The Complete Chloroplast Genome of Onobrychis Gaubae (Fabaceae- Papilionoideae): Comparative Analysis with Related IR-lacking Clade Species

doi:10.21203/rs.3.rs-290026/v2

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Research Article

The Complete Chloroplast Genome of Onobrychis Gaubae (Fabaceae- Papilionoideae): Comparative Analysis with Related IR-lacking Clade Species

https://doi.org/10.21203/rs.3.rs-290026/v2

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published 19 Feb, 2022

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Background: Plastome (Plastid genome) sequences provide valuable markers for surveying the evolutionary relationships and population genetics of plant species. Papilionoideae (papilionoids) has different nucleotide and structural variations in plastomes, which makes it an ideal model for genome evolution studies. Therefore, by sequencing the complete chloroplast genome of Onobrychis gaubae in this study, the characteristics and evolutionary patterns of plastome variations in IR-loss clade were compared.

Results: In the present study, the complete plastid genome of O. gaubae, endemic to Iran, was sequenced using Illumina paired-end sequencing and was compared with previously known genomes of the IRLC species of legumes. The O. gaubae plastid genome was 122,873 bp in length and included a large single-copy (LSC) region of 81,486 bp, a small single-copy (SSC) region of 13,805 bp and one copy of the inverted repeat (IR_b) of 29,100 bp. The genome encoded 110 genes, including 76 protein-coding genes, 30 transfer RNA (tRNA) genes and four ribosome RNA (rRNA) genes and possessed 83 simple sequence repeats (SSRs) and 50 repeated structures with the highest proportion in the LSC. Comparative analysis of the chloroplast genomes across IRLC revealed three hotspot genes (ycf1, ycf2, clpP) which could be used as DNA barcode regions. Moreover, seven hypervariable regions (trnL(UAA)-trnT(UGU), trnT(GGU)-trnE(UUC), ycf1, ycf2, ycf4, accD and clpP) were identified between Onobrychis species, which could be used to distinguish the Onobrychis species. Phylogenetic analyses revealed that O. gaubae is closely related to Hedysarum. The complete O. gaubae genome is a valuable resource for investigating evolution of Onobrychis species and can be used to identify related species.

Conclusions: Our results reveal that the plastomes of the IRLC are dynamic molecules and show multiple gene losses and inversions. The identified hypervariable regions could be used as molecular markers for resolving phylogenetic relationships and species identification and also provide new insights into plastome evolution across IRLC.

Plant Physiology and Morphology

Plant Molecular Biology and Genetics

Plastome

IRLC

Onobrychis

Hypervariable region

Phylogenetic relationship

Chloroplast is a vital organelle in plant cells that has an important role in plant carbon fixation and numerous metabolic pathways^1,2. In angiosperms, the chloroplast genome (plastome) typically has a circular structure that ranges from 120 to 180 kb in length. Plastomes mostly exhibit a quadripartite structure in which a pair of inverted repeats (IRa and IRb; usually around 25 kb, but can vary from 7 to 88 kb each) separate the large single-copy (LSC, ca. 80 kb) and the small single-copy (SSC, ca. 20 kb) regions^1,2. Most plastomes encodes 80 protein-coding genes primarily involved in photosynthesis and other biochemical processes along with 30 tRNA and 4 rRNA genes^3,4. In contrast to mitochondrial and nuclear genomes, the plastomes across seed plants are highly conserved with respect to gene content, structure and organization^5,6. However, mutations including duplication, rearrangements, and losses have been reported at the genome and gene levels among some angiosperm lineages, including Asteraceae⁷, Campanulaceae⁸, Onagraceae⁹, Fabaceae¹⁰ and Geraniaceae¹¹.

Fabaceae (legumes) is the third largest family of angiosperms which shows the most extensive structural variation¹². Currently accepted classification of the legumes based on plastid gene matK includes six subfamilies: Caesalpinioideae, Cercidoideae, Detarioideae, Dialioideae, Duparquetioideae, and Papilionoideae¹³. Gene content and gene order in plastomes of subfamilies are highly conserved and similar to the ancestral angiosperm genome organization except for Papilionoideae, which exhibits numerous rearrangements and gene/intron losses and have a smaller genome⁵. In this subfamily, a loss of one of the IR¹⁴, the presence of many repetitive sequences¹⁵ and the presence of a localized hypermutable region^15,16 have been documented. The Papilionoideae is further divided into six major clades: the Genistoids, Dalbergioids, Mirbelioids, Millettioids, Robinioids and the inverted-repeat lacking clade (IRLC)¹⁴. IRLC is the largest legume lineage which contains over 4000 species in 52 genera and eight tribes^14,17−19. Recently, with the advent of next generation sequencing (NGS) technology, plastomes of several taxa from different tribes in this clade have been sequenced. The majority of IRLC plastomes sequenced to date were restricted to the tribes Fabeae, Trifolieae and Caraganeae. Thus it is essential to investigate the members from other lineages to better understand plastome evolution within the IRLC, and more broadly within Papilionoideae. In the tribe Hedysareae²⁰ with nine genera, the plastomes of some Hedysarum species and only one species of Onobrychis (O. viciifolia within subgenus Onobrychis) have been reported. In the present study, the complete plastome of O. gaubae Bornm. belonging to Hedysareae was sequenced (GenBank accession number: ?). Onobrychis, which is the second largest genus after Hedysarum, is composed of two subgenera (Onobrychis and Sisyrosema) and has more than 130 species. This genus mainly found throughout temperate and subtropical regions of Eurasia, N and NE Africa²¹. Onobrychis gaubae belongs to the subgenus Sisyrosema, is a polymorphic species restricted to the southern slopes of Alborz mountain range, Iran^22,23.

