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The chloroplast genome evolution of Venus slipper (Paphiopedilum): IR expansion, SSC contraction, and highly rearranged SSC regions

BMC Plant Biology volume 21, Article number: 248 (2021) Cite this article

Abstract

Background

Paphiopedilum is the largest genus of slipper orchids. Previous studies showed that the phylogenetic relationships of this genus are not well resolved, and sparse taxon sampling documented inverted repeat (IR) expansion and small single copy (SSC) contraction of the chloroplast genomes of Paphiopedilum.

Results

Here, we sequenced, assembled, and annotated 77 plastomes of Paphiopedilum species (size range of 152,130 – 164,092 bp). The phylogeny based on the plastome resolved the relationships of the genus except for the phylogenetic position of two unstable species. We used phylogenetic and comparative genomic approaches to elucidate the plastome evolution of Paphiopedilum. The plastomes of Paphiopedilum have a conserved genome structure and gene content except in the SSC region. The large single copy/inverted repeat (LSC/IR) boundaries are relatively stable, while the boundaries of the inverted repeat and small single copy region (IR/SSC) varied among species. Corresponding to the IR/SSC boundary shifts, the chloroplast genomes of the genus experienced IR expansion and SSC contraction. The IR region incorporated one to six genes of the SSC region. Unexpectedly, great variation in the size, gene order, and gene content of the SSC regions was found, especially in the subg. Parvisepalum. Furthermore, Paphiopedilum provides evidence for the ongoing degradation of the ndh genes in the photoautotrophic plants. The estimated substitution rates of the protein coding genes show accelerated rates of evolution in clpP, psbH, and psbZ. Genes transferred to the IR region due to the boundary shift also have higher substitution rates.

Conclusions

We found IR expansion and SSC contraction in the chloroplast genomes of Paphiopedilum with dense sampling, and the genus shows variation in the size, gene order, and gene content of the SSC region. This genus provides an ideal system to investigate the dynamics of plastome evolution.

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Background

High throughput sequencing technology has made obtaining chloroplast genome (plastome) sequences more practical [1]. Many studies have used plastome data to address the phylogenetic relationships among land plants, chloroplast genome evolution, and patterns and rates of nucleotide substitutions [2,3,4,5]. These studies indicate that the chloroplast genome has striking variations in genome size, genome structure, and gene substitution rate across the angiosperms.

Though the average chloroplast genome of land plants is 151 kb, the average inverted repeat (IR) region is 25 kb [6, 7]. Published studies show extensive variation in the plastome size and length of the IR region [e.g. 4, 8]. The plastome size of Pelargonium transvaalense is 242,575 bp, with an IR region of 87,724 bp [8]. On the other hand, IR region loss has been detected in all lineages across land plants [9]. High throughout sequencing provides a good opportunity to test the plastome evolution in more groups.

Paphiopedilum (Venus slipper) is the largest genus of slipper orchids, mainly distributed in southeast Asia, with half of these species growing on islands. The sequenced plastomes of Paphiopedilum show the expansion of the IR region, while the SSC regions are greatly reduced in size and gene content [10,11,12,13]. Even typical SSC genes such as ycf1, psaC, and ndhD have been transferred to the IR region in Paphiopedilum armeniacum [10]. However, previous studies of Paphiopedilum plastomes are based on sparse taxon sampling, and the pattern of IR expansion/SSC contraction at the genus level are unknown. Paphiopedilum provides a unique opportunity to study the dynamics of the boundary shift impact on plastid genome structure and sequence evolution.

Additionally, there are still unresolved phylogenetic questions in Paphiopedilum. For instance, previous chloroplast markers cannot solve the deep phylogenetic relationship [14,15,16]. Resolving relationships in Paphiopedilum will promote the study of speciation of the genus. Recent phylogenetic analyses indicated widespread reticulate evolution of the genus and that sect. Cochlopetalum is not monophyletic in the chloroplast gene tree [14, 15]. Besides, there is also systematic uncertainty of four taxa (P. canhii, P. fairrieanum, P. hirsutissimum, and P. rungiyasanum) with unusual morphologies that fall outside the established section/subgenera groupings [15, 17]. For example, P. canhii is a newly described species from Vietnam, proposed to the subgeneric status [18, 19]. Given that Paphiopedilum is a genus with reticulate evolution and clonal propagation, current phylogenetic studies of the genus are rather limited and the effect of these events on the evolution of the chloroplast genome is unknown. Paphiopedilum is a suitable system to study the evolution of the chloroplast genome and to test whether whole plastome sequences can resolve the phylogeny of the genus.

