ranunculus genomes provides useful information regarding species identification and evolution that could guide future phylogenetic analyses.


Introduction
The family Ranunculaceae is distributed worldwide, comprising 59 genera and more than 2500 species [1]. Among them, Ranunculus (buttercups) is the largest genus, comprising approximately 600 species, and is classified into 20 taxonomic sections [2,3]. Ranunculus inhabits tropical and temperate regions within temperate forests, arctic tundra, mountainous regions, freshwater systems, and terrestrial ecosystems [3,4]. Genetically, this genus is diverse with a wide distribution, making its classification highly complex [1]. In particular, taxonomic difficulties have occurred due to the morphological and ecological diversity of traits that have arisen during the evolution of Ranunculus in different environments [5,6].
R. sceleratus L. is an annual or perennial herbaceous plant in the family Ranunculaceae [7] that primarily inhabits wetlands or slow rivers and is distributed worldwide, including throughout Republic of Korea [8]. Dried and fresh R. sceleratus is reportedly used in the treatment of esophageal and breast cancer, as well as malaria-induced jaundice [8,9]. Additionally, as a medicinal plant, it can suppress the occurrence of degenerative diseases by acting as an antibacterial agent and antioxidant [10]. Indeed, plants of the genus Ranunculus have been reported to protect the liver and exhibit anti-inflammatory and anti-malarial effects [11][12][13][14][15][16]. They have pharmacological activity and potentially high value

Plant Sampling and Sequencing
The leaves of R. sceleratus were collected from Yeongi-gun, Chungcheongnam-do, Republic of Korea (36 • 37 32 N, 127 • 17 16 E). The identification and collection of plants were conducted by the author Sun-Yu Kim. The voucher number was NNIBRVP70282, which was deposited with the Nakdonggang National Institute of Biological Resources. Total genomic DNA extraction was performed on R. sceleratus leaves with the DNeasy ® Plant Mini kit (Qiagen) according to the manufacturer's instructions. An Illumina HiSeq 2500 sequencing library was prepared according to the manufacturer's protocol. The DNA library was sequenced with a 150 bp paired-end.

Chloroplast Genome Assembly and Annotation
A short raw read was assembled to obtain the R. sceleratus cp genome, which was then assembled using NOVOPlasty ver. 4.3.1 software [35]. The cp genome was verified for sequence identity using GetOrganelle ver. 1.7.7 [36]. Both assemblers confirmed the sequence of the same cp genome. The complete R. sceleratus cp genome was annotated using CPGAVAS2 [37]. Ranunculus seleratus from Republic of Korea was annotated using Ranunculus seleratus from China (MK253452). Areas with errors in the annotations were manually corrected using Geneious ver. 11.0.1 software. Finally, the annotated cp genome sequence was uploaded to GenBank under the accession number ON755204 using NCBI BankIt. An image of the cp genome for R. sceleratus from Republic of Korea was drawn using OrganellarGenome DRAW [38].
Repeat Finder of Geneious ver. 11.0.1 was used to identify the types of long repeat sequences, i.e., palindromic, forward, reverse, and complement. Repeat sequence identification conditions were set to a minimum repeat size of 20 bp and 100% sequence identity.

Phylogenetic Analysis
To construct phylogenetic trees, 13 cp genomes of the species belonging to the family Ranunculaceae were downloaded from NCBI GenBank. The dataset was constructed by arranging the genome and gene sequences in the same order. The datasets were then realigned using MAFFT ver. 7.490 [40], and a phylogenetic tree was constructed using two methods: maximum likelihood (ML) and Bayesian inference (BI). The ML method used jModelTest ver. 2.1.7 [41] to select the appropriate optimal model (GTR + G + I) from the dataset and constructed the phylogenetic tree using PhyML ver. 3.0 [42]. Moreover, 1000 bootstrap replicates were included for the ML tree. The BI method reconstructed a phylogenetic tree using MrBayes ver. 3.2.7 [43] under the GTR + G + I model (lset nst = 6, Rates = invgamma), which is the optimal base substitution model. The Markov Chain Monte Carlo (MCMC) algorithm used in the analysis was repeated for 10 million MCMC and sampled every 100 generations, with the first 25% discarded as burn-in. The posterior probability values for each node were determined by consensus on multiple trees. Additionally, the phylogenetic trees drawn by both methods were constructed by consensus. The support for the ML and BI consensus trees was expressed as bootstrap values and posterior probabilities, respectively.