The main goal of this study is to assemble the chloroplast genome of O. gaubae, and to annotate the genome and characterize its structure to provide new genomic resource of this species. We also performed comparative analyses of the genome and phylogenetic reconstruction to evaluate the sequence divergence in the plastomes across the IR-lacking clade.

Characteristics of the chloroplast genome of O. gaubae

The number of paired-end raw reads obtained by the Illumina HiSeq 2000 system is 43,189,861 for O. gaubae sample. The plastid genome of O. gaubae with 122,873 bp in length and having only one copy of the IR region, which is in accordance with the reports and its genome structure is similar to those of other IRLC species. In this context, the lack of rps16 and rpl22 genes and intron 1 of clpP in the plastome of O. gaubae should be noted; these genes, are absent from the chloroplast genomes of entire IRLC^24-26. The assembled chloroplast genome of O. gaubae contained 110 genes, including 76 protein-coding genes, 30 transfer RNA (tRNA) genes and four ribosome RNA (rRNA) genes (Fig. 1, Table 1). The LSC (81,066 bp), SSC (13,777 bp) and IR (28,802 bp) regions along with the locations of 110 genes in the chloroplast genome are shown in Fig. 1.

Table 1

Genes predicted in the chloroplast genome of O. gaubae.

Category of genes	Group of genes	Name of genes
Self-replication	Large subunit of ribosomal proteins	rpl14, rpl16, rpl2, rpl20, rpl23, rpl32, rpl33, rpl36
	Small subunit of ribosomal proteins	rps2, rps3, rps4, rps7, rps8, rps11, rps12, rps14, rps15, rps18, rps*19
	DNA-dependent RNA polymerase	rpoA, rpoB, rpoC1,rpo*C2
	Ribosomal RNA genes	rrn16S, rrn23S, rrn 4.5S, rrn 5S
	Transfer RNA genes	30 trn genes (5 contain an intron)
Genes for photosynthesis	Subunits of NADH-dehydrogenase	ndhA, ndhB, ndhC, ndhD, ndhE, ndhF, ndhG, ndhH, ndhI, ndhJ, ndhK
	Subunits of photosystem I	psaA, psaB, psaC, psaI, psaJ
	Subunits of photosystem II	psbA, psbB, psbC, psbD, psbE, psbF, psbH, psbI, psbJ, psbK, psbL, psbM, psbN, psbT, psbZ
	Subunits of cytochrome b/f complex	petA, petB, petD, petG, petL, petN
	Subunits of ATP synthase	atpA, atpB, atpE, atpF, atpH, atp*I
	Subunit of rubisco	rbcL
Other genes	Maturase K	matK
	Envelope membrane protein	cemA
	Subunit of Acetyl-CoA-carboxylase	accD
	C-type cytochrome synthesis gene	ccsA
	Protease	clpP*
Genes of unkown function	Conserved hypothetical chloroplast open reading frames	ycf1, ycf2, ycf4, ycf3**
The number of asterisks after the gene names indicates the number of introns contained in the genes.

A total of 16 genes contained a single intron, whereas ycf3 exhibits two introns (Additional File 1: Table S1). The rps12 gene is a trans-splicing gene which does not have introns in the 3'-end. The trnK-UUU has the largest intron encompassing the matK gene, with 2,495 bp, whereas the intron of trnL-UAA is the smallest (542 bp). The O. viciifolia (the only other species of Onobrychis which cp genome has been sequenced) plastome with 121,932 bp in length showed similar conserved gene contents, order and orientations to O. gaubae. The most important structural differences in the O. viciifolia plastid genome compared to the O. gaubae are the lack of the atpF intron and inversion of ycf2 gene.

The length of plastome in the IRLC taxa in this study ranging from 121,020 to 130,561 bp. All plastomes exhibited the typical structure of IR-loss clade composed of LSC region (79,916 to 87,193), SSC region (13,383 to 14,187) and only one inverted repeat region (27,604 to 30,487) (Table 2).

Table 2

Chloroplast genome information from sampled IRLC species and the newly assembled O. gaubae.

Species	Size (bp)	LSC (bp)	GC (%) (LSC)	SSC (bp)	GC (%) (SSC)	IR (bp)	GC (%) (IR)	GC (%) Total
Astragalus mongholicus	123,582	80,986	33.4%	13,773	29.9%	28,823	38.1%	34.1%
Caragana microphylla	130,029	85,436	33.3%	14,106	30.4%	30,487	38.8%	34.3%
Carmichaelia australis	122,805	80,588	33.5%	14,074	30.2%	28,143	38.6%	34.3%
Cicer arietinum	125,319	82,583	33%	13,820	29.9%	28,916	38.3%	33.9%
Galega officinalis	125,086	82,915	33.2%	13,347	30.5%	28,824	38.7%	34.2%
Glycyrrhiza glabra	127,943	84,714	33.1%	14,187	30.1%	29,042	39.6%	34.2%
Hedysarum semenovii	123,407	80,288	34.1%	13,679	30.5%	29,440	38.9%	34.9%
Lens culinaris	122,967	81,659	33.7%	13,833	30.2%	27,604	38.7%	34.4%
Lessertia frutescens	122,700	80,698	33.4%	13,750	29.9%	28,252	38.4%	34.2%
Medicago sativa	125,330	83,756	32.9%	13,383	30.2%	28,191	38.6%	34%
Melilotus albus	127,205	84,279	32.7%	13,806	29.8%	29,120	38.1%	33.6%
Meristotropis xanthioides	127,735	84,629	33.1%	14,150	30.1%	28,956	39.6%	34.2%
Onobrychis gaubae	122,873	79,968	33.7%	13,805	30.5%	29,100	38.8%	34.5%
Onobrychis viciifolia	121,932	78,986	33.8%	13,821	30.4%	29,125	38.8%	34.6%
Oxytropis bicolor	122,461	80,170	33.5%	14,017	30%	28,274	38.3%	34.2%
Tibetia liangshanensis	123,372	79,916	33.9%	13,513	30.6%	29,943	38.6%	34.7%
Wisteria floribunda	130,561	87,193	33.2%	14,127	30%	29,628	39.4%	34.4%
LSC: Large Single Copy, SSC: Small Single Copy, IR: Inverted Repeat.