Comparative CP Genome Analysis and Screening of Barcoding Regions
Chloroplast genome sequences for Ranunculus species were downloaded from NCBI (Table S1) and rearranged using MAFFT ver. 7.490 [40]. For the aligned sequence, the coding sequence (CDS) present for each species was extracted using Geneious ver. 11.0.1 [44]. Additionally, the intergenic regions in 12 cp genomes were extracted and sequenced. To analyze the nucleotide diversity (Pi), the CDS and intergenic regions of the cp genome for Ranunculus were analyzed using DnaSP ver. 6.12 software [45]. Ranunculus species genomes were compared using mVISTA software (http://genome.lbl.gov/vista/index.shtml, accessed on 2 March 2022) and visualized in shuffle-LAGAN mode.

Specific Barcode Region Primer Design
Primers for effective molecular markers were designed based on the identifiable intergenic region of Ranunculus species. First, Ranunculus austro-oreganus L.D. Benson, R. bungei Steud., R. cantoniensis DC., R. japonicas Thunb., R. macranthus Scheele, R. occidentalis Nutt., R. pekinensis (L.Liou) Luferov, R. reptans L., R. repens L., R. sceleratus L., and R. yunnanensis Franch were downloaded from GenBank. Additionally, a dataset including the cp genome of R. sceleratus from Republic of Korea assembled in this study was used for subsequent analysis. A total of 12 cp genomes were multi-aligned using MAFFT ver. 7.490 [40]. Among the DNA barcode regions (coding sequence and intergenic), 13 CDS (Pi ≥ 0.02) and 48 intergenic regions (Pi ≥ 0.040) with high nucleotide diversity were selected as hotspots. The PCR amplified product size range was 150-1500 bp.
To evaluate the success of species identification for specific barcode markers, two methods were employed, namely, the phylogenetic tree-based method and the sequence similarity-based method. The phylogenetic tree method analyzed two phylogenetic trees (32 specific barcode region sequences), ML and BI, and judged whether they were consistent with the tree drawn based on the cp genome. The similarity-based method constructed a local database using BLAST 2.2.29+; and the sequences were run through BLAST to perform the query. Identification of species barcoding markers was considered successful when the concordance for homogeneity was 100%.

Positive Selection Analysis
Extraction of the aligned CDS for Ranunculus was performed using Geneious ver. 11.0.1 [44]. For positive selection analysis, the optimized branch site model method was applied using the CODEML program of the PAML ver. 4.10.6 package [46,47]. We implemented Bayes Empirical Bayes (BEB) and Naive Empirical Bayes (NEB) methods to identify specific amino acid sites in potential positive selection genes to calculate posterior probabilities. Codon sites with high posterior probability (P > 0.5) were considered positively selected sites [48,49]; positive selection: ratio ω > 1, neural selection: ω = 1, and negative selection: ω < 1 [50]. An alternative branching site model (model = 2, NSsites = 2, Fix = 0) and a neutral branching site model (model = 2, NSsites = 2, Fix = 1, Fix ω = 1) were applied to identify positively selected sites. Genes with a p-value < 0.05 and a positively selected site were considered a positive selection. Visualization of the amino acid sequence portions of positively selected genes was performed using Jalview ver. 2.11.2 software [51].

CP Genome Characterization for Ranunculus Species
Whole-genome sequencing of R. sceleratus from Republic of Korea produced 958 million reads using the HiSeq 2500 platform, yielding a total of 144 GB ( Table 1). The complete cp genome of R. sceleratus from Republic of Korea was obtained by assembly based on the reference genome of R. sceleratus from China (NCBI Accession No.: MK253452). The cp genome of R. sceleratus from Republic of Korea was 156,329 bp in length and had a typical quadripartite structure ( Figure 1). The cp genome contained large single-copy (LSC, 85,840 bp), small single-copy (SSC, 19,885 bp), and inverted repeat (IR) regions; the IR comprised two copies of IRs a and b and was 25,302 bp in size (Table 1). gle-copy (SSC, 19,885 bp), and inverted repeat (IR) regions; the IR co of IRs a and b and was 25,302 bp in size (Table 1).   Although the cp genome of higher plants is highly conserved, the extension and contraction between the IR, LSC, and SSC regions cause differences in genome length between species [26,33,34,52,53]. In the case of Ranunculus species, a change in gene position due to the expansion and contraction of IR and SSC was observed. More specifically, in nine species (except R. austro-oreganus and R. occidentalis), the atpH gene was located between the atpF and atpI genes in the 1.3 kb LSC region ( Figure 2). In contrast, the atpH gene in R. austrooreganus and R. occidentalis was located between the ycf1 and trnN-GUU (11.5 kb) genes in the SSC region. Structural variations in IR and SSC can result in gene rearrangement [54,55]. In this study, the SSC region lengths of R. austro-oreganus (21,249 bp) and R. occidentalis (21,269 bp) were extended by more than~2 kb compared to the SSC region lengths of R. japonicus and R. macranthus (18,909 bp) and, thus, might represent a major cause of atpH gene rearrangement [54,55]. No contraction or expansion was detected in the IR and SSC regions between No contraction or expansion was detected in the IR and SSC regions between R. sceleratus from Republic of Korea and China; however, a small 5 bp sequence difference was observed in the LSC region (Table S2). Given that the cp genome tends to be highly conserved across species [56][57][58], it was estimated that low levels of sequence variation exist between the Republic of Korean and Chinese species. Hence, these minor sequence differences in the cp genome serve as useful markers for differentiating geographic populations [27]. This marker can potentially be used for population identification, but further population studies are needed to verify this.
The GC content of the cp genome was 37.9%, which corresponded with that of R. sceleratus from China; 12 other Ranunculus species genomes were reported to have a similar GC content (37.7-37.9%; Table S3). The GC content of the LSC region of the 13 cp genome was 35.9-36.2%; that of the SSC region was 31.0-31.7%, and that of the IR region was 42.8-43.6%, with the largest difference in GC content in the IR region. The cp genome was classified into self-replication, photosynthesis, and other genes, as it is in other plants (Table S2). The Ranunculus species, excluding R. austro-oreganus, R. occidentalis, and R. sceleratus from China, carried 112 genes. The genetic and structural makeup of most species was similar. That is, R. sceleratus from Republic of Korea and China carried infA, while R. austro-oreganus and R. occidentalis had ycf15 genes (Table S2). Notably, the infA gene has been lost in many angiosperms, and in certain plants, it has been transferred to the nucleus [59,60]. Loss of the genome may be a low-cost strategy for rapid replication in adverse environmental conditions [61].

Long Repeats
Long repeats are important hotspots for genetic rearrangement and population variation in the cp genome [62][63][64]. A total of 402 repeats were identified for 12 Ranunculus cp genomes (Table S4), including 148 forward repeats, 127 reverse repeats, 96 complementary repeats, and 31 palindromic repeats. Regarding the number of repeats by species, R. japonicus contained the most (64), while R. yunnanensis had the fewest (18; Figure 3). According to the number of iterations, the largest number of species contained 42 repeats ( Figure 4, Table S5). Notably, most species could be identified based on the total number of repeats, excluding R. austro-oreganus (31), R. occidentalis (31), R. bungei (34), and R. reptans (34 ; Table S4). However, species with the same number of repeats could be identified based on the type of repeats and the ratio of repeats between species (Table S5). Repeat sequences rearrange the cp genome [63] and reportedly enable interspecies identification owing to their random distribution [63,65]. was classified into self-replication, photosynthesis, and other genes, as it is in other plants (  (Table S2). Notably, the infA gene has been lost in many angiosperms, and in certain plants, it has been transferred to the nucleus [59,60]. Loss of the genome may be a low-cost strategy for rapid replication in adverse environmental conditions [61].