Gene order and gene/intron content in plastomes of all IRLC taxa are highly conserved. The overall GC content of the O. gaubae chloroplast genome sequence was 34.5%, which is consistent with other IRLC species, whose plastomes have GC-contents ranging from 33.6% to 35.1% (Table 2). Different GC content occurs in the LSC (32. 7% - 34.1%), SSC (29.8% - 30.6%) and IR (38.1% - 39.6%) regions (Table 2).

Codon usage bias

The total coding DNA sequences (CDSs) were 81,121 bp in length and encoded 75 genes including 24,765 codons which belonged to 61 codon types. Codon usage was calculated for the protein-coding genes present in the O. gaubae cp genome. Phenylalanine was the most abundant amino acid, whereas Alanine showed the least abundance in this species (Additional File 1: Table S2). Most protein-coding genes employ the standard ATG as the initiator codon. Among the O. gaubae protein-coding genes, three genes used alternative start codons; ACG for psbL and ndhD, and GTG for rps8. Similar codon usage pattern was exhibited in the O. viciifolia species (Additional File 1: Table S3).

The chloroplast genomes of the IRLC were analyzed for their codon usage frequency according to sequences of protein-coding genes and relative synonymous codon usage (RSCU). RSCU is an important indicator to measure codon usage bias in coding regions. This value is the ratio between the actual observed values of the codon and the theoretical expectations. If RSCU=1, codon usage is unbiased; if RSCU>1, specific codon frequency is higher than other synonymous codons, otherwise, the frequency is low^27,28. The total number of codons among protein-coding genes in the IRLC species varies from 20,381 codons in Hedysarum taipeicum (as the smallest number) to 24,765 codons in O. gaubae. The most often used synonymous codon was AUU, encoding isoleucine, and the least used was CGC/CGG, encoding arginine (Additional File 2: Table S4). In the IRLC, the standard AUG codon was usually the start codon for the majority of protein-coding genes and UAA was the most frequent stop codon among three stop codons. Methionine (AUG) and tryptophan (UGG) showed RSCU = 1, indicating no codon bias for these two amino acids. The highest RSCU value was for UUA (~2.04) in leucine and the lowest was GGC (~0.35) in glycine. Leucine preferred six codon types (UUA, UUG, CUU, CUC, CUA, and CUG) and actually showed A or T (U) bias in all synonymous codons (Additional File 2: Table S4). The result of distributions of codon usage in the IRLC species showed that RSCU>1 was recorded for most codons that ended with an A or a U, except for UUG codon, resulting in the bias for A/T bases. As well as, more codons with the RSCU value less than one, ended with base C or G. So, there is high A/U preference in the third codon of the IR-loss clade coding regions, which is a common phenomenon in cp genomes of higher plants²⁹.

Analysis of repeats

Repeat analysis of the O. gaubae plastome identified 50 repeat structures with lengths ranging from 30 bp to 179 bp. These structures included 29 forward repeats with lengths in the range of 30-179 bp, 19 palindromic repeats of 30-81 bp and two reverse repeats with a length of 31 and 37 bp (Additional File 3: Table S5). Among the 50 repeats, 66% are located in the LSC region, 18% in the IR region and 16% in the SSC region. Also, most of the repeats (42%) were found in coding region (accD, psaA, psaB, psbC, psbJ, ycf1, ycf2, ycf4, rps12, trnR-UCU, trnK-UUU), 40% were distributed in the intergenic spacer regions (IGS) and 18% were located in the introns (ndhA, rpl16, rps12, petB, ycf3). The pattern of repeat structures (both in frequency and location) in O. gaubae is similar to that of O. viciifolia (Additional File 3: Table S6). In the majority of the studied IRLC species, the most frequently observed repeats were forward, then palindromic, and the least was the reverse (Fig. 2).

The forward type was the most abundant repeat with length in the range 30-50 bp in all the IRLC species. The longest repeats were also of the forward type, with length of 560 bp were detected in the Hedysarum taipeicum, followed by Vicia sativa of 517 bp and Caragana microphylla of 455 bp, which were much longer than other species studied.

Simple sequence repeats (SSRs), or microsatellites, are a type of tandem repeat sequences which contain 1-6 nucleotide repeat units and have wide distribution throughout the genome^28,30. Accordingly, microsatellites play a crucial role in the genome recombination and rearrangement. These nucleotide motifs show a high level of polymorphism that can be widely used for phylogenetic analysis, population genetics and species authentication^28,31. A total of 83 SSRs were detected in the O. gaubae plastome, which were composed by a length of at least 10 bp. Among them, 47 (56.62%) were mono-repeats, 24 (28.91%) were di-repeats, 6 (7.22%) were tri-repeats, five (6.02%) were tetra-repeats and one were penta-repeats (1.2%). No hexanucleotide SSRs existed in O. gaubae genome (Additional File 3: Table S7). Onobrychis viciifolia with 101 SSRs including 50 mono-repeats (49.5%), 30 (29.7%) di-repeats, nine (8.91%) tri-repeats, 11 (10.89%) tetra-repeats and one penta-repeat (0.99%), exhibited similar SSR distribution pattern in the plastome (Additional File 3: Table S8). The number of SSRs in the IRLC cp genomes (cpSSRs) ranged from 68 (Vicia sativa and Lens culinaris) to 151 (Melilotus albus) across the IRLC species (Fig. 3A). The mononucleotide repeats (P1) were identified at a much higher frequency, which varied from 45 (Tibetia liangshanensis, Glycyrrhiza glabra) to 93 (Melilotus albus) (Fig. 3B).

In the mononucleotide repeats, A/T motifs were the most abundant but no G/C motif was detected in the cp genome. Likewise, the majority of the dinucleotides and trinucleotides were found to be particularly rich in AT sequences. Therefore, the AT richness in the SSRs of the chloroplast genome of O. gaubae is consistent with the results of previous studies^28,32 which have shown that in the cp genome, SSRs generally composed of polythymine (poly T) or polyadenine (poly A) repeats³³.

Sequence divergence analysis

The average nucleotide diversity (Pi) among the protein-coding genes of 23 species of the IRLC was estimated to be 0.05736. Furthermore, comparison of nucleotide diversity in the LSC, SSC and IR regions indicated that the IR region exhibits the highest nucleotide diversity (0.11549) and the SSC region shows the least (0.04132). We detected three hyper-variable regions with Pi values>0.1 among the IRLC species; ycf1 and ycf2 from IR region and clpP from LSC region (Fig. 4).

These genes might be undergoing rapid nucleotide substitution in IR-loss clade at the genus and species levels. Among these, ycf1 encoding a protein of 1800 amino acids has the highest nucleotide diversity (0.18745). The average nucleotide diversity was also investigated between two Onobrychis plastid genome sequences. The average value of Pi between the Onobrychis species was estimated to be 0.05632 (Additional File 4: Fig. S1). High nucleotide variations were observed for the protein-coding regions ycf1, ycf2, clpP, accD and ycf4 and intergenic regions such as trnL(UAA)-trnT(UGU) and trnT(GGU)-trnE(UUC). Sliding window analysis results revealed the same variable regions in the cp genome of the two Onobrychis species.

Moreover, we compared whole chloroplast genome sequences of different taxa of IRLC to analyze gene order and content with mVISTA. We found that, similar to other plant species, the gene coding regions were more conserved than the noncoding regions (Additional File 5: Fig. S2). High nucleotide variations were observed across IRLC for the protein-coding regions ycf1, ycf2 and clpP. Similar results were also obtained from calculation of nucleotide diversity (Pi).

Ka/Ks analysis

In this study, the non-synonymous (Ka) to synonymous (Ks) rate ratio (Ka/Ks) was estimated for 75 protein-coding genes across the 28 IRLC species analyzed (Additional File 6: Table S9). In general, the Ka/Ks values were lower than 0.5 for almost all genes. The ycf4 gene which is involved in regulating the assembly of the photosystem I complex had the highest nonsynonymous rate, 0.165691, while the ycf1 gene with unknown functions had the highest synonymous rate, 0.181067. The Ka/Ks ratio (denoted as ω) is widely used as an estimator of selective pressure for protein-coding genes. An ω > 1 indicates that the gene is affected by positive selection, ω < 1 indicates purifying (negative) selection, and ω close to 1 indicates neutral mutation³⁴. In the present study, the Ka/Ks ratio was calculated to be 0 for psbL gene which encodes one of the subunits of photosystem II. The Ka/Ks ratio indicates purifying selection in 73 protein-coding genes. The highest Ka/Ks ratio which indicates positive selection was observed in accD gene which encodes a subunit of the acetyl-CoA carboxylase (ACCase) enzyme.

Prediction of RNA editing sites

RNA editing as a post-transcriptional modification process, mainly occurs in chloroplasts and mitochondrial genomes. In higher plants, some chloroplast RNA editing sites which provide a way to create transcript and protein diversity are conserved²⁸.

RNA editing sites of O. gaubae plastid genes were predicted using Prep-CP prediction tool (Additional File 7: Table S10). In total, 58 editing sites were present in 19 chloroplast protein-coding genes and all of the editing sites were C-to-U conversions (Additional File 7: Table S10). Among them, nine editing sites, the highest number, were found in the region encoding ndhB gene followed by seven editing sites in petB. There were six editing sites detected each in ndhA and rpoB genes. accD, ndhG and petD had three editing sites, and ndhD and ndhF had two editing sites. Two editing sites were also found in ccsA, matK and rpoC1 genes. The remaining seven genes had only one editing site. The results showed that ndh genes exhibited the most abundant editing sites which were nearly 39.6% of the total editing sites. Furthermore, we predicted 65 RNA editing sites out of 22 plastid genes in chloroplast genomes of O. viciifolia. In this species, the highest number of editing sites belongs to the petB, rpoC1 and ndhB genes with 9, 8 and 7 sites, respectively (Additional File 7: Table S11). In these two Onobrychis species, the amino acid conversion from S to L occurred most frequently, while R to W occurred least.

Phylogenetic analysis

Phylogenetic relationships within the IRLC were reconstructed using the representative taxa (28 species from different tribes) and two species as outgroup based on 75 protein-coding genes of their chloroplast genomes. The total concatenated alignment length from the 75 protein-coding genes was 87,455 bp. The maximum likelihood (ML) analysis resulted in a well-resolved tree and the Bayesian inference yielded a well-resolved topology with high support values (Fig. 5).