Long Repeats
Long repeats are important hotspots for genetic rearrangement and population variation in the cp genome [62][63][64]. A total of 402 repeats were identified for 12 Ranunculus cp genomes (Table S4), including 148 forward repeats, 127 reverse repeats, 96 complementary repeats, and 31 palindromic repeats. Regarding the number of repeats by species, R. japonicus contained the most (64), while R. yunnanensis had the fewest (18; Figure 3). According to the number of iterations, the largest number of species contained 42 repeats ( Figure 4, Table S5). Notably, most species could be identified based on the total number of repeats, excluding R. austro-oreganus (31), R. occidentalis (31), R. bungei (34), and R. reptans (34 ; Table S4). However, species with the same number of repeats could be identified based on the type of repeats and the ratio of repeats between species (Table S5). Repeat sequences rearrange the cp genome [63] and reportedly enable interspecies identification owing to their random distribution [63,65].

SSR Analysis
In this study, 53 (Table S6). The motif distribution of the Ranunculus species was 58.21% mono-nucleotide, 17.34% di-nucleotide, 15.69% tetra-nucleotide, 6.02% tri-nucleotide, 2.37% penta-nucleotide, and 0.36% hexa-nucleotide (Table S6). The number of SSRs throughout the genome is generally large in the order of di-, mono-, tri-, and tetra-nucleotides [66][67][68]. However, herein, the SSR distribution in the cp genome was large in the order of mono-, di-, tetra-, and tri-nucleotides, with a difference observed in the distribution of SSR between the genome and cp genome. This appears to be a feature of SSR distribution in the cp genome of plants [69,70].
In the cp genome of R. sceleratus from Korea, 64.15% (34 SSRs) of the SSRs were located in the LSC region, followed by 28.30% (15 SSRs) in the SSC region and 7.55% (4 SSRs) in the IR (Table S6). The R. sceleratus from Korea and China exhibited small intraspecies sequence differences in the LSC region, which was located between the ndhC and trnV-UAC genes. However, an indel difference in the TAAAG repeat sequence was detected ( Figure S1). Hence, the region between these two genes may be useful as a genetic marker to distinguish R. sceleratus species from Korea and China.
The number of mono-, di-, and tetra-nucleotide repeats was highly variable in the cp genome of Ranunculus species. In fact, if the total number of SSRs among species was the same, it was possible to distinguish between species based on differences in SSR distribution (R. austro-oreganus: 47, R. japonicus: 47, R. cantoniensis: 45, and R. occidentalis: 45). SSR variations in the cp genomes of nine Ranunculus taxa were determined to be useful for comparing phylogenetic relationships through genetic polymorphisms at the population and species level, suggesting their importance for SSR studies.

Phylogenetic Analysis
Datasets used for phylogenetic analysis were classified and constructed to include the cp genome, coding sequences, intergenic regions, and specific barcode regions. ML and BI tree analysis of the four datasets showed that Ranunculus was generated as a single clade; the bootstrap value was strongly supported at 100% for all clades ( Figure 5).
In the cp genome of R. sceleratus from Republic of Korea, 64.15% (34 SSRs) of the SSRs were located in the LSC region, followed by 28.30% (15 SSRs) in the SSC region and 7.55% (4 SSRs) in the IR (Table S6). The R. sceleratus from Republic of Korea and China exhibited small intraspecies sequence differences in the LSC region, which was located between the ndhC and trnV-UAC genes. However, an indel difference in the TAAAG repeat sequence was detected ( Figure S1). Hence, the region between these two genes may be useful as a genetic marker to distinguish R. sceleratus species from Republic of Korea and China.
The number of mono-, di-, and tetra-nucleotide repeats was highly variable in the cp genome of Ranunculus species. In fact, if the total number of SSRs among species was the same, it was possible to distinguish between species based on differences in SSR distribution (R. austro-oreganus: 47, R. japonicus: 47, R. cantoniensis: 45, and R. occidentalis: 45). SSR variations in the cp genomes of nine Ranunculus taxa were determined to be useful for comparing phylogenetic relationships through genetic polymorphisms at the population and species level, suggesting their importance for SSR studies.