The ML and Bayesian trees were largely congruent. The reconstructed phylogeny is in agreement with previous studies^5,14,25,35 indicated that IRLC was monophyletic and consisted of several clades. As shown in the previous studies, Glycyrrhiza + Meristotropis were monophyletic, along with the tribe Wisterieae was sister to the rest of the IRLC^19,36. Then, there are two major clades: clade I and II (Fig. 5). Clade I comprises tribes Caraganeae¹⁷, Hedysareae²⁰ and Coluteae¹⁸ as well as genera Oxytropis and Astragalus. Our results showed a close relationship between O. gaubae and O. viciifolia and Hedysarum species and confirmed O. gaubae phylogenetic position in the tribe Hedysareae. Furthermore, our plastid DNA analyses which are consistent with the previous study³⁵, show that Oxytropis is sister to the tribe Coluteae. In clade II, tribe Cicereae is the basal branch and formed a sister group relationship with the paraphyletic Trifolieae and the monophyletic tribe Fabeae.

General features of the O. gaubae plastid genome

In our study, we determined the first complete chloroplast genome sequence of O. gaubae within O. subgenus Sisyrosema using the Illumina platform and deposited in the NCBI Genbank. Our assembly and annotation results showed that the length of the cp genome is 122,873 bp and its structure is similar to those of other IRLC species. Gene number, order, and type were found to be very similar between O. gaubae and O. viciifolia. In other words, the two Onobrychis plastid genomes were relatively conserved with the IR region more conserved than LSC and SSC regions. In this regard, one of the structural changes detected in the O. viciifolia cp genome is the loss of the atpF intron; whereas, O. gaubae possesses this intron. The atpF gene of O. gaubae is 1261 bp long with one intron of 702 bp, exon 1 of 144 bp and exon 2 of 415 bp. While the atpF gene of O. viciifolia is 558 bp long. The atpF gene has a conserved group II intron which has been found in the most previously sequenced land plant plastomes³⁷. The atpF intron is rarely lost in flowering plants but some intronless chloroplast genomes have been reported, including Manihot (Euphorbiaceae)³⁷, Passiflora (Passifloraceae)³⁸ and several taxa across IRLC (Colutea nepalensis, Lessertia frutescens, Oxytropis bicolor, O. recemosa and Sphaerophysa salsula)³⁵. It has been suggested that recombination between an edited mRNA and the atpF gene may be a possible reason for the lack of intron³⁷. Structural variations such as intron presence/absence can be useful as a molecular marker and provide informative characters at low taxonomic levels in phylogenetic studies²⁵. Another structural change in plastome of O. viciifolia is the inversion of ycf2 gene. Plastome inversions due to the relative rarity and easily determined homology (no homoplasy), are highly valuable and useful in phylogenetic studies⁵. The main cause of inversions is not fully understood, but intramolecular recombination between dispersed short inverted/direct repeats and tRNA genes is an accepted explanation^39,40.

It was previously reported that the plastomes of Papilionoideae, particularly IR-loss clade, are not conserved in their genomic structure in terms of gene order and gene content and exhibit numerous rearrangements and gene/intron losses^5,24,25. In this context, our results showed that the lengths of the IRLC plastid genomes ranged from 121,020 to 130,561 bp. This suggests that the IRLC cp genomes may have undergone different evolutionary processes such as gene/intron loss, insertion/deletion and IR/LSC/SSC expansion/contraction⁴¹. The plastomes among the IRLC taxa were similar in GC content but higher GC contents were usually detected in the IR region compared to the other regions of cp genome, which is mainly due to the presence of rRNA genes (rrn23, rrn16, rrn5, rrn4.5) with high GC content (50%-56.4%) in IRs ^6,32. One of the factors that shape codon usage biases in different organisms is the GC content in codon positions. Codon usage bias indicates the importance of molecular evolutionary phenomena. As mentioned above, codon usage patterns are similar between two Onobrychis species and also across IRLC.

In this study, we found many repeat regions including forward repeats, palindromic repeats and reverse repeats, of which forward types were the most frequent in the IR-loss clade. Furthermore, in the IRLC, repeat sequences involved in genome rearrangement, were mainly distributed in non-coding regions (IGS). Repeat structures induce indels and substitutions resulting in the mutation hotspot in the reconfiguration of genome⁶; therefore, these repeats can provide valuable information for phylogenetic and population studies²⁸. Moreover, we detected SSRs in two Onobrychis species and representative species of the IRLC. In the IR-loss clade, mononucleotide repeats were highly abundant and were mostly composed of AT than G/C repeats. Strong AT bias in SSR loci was also observed in other legumes such as Vigna radiate⁴², Arachis hypogaea⁴³ and Stryphnodendron adstringens³² which, like other plastomes of species, may contribute to the bias in base composition⁶. The results showed that SSR loci of LSC regions appeared more frequently than in SSC or IR regions, which may be hypothesized that this phenomenon is relevant to the lack of one IR region in IR-loss clade. In general, cpSSRs show abundant variation and might provide useful information for detecting intra- and interspecific polymorphisms at the population level^30,33.

Divergent hotspots in the IRLC chloroplast genomes

In this study, the sequence divergence regions that could be used as effective molecular markers were identified based on mVISTA and sliding window analysis. In the IR-loss clade, the divergence was distributed in the LSC and IR regions. Three highly variable regions (clpP, ycf1, ycf2) were observed with the higher Pi values and were located at LSC and IR regions, respectively. The gene ycf1 with the highest nucleotide diversity is more variable than matK and it can be useful for molecular systematics at low taxonomic levels^44,45. Furthermore, variable regions including ycf1, ycf2, clpP, accD and ycf4 (as the protein-coding regions) and trnL(UAA)-trnT(UGU) and trnT(GGU)-trnE(UUC) (as the intergenic regions) could be ideal as molecular markers to distinguish the Onobrychis species. Several studies^14,18,46analyzed the phylogenetic reconstructions of the IRLC species at various taxonomic levels based on different fragments of plastid coding genes such as matK, ndhF and rbcL, the nuclear ribosomal ITS and the combined sequences of these genes/spacers. We could use the highly variable regions acquired from this study to develop the potential phylogenetic markers which can be useful for species authentication and reconstruction of phylogeny within different tribes/genera of IR-loss clade in further studies.

Papilionoideae, in particular the IRLC, displays structural variations which provide informative characters to increase phylogenetic resolution and make the taxon an excellent model for genome evolution studies^5,25. The plastomes of several members of the IRLC have regions with significant variation and rearrangement and accelerated mutation rates, including loss of introns from rps12 and clpP genes²⁴, absence of rps16 gene²⁶ and transfer/loss of rpl22 to the nucleus²⁴. Numerous studies have also shown some other rearrangements in some IRLC taxa, such as loss of accD gene in six species of Trifolium^10,25, loss of rpl23 and rpl33 genes in some species of Lathyrus, Pisum and Vicia³¹ and loss of ycf4 gene in some species of Lathyrus and Pisum^15,16. As revealed in other studies, there are several reasons for occurrence of rearrangements in the plastome, such as the lack of one IR region, variable IR region size and many tandemly repeated sequences⁴⁷. For example, the loss of the rps16 gene was probably due to the presence of a nuclear rps16 copy, which contributed to pseudogenization of the plastid copy⁴⁷. Likewise, the lack or expansion of the accD gene was explained by the presence of tandemly repeated sequences^6,15.

Positive selection analysis

Moreover, we estimated the Ka/Ks for each gene in DnaSP v.6.12. Acceleration of the evolutionary rate was observed only in the accD gene. Some studies have investigated whether selective pressure is acting on a particular protein-coding gene in different genera/tribes of IR-loss clade. For instance, tests for positive selection suggested that Lathyrus, Pisum and Vavilovia, all belonging tribe Fabeae, have undergone adaptive evolution in the ycf4 gene^15,16.

Legumes chloroplast genome, and in particular IRLC, have regions with high mutation rates, including rps16-accD-psaI-ycf4-cemA region. rps16 gene was lost from cpDNA in the common ancestor of the IR-loss clade¹⁵. accD coding region was completely absent in the T. subgenus Trifolium and has nuclear copies in Medicago truncatula and Cicer arietinum²⁵. Three consecutive genes psaI-ycf4-cemA is situated in a local mutation hotspot and has been lost in some species of Lathyrus^15,16.

Plastid RNA editing prediction

RNA editing is one of the post-transcriptional events which converts cytidine (C) to uridine (U) or U to C at specific sites of RNA molecules and modifies the genetic information from the genome in the plastids and mitochondria of land plants. RNA editing serves as a mechanism to correct missense mutations of genes by inserting, deleting and modifying nucleotides in a transcript⁴⁸. In this study, between two species of Onobrychis (O. gaubae and O. viciifolia) and also in the IRLC genera, the most editing sites were observed for ndh genes. In this regard, the highest number of plastid editing sites was found in the ndh group genes in flowering plants⁴⁸. Moreover, the ndh genes encoding for a thylakoid Ndh complex, have been lost or pseudogenized in different species of algae, bryophytes, pteridophytes, gymnosperms, monocots, eudicots, magnoliids, and protists^49-51. The RNA editing is probably important for the NDH protein complex function and may also lead to improved photosynthesis and display positive selection during evolution⁴⁸.

Phylogenetic relationships

With the use of the whole cp genome coding sequence from 28 representative species of the IR-loss clade, a highly consistent topology was recovered by ML and Bayesian analyses (Fig. 5). The monophyly of the IRLC and all its tribes was consistent with all previous studies^5,14,25,35. Wisterieae together with Glycyrrhiza and Meristotropis were the first diverging lineage as sister to the remaining taxa. Tribes Caraganeae and Hedysareae were grouped together. Many previous studies showed that Astragalus was sister to the genus Oxytropis but recent study on the chloroplast phylogenomics of Astragalus reported that Astragalus is a monophyletic clade and Oxytropis is sister to the Coluteoid clade³⁵, which is in agreement with the present study. Cicereae + Trifolieae + Fabeae formed well supported sister groups. The results of the present study suggest that there is no conflict between the phylogeny made by whole cp genome and that inferred by individual gene datasets. Therefore, a phylogenetic reconstruction for IR-loss clade species studied here showed that plastid genome database will be a helpful resource for molecular phylogeny at the higher taxonomic level (generic to tribal rank).

In this study, the complete plastome sequence of O. gaubae (122,873 bp) was determined. The gene contents and gene orientation of O. gaubae plastome are similar to those found in the plastid genome of other IRLC species. Comparison of plastomes across IRLC showed that the coding regions are more conserved than non-coding regions and IR is more conserved than LSC and SSC regions. The present study also analyzed genetic information in the IRLC plastomes including the distribution and location of repeat sequences and SSRs, codon usage, RNA editing prediction, hotspot regions and phylogenomic analysis. Moreover, we identified three hotspot genes (ycf1, ycf2, clpP) which provided sufficient genetic information for species identification and phylogenetic reconstruction of the IRLC species. Seven hypervariable regions including ycf1, ycf2, clpP, accD and ycf4 (as the protein-coding regions) and trnL(UAA)-trnT(UGU) and trnT(GGU)-trnE(UUC) (as the intergenic regions) were also identified between Onobrychis species, which could be used to distinguish species. Finally, the data obtained from this study could provide a useful resource for further research on tribe Hedysareae and also IR-loss clade at the genomic scale.