Phylogenetic Analysis
Datasets used for phylogenetic analysis were classified and constructed to include the cp genome, coding sequences, intergenic regions, and specific barcode regions. ML and BI tree analysis of the four datasets showed that Ranunculus was generated as a single clade; the bootstrap value was strongly supported at 100% for all clades ( Figure 5). The first clade comprised six species (R. austro-oreganus, R. occidentalis, R. japonicus, R. cantoniensis, R. macranthus, and R. repens), while the second comprised R. sceleratus from China (MK253452), R. sceleratus from Republic of Korea (ON755204), R. bungei, R. pekinensis, and R. yunnanensis. Ranunculus sceleratus from Republic of Korea was most closely related to R. sceleratus from China. tionships within Ranunculus. The current study also demonstrated the potential of usin the complete cp genome to resolve phylogenetic relationships [4,26,32,71]. Although t cp genome is phylogenetically informative, the closely related R. austro-oreganus, R. oc dentalis, R. bungee, and R. pekinensis required high-resolution specific barcodes for speci identification.
The CDS phylogenetic tree did not clearly differentiate between these four close related species; however, the intergenic regions and 16 specific barcode regions facilitat their identification. In particular, the tree constructed based on specific barcode regio efficiently differentiated the species using a small number of intergenic regions.

Barcoding Region Screening of Genome Divergence Regions
Genome comparison plots were created using mVISTA to assess the sequence sim larity of the cp genomes between Ranunculus species ( Figure S2). The aligned gene po tions among Ranunculus species showed that the positions and sequences of genes, exce atpH, were conserved. When comparing the same species, relatively no difference w detected in the sequences (R. sceleratus from Korea and China); however, a clear diffe ence in sequences was observed between Ranunculus species. Notably, the IR was rel tively more conserved than the LSC and SSC regions, which agrees with results for oth plant species.
We multi-aligned the cp genome sequence of the Ranunculus genus and screened t protein-coding regions, intergenic regions, and intronic regions to identify unique r gions within the genome. DnaSP analysis searched for polymorphic genes in coding s quences and intergenic regions. The nucleotide diversity (Pi) values ranged from 0.0 (rpl23) to 0.043 (ccsA). The most polymorphic genes were ccsA, matK, rpl32, rps3, ndh rps15, ndhG, ndhA, psbH, accD, clpP, atpF, and ndhH (Pi ≥ 0.02), and the least polymorph The complete cp genome has been proposed to be useful for taxonomic reconstruction [26,32] and utilized in previous studies to resolve interspecies phylogenetic relationships within Ranunculus. The current study also demonstrated the potential of using the complete cp genome to resolve phylogenetic relationships [4,26,32,71]. Although the cp genome is phylogenetically informative, the closely related R. austro-oreganus, R. occidentalis, R. bungee, and R. pekinensis required high-resolution specific barcodes for species identification.
The CDS phylogenetic tree did not clearly differentiate between these four closely related species; however, the intergenic regions and 16 specific barcode regions facilitated their identification. In particular, the tree constructed based on specific barcode regions efficiently differentiated the species using a small number of intergenic regions.

Barcoding Region Screening of Genome Divergence Regions
Genome comparison plots were created using mVISTA to assess the sequence similarity of the cp genomes between Ranunculus species ( Figure S2). The aligned gene positions among Ranunculus species showed that the positions and sequences of genes, except atpH, were conserved. When comparing the same species, relatively no difference was detected in the sequences (R. sceleratus from Republic of Korea and China); however, a clear difference in sequences was observed between Ranunculus species. Notably, the IR was relatively more conserved than the LSC and SSC regions, which agrees with results for other plant species.
Herein, when the sequence diversity of 11 Ranunculus species was compared, the noncoding region was more variable than the coding region ( Figure 6). Similarly, in angiosperms, the nucleotide diversity is higher within the intergenic region than within the coding region [27,[72][73][74]. Certain hotspots within these variable regions were used to design primers to differentiate Ranunculus species from others, demonstrating that these regions can be utilized as potential barcoding markers. More specifically, within the protein-coding region, 13 regions with a Pi ≥ 0.02 and 48 intergenic regions with a Pi ≥ 0.040 were set as hotspot regions.

Barcode Validation of Hotspot Regions in the Genome
Among the 48 hotspot regions, those useful as specific barcodes were selected based on BLAST and phylogenetic tree analyses. The specific barcode regions ndhG-ndhI, petN-psbM, atpF-atpI, ndhC-trnV-UAC, trnT-GGU-psbD, and psbE-petL had a BLAST-based species identification rate of 100%. For the remaining 10 markers, the species identification rate using BLAST was 83.3-91.7% ( Table 2).
Herein, when the sequence diversity of 11 Ranunculus species was compared, the noncoding region was more variable than the coding region ( Figure 6). Similarly, in angiosperms, the nucleotide diversity is higher within the intergenic region than within the coding region [27,[72][73][74]. Certain hotspots within these variable regions were used to design primers to differentiate Ranunculus species from others, demonstrating that these regions can be utilized as potential barcoding markers. More specifically, within the proteincoding region, 13 regions with a Pi ≥ 0.02 and 48 intergenic regions with a Pi ≥ 0.040 were set as hotspot regions.
With the exception of R. austro-oreganus and R. occidentalis, the first group of markers to distinguish between nine Ranunculus species included trnT-UGU-trnL-UAA and trnK-UUU-rps16. The second group of markers to distinguish between nine Ranunculus species, with the exception of R. bungei and R. pekinensis, included petG-trnW-CCA, rpl32-trnL-UAG, rpl16-rps3, rps16-trnQ-UUG, accD-psaI, trnG-GCC-trnfM-CAU, and psbZ-trnG-GCC. With the exception of R. macranthus and R. repens, the third group of markers to distinguish between nine Ranunculus species included rps8-rpl14. Combining two or more markers provides a higher rate of species identification than a single marker [76,77]. Therefore, to enhance species identification, ML and BI phylogenetic tree construction and BLAST were performed to analyze the species identification rate by combining the first, second, and third group markers.
The combination of specific barcode markers identified seven-fold more variant sites than conventional single specific barcode markers. Given that markers have different rates of nucleotide variation at different loci, when assessing closely related species, the application of combination markers may be more advantageous for species identification [78]. The conventional barcoding markers, rbcL, psbA-trnH-GUG, and trnL-UAA-trnF-GAA, showed a low 75% species identification rate using BLAST and phylogenetic tree-based methods (Table 3). Therefore, the combination of specific barcode markers and single specific barcode markers with high species resolution developed in this study can replace the existing barcoding markers. trnL-UAA-trnF-GAA 418 75 75 75 Primers were designed for the flanking regions of these markers to enable the use of 16 hotspot regions as specific barcodes ( Table 4). The 100% species identifiable markers for Ranunculus showed high resolution for genetically closely related species, such as R. austro-oreganus, R. occidentalis, R. bungei, and R. pekinensis. Accordingly, these markers can be effectively applied for the identification of very closely related species of the genus Ranunculus.

Positive Selection Analyses
Positive selection analysis was performed on 66 CDS. In the likelihood ratio analysis, most p-values for all genes were >0.05, i.e., insignificant. In contrast, in the BEB test, atpF, ndhE, ndhF, rpl23, rpoA, rps4, and ycf4 showed posterior probabilities ≥ 0.7, with ndhE, ndhF, and rpl23 having values ≥ 0.9. In the NEB method, positively selected sites in atpF, ndhF, rps4, and rpoA genes were detected as ≥0.8 (Table S8). In a previous study, codon regions with high posterior probability based on BEB and NEB analyses were considered positive selection genes [48]. Therefore, we ultimately selected atpF, ndhE, ndhF, rpl23, rpoA, and rps4 as positive selection genes. The six amino acid characteristics between Ranunculus species and other genera are shown in Figure 7.  The atpF gene, which is required for electron transport and photophosphorylation during photosynthesis, encodes the H + -ATP synthetase subunit [79]. The rpoA gene encodes a plastid-encoded RNA polymerase (PEP) that regulates gene expression [80][81][82]. The rpl and rps genes encode large and small subunits of ribosomal proteins and are differentially expressed according to abiotic and biotic environmental factors by regulating plant growth and transcription [83,84].