Chloroplast DNA extraction and sequencing

The young leaves of O. gaubae were collected from the southern slopes of Alborz mountain range in Tehran, Iran. It was identified by Professor S. Kazempour-Osaloo. This species were preserved in the Tarbiat Modares University Herbarium (TMUH) (voucher code: 2016-1). Permission was not necessary for collecting the samples, which has not been included in the list of national key protected plants. The fresh leaves were immediately dried with silica gel for further DNA extraction. Our experimental research, including the collection of plant materials, are complies with institutional, national or international guidelines. Genomic DNA was extracted from dried leaves using a DNeasy Plant Kit (Qiagen) according to the manufacturer’s instructions. DNA quality and quantity were confirmed using 1% agarose gel electrophoresis and the resulting DNA was sequenced using the Illumina HiSeq-2000 platform at Iwate Biotechnology Research Center. The paired-end libraries were constructed according to the manufacturer’s protocol (Illumina Inc., San Diego, CA). In total, 43,189,861 paired-end reads each comprising 100-bp sequence were obtained.

Genome Assembly and Annotation

Using the complete plastid genome of Onobrychis viciifolia (MW007721) as the reference, the paired-end reads of O. gaubae were filtered and assembled in to a complete plastome using Fast-Plast (https://github.com/mrmckain/Fast-Plast)⁵². Furthermore, we compared the chloroplast genome of O. gaubae with the complete chloroplast sequence of other Hedysareae species (Hedysarum and Alhagi species). Gaps in the cpDNA sequences were filled by PCR amplification and Sanger sequencing. The de novo assembled chloroplast genomes were annotated by GeSeq⁵³. We used the online tRNAscan-SE service⁵⁴ to improve the identification of tRNA genes. To detect the number of matched reads and the depth of coverage, raw reads were remapped to the assembled plastomes with Bowtie2⁵⁵ as implemented in Geneious v.9.0.2. The entire chloroplast genome sequences of O. gaubae was deposited in GenBank (Accession Number:?).

Codon usage

Codon usage was determined for all protein-coding genes. The codon usage analysis was performed in the web server Bioinformatics (https://www.bioinformatics.org/sms2/codon_usage.html). Furthermore, the relative synonymous codon usage (RSCU) values were determined with MEGA X⁵⁶, which was used to reveal the characteristics of the variation in synonymous codon usage.

Characterization of repeat sequences

REPuter was used to identify forward repeats, reverse sequences, complementary and palindromic sequences, with a minimal size of 30 bp, hamming distance of 3 and over 90% identity. Simple sequence repeats (SSRs) were detected using the microsatellite identification tool MISA (available online: http://pgrc.ipk-gatersleben.de/misa/misa.html). The minimum numbers of the SSR motifs were 10, 5, 4, 3, 3 and 3 for mono-, di-, tri-, tetra-, penta-, and hexanucleotide repeats, respectively.

Divergent hotspots identification and synonymous (Ks) and non-synonymous (Ka) substitution rate analysis

To assess the nucleotide diversity (Pi) among the plastid genomes of the representative species of the IRLC, the whole chloroplast genome sequences were aligned using MAFFT⁵⁷ on XSEDE v.7.402 in CIPRES Science Gateway⁵⁸. A sliding window analysis was conducted to determine the nucleotide diversity of the chloroplast genome using DnaSP v.6.12 software⁵⁹. The window length was set to 800 bp and the step size was 200 bp. Furthermore, the protein-coding regions of the 28 chloroplast genomes were used to evaluate evolutionary rate variation within the IRLC. Thus, we aligned the 75 protein-coding genes separately using MAFFT and then estimated the synonymous (Ks) and non-synonymous (Ka) substitution rates, as well as their ratio (Ka/Ks) using DnaSP v.6.12 software.

Genome comparison

To investigate divergence in chloroplast genomes, identity across the whole cp genomes was visualized using the mVISTA viewer in the Shuffle-LAGAN mode⁶⁰ among the 19 IRLC accessions using Glycyrrhiza glabra as the reference.

Prediction of potential RNA editing sites

Thirty-five protein-coding genes of O. gaubae were used to predict potential RNA editing sites using the Predictive RNA Editor for Plants (PERP)-Cp web server (http://prep.unl.edu)⁶¹ with a cutoff value of 0.8.

Phylogenetic reconstruction

Seventy-five protein-coding genes were recorded from 28 species within IRLC, as well as from two outgroups (Robinia pseudoacacia L. and Lotus japonicus (Regel) K.Larsen). All genes sequences were obtained from GenBank (Additional File 8: Table S12). The concatenated data were analyzed using maximum likelihood and Bayesian inference methodologies. Prior to maximum likelihood and Bayesian analyses, a general time reversible and gamma distribution (GTR + G) model was selected using the MrModeltest2.2⁶² under the Akaike Information Criteria (AIC)⁶³. Maximum likelihood analyses were performed using the online phylogenetic software W-IQ-TREE⁶⁴ available at http://iqtree.cibiv.univie.ac.at. Nodes supports were calculated via rapid bootstrap analyses with 5000 replicates. Bayesian inference was performed using MrBayes v.3.2 in the CIPRES⁵⁸ with the following settings: Markov chain Monte Carlo simulations for 5,000,000 generations with four incrementally heated chains, starting from random trees and sampling one out of every 1,000 generations. The first 25% of the trees were regarded as burn-ins. The remaining trees were used to construct a 50% majority-rule consensus tree and to estimate posterior probabilities. Posterior probabilities (PP) > 0.95 were considered as significant support for a clade.