Conclusions
The chloroplast ndh gene forms a thylakoid NADH dehydrogenase (Ndh) complex homologous to the encoded polypeptide and mitochondrial complex I [85][86][87]. As such, ndh plays an essential role in photosynthesis in higher plants [88]. In particular, the Ndh complex generated from ndh protects against photooxidation-related stress and maintains an optimal photophosphorylation rate [88]. However, the ndh gene is presumably not required in mild, unstressed environments and exhibits rapid changes in function under light-related stress [88][89][90]. In this study, ndhE, ndhF, rpl23, atpF, rps4, and rpoA were confirmed to have high positive selection sites. Considering that various light-related stressful environments alter the biochemical composition and morphology of plants during acclimatization [91], it is inferred that these positive selection sites represent mutations in gene function related to environmental resistance as a light-related stress response in Ranunculus species. Therefore, genes that are positively selected and adapted to these genes could represent adaptation to new environmental conditions for Ranunculus species.

Conclusions
Ranunculus sceleratus (family: Ranunculaceae) is a medicinally and economically important plant; however, gaps in taxonomic and species identification limit its practical applicability. The aim of this study was to elucidate the chloroplast genome of R. sceleratus from Republic of Korea, identify how it differed from the chloroplast genome of R. sceleratus from China, and identify potential specific barcodes through genome comparisons in the genus Ranunculus. The chloroplast genome was assembled from Illumina HiSeq 2500 sequencing raw data. The genome was 156,329 bp and had a typical quadripartite structure comprising a small single-copy region, a large single-copy region, and two inverted repeats. Fifty-three simple sequence repeats were identified in the four quadrant structural regions. The region between the ndhC and trnV-UAC genes could be useful as a genetic marker to distinguish between R. sceleratus populations from Republic of Korea and China. The Ranunculus species formed a single lineage. To differentiate between Ranunculus species, we identified 16 hotspot regions and confirmed their potential using specific barcodes based on phylogenetic tree and BLAST-based analyses. The ndhE, ndhF, rpl23, atpF, rps4, and rpoA genes were considered positively selected owing to their high posterior probability. The amino acid site varied between Ranunculus species and other genera. Comparison of the Ranunculus genomes provides useful information regarding species identification and evolution that could guide future phylogenetic analyses.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/genes14061149/s1, Figure S1: The alignment of the chloroplast sequence was based on the chloroplast genome of R. sceleratus from Republic of Korea and China; Figure S2: The alignment of the chloroplast sequence was based on the chloroplast genome of R. sceleratus from Republic of Korea; Figure S3: Maximum Likelihood and Bayesian Inference phylogenetic trees of specific barcode region markers; Figure S4: Maximum Likelihood and Bayesian Inference phylogenetic trees of combination markers of intergenic regions; Table S1: List of species information used for comparative chloroplast genome analysis of Ranunculus; Table S2: List of genes in the Ranunculus species; Table S3: Comparative information of the cp genome structure of the species in the Ranunculus genus; Table S4: Repeat type information of Ranunculus species; Table S5: Distribution by species according to the long repeat sequence; Table S6: SSR region information of the species cp genome of the genus Ranunculus; Table S7: Sequence and tree-based species identification rate of combinatorial markers of the genus Ranunculus; Table S8: The potential positive selection test based on the branch-site model of Ranunculus.

Data Availability Statement:
The genome sequence data that support the findings of this study are openly available from NCBI GenBank (https://www.ncbi.nlm.nih.gov/ accessed on 2 March 2022) under the accession no. ON755204.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.