SSR: Simple sequence repeat; cp: Chloroplast; IRs: Inverted repeats; LSC: Large single-copy; SSC: Small single-copy; IRLC: Inverted repeat lacking clade; ML: Maximum-likelihood; Ks: Synonymous substitution rates; Ka: Nonsynonymous substitution rates; RSCU: Relative synonymous codon usage; DnaSP: DNA sequence polymorphism; NCBI: National Center for Biotechnology; Pi: Nucleotide diversity/polymorphism; GTR: General time reversible; ITS: Internal transcribed spacer of ribosomal DNA; rRNA: Ribosomal RNA; tRNA: Transfer RNA.

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Availability of Data and materials

Sequences used in this study are available from the National Center for Biotechnology Information (NCBI) (see Additional file 8: Table S12). Annotated sequence of plastome of O. gaubae were submitted to GenBank under ? accession number. Sample of O. gaubae is saved at the Tarbiat Modares University Herbarium, Tehran, Iran.

Competing interests

The authors declare that they have no competing interests.

Funding

This study was supported by Kyoto University, Iwate Biotechnology Research Center and Tarbiat

Modares University. The funders had no role in the design of the study, analysis of data, decision to publish and in manuscript preparation.

Authors’ contributions

M. M. and S. K. O. conceived the idea, designed the study and carried out the plant sampling; M. M., A. O. and M. S. extracted chloroplast DNA for next generation sequencing, A. O. and M. S. assembled the genome, M. M. and A. O. performed the manual genome annotation, M. M. performed the phylogenetic and computational analyses, M. M. wrote the paper. R. T. and S. K. O. edited and reviewed the paper. All authors have read and approved the final manuscript.

Acknowledgements

Not applicable.

Author details

¹Department of Plant Biology, Faculty of Biological Sciences, Tarbiat Modares University, Tehran 14115-154, Iran. ²Graduate School of Agriculture, Kyoto University, Kyoto 617-0001, Japan. ³Iwate Biotechnology Research Center, Kitakami, Iwate 024-0003, Japan.

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No competing interests reported.

AdditionalFile1.docx
Additional file 1: Table S1. Genes with intron in the O. gaubae chloroplast genome, including the exon and intron length. Table S2. Codon usage for O. gaubae chloroplast genome. Table S3. Codon usage for O. viciifolia chloroplast genome.
AdditionalFile2.xlsx
Additional file 2: Table S4. Putative preferred codons in the IRLC plastid genomes. RSCU = relative synonymous codon usage.
AdditionalFile3.docx
Additional file 3: Table S5. Forward, Reverse and Palindromic repeat sequences in the O. gaubae chloroplast genome. Table S6. Forward, Reverse and Palindromic repeat sequences in the O. viciifolia chloroplast genome. Table S7. Distribution of simple sequence repeat (SSR) in the O. gaubae chloroplast genome. Table S8. Distribution of simple sequence repeat (SSR) in the O. viciifolia chloroplast genome.
AdditionalFile4.tiff
Additional file 4: Figure S1. Nucleotide variability (%) values between O. gaubae and O. viciifolia species.
AdditionalFile5.pdf
Additional file 5: Figure S2. Sequence identity plot comparing the IRLC chloroplast genomes with Glycyrrhiza glabra as a reference.
AdditionalFile6.docx
Additional file 6: Table S9. The Ka, Ks and Ka/Ks ratio of IRLC chloroplast genome for individual genes and region.
AdditionalFile7.docx
Additional file 7: Table S10. Prediction of RNA editing sites in chloroplast genes of O. gaubae. Table S11. Prediction of RNA editing sites in chloroplast genes of O. viciifolia.
AdditionalFile8.docx
Additional file 8: Table S12. Accession number and sampled chloroplast genomes obtained from GenBank.

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published 19 Feb, 2022

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Editorial decision: Major revision
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Reviewers agreed at journal
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The Complete Chloroplast Genome of Onobrychis Gaubae (Fabaceae- Papilionoideae): Comparative Analysis with Related IR-lacking Clade Species

Status:

Journal Publication

Version 2

Abstract

Figures

Background

Results

Characteristics of the chloroplast genome of O. gaubae

Codon usage bias

Analysis of repeats

Sequence divergence analysis

Ka/Ks analysis

Prediction of RNA editing sites

Phylogenetic analysis

Discussion

General features of the O. gaubae plastid genome

Divergent hotspots in the IRLC chloroplast genomes

Positive selection analysis

Plastid RNA editing prediction

Phylogenetic relationships

Conclusions

Methods

Chloroplast DNA extraction and sequencing

Genome Assembly and Annotation

Codon usage

Characterization of repeat sequences

Divergent hotspots identification and synonymous (Ks) and non-synonymous (Ka) substitution rate analysis

Genome comparison

Prediction of potential RNA editing sites

Phylogenetic reconstruction

Abbreviations

Declarations

Ethics approval and consent to participate

Consent for publication

Availability of Data and materials

Competing interests

Funding

Authors’ contributions

Acknowledgements

Author details

References

Competing Interests

Supplementary Files

Status:

Journal Publication

Version 2