In an effort to answer these unresolved questions, we used a genome skimming method to sequence 77 chloroplast genomes of Paphiopedilum. We used comparative genomics to study the evolution of the Paphiopedilum chloroplast genome and included four Paphiopedilum plastome sequences reported in previous studies [10, 12, 13, 20]. We analysed the sequences of all the shared protein coding genes from 81 Paphiopedilum samples to study patterns of evolutionary rates of the chloroplast genome. The goals of this study are to 1) reconstruct the phylogenetic relationships of the genus, especially the systematic positions of the phylogenetically unstable taxa (P. canhii, P. fairrieanum, P. hirsutissimum, and P. rungiyasanum), and test whether the chloroplast genome could resolve the recently diverged taxa of the genus; 2) characterize the chloroplast genome evolution pattern of Paphiopedilum; and 3) calculate the substitution rate of the coding genes, evaluate the impact of IR/SSC boundary shift, and test whether the genes transferred to the IR region due to boundary shift have decreased substitution rates.

Results

Plastomes of Paphiopedilum

We obtained 66 full plastome sequences. The other 11 plastome sequences had one or two gaps located in regions of high AT content within the three intergeneric regions (trnS-trnG, trnE-trnT, and trnP-psaJ). The obtained plastid genome sequences were deposited in GenBank (accession Nos. MN587749 – MN587825) (Table S1). The mean coverage depth of the sequenced plastomes was over 3000-fold (Table S1). We included four published plastome sequences, namely, P. armeniacum [10], P. dianthum [12], P. malipoense [20], and P. niveum [11], in subsequent analyses, yielding a total of 81 genomes of the genus Paphiopedilum. The downloaded plastome sequence of Phragmipedium longifolium [10] was used as the outgroup.

The genome size of Paphiopedilum ranged from 152,130 bp in P. tigrinum to 164,092 bp in P. emersonii (Table S1), P. emersonii had the largest number of genes (134 genes) and one of the shortest SSC regions (660 bp). The plastid genome of the genus shows typical quadripartite structure, with two identical copies of IR separated by a LSC region and an SSC region (Fig. S1). Compared with the plastome of other angiosperms, Paphiopedilum has numerous expansions of IRs. The length of the IR region was enlarged to 31,743 bp – 37,043 bp, with the length of the IR region of eight samples larger than 35 kb (Table S1), while the length of the SSC region contracted to 524 bp – 5916 bp (Fig. 1, Table S1). In addition, the SSC regions of Paphiopedilum are hotspots for gene transfer, loss, and rearrangement (Figs. 2, 3 and 4). The size variation in the SSC region mainly results from the transfer of typical SSC genes to IR regions and the loss/pseudogenization of ndh genes. Subg. Parvisepalum has a relatively larger SSC region than the other species in the genus (Figs. 1 and 2, Table S1). Pearson’s correlation analysis suggested that the length of IR region and SSC region are strongly correlated (r = – 0.8050, P < 0.001).

Fig. 1
figure 1

The length of plastid genome size, LSC, SSC, and IR regions were plotted on the ML tree of Paphiopedilum. LSC, SSC, and IR regions were scaled differently. The six samples in bold only have trnL-UAG preserved in the SSC region

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Fig. 2
figure 2

The variation in SSC regions of Paphiopedilum. The scaled representations of SSC types are plotted on the ML tree. The five species in bold lost all the functional ndh genes

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Fig. 3
figure 3

Extent of the gene rearrangements of 15 Paphiopedilum chloroplast genomes. Locally collinear blocks of the sequences are colour-coded and connected by lines

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Fig. 4
figure 4

Shift of the LSC/IR boundaries and IR/SSC boundaries of Paphiopedilum

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The gene order was conserved and composed of 127 to 134 genes, including 76 to 81 protein coding genes, 38 to 39 tRNA genes, eight rRNAs, three to eight pseudogenes, and 20 to 25 genes was duplicated in the IR region (Tables S1 and S2). In addition to the duplication of the IR regions, trnG-GCC was duplicated in P. exul and P. aff. exul, while trnQ-UUG was duplicated in P. charlesworthii, P. tigrinum, and P. barbigerum var. lockianum. The two copies of trnG-GCC have one nucleotide variation, and the two copies of trnQ-UUG have eight nucleotide variations. Gene density ranged from 0.78 to 0.84, and P. tigrinum had the shortest plastome size and the highest gene density (Table S1). The GC content of the plastome genome ranged from 34.7% to 36.3%, and the GC content of the protein-coding genes ranged from 29.7% to 46.1%.

In addition, we found 17 genes containing introns, including six tRNA genes (trnA-UGC, trnG-UCC, trnL-UAA, trnI-GAU, trnK-UUU, and trnV-UAC) and 11 protein coding genes (atpF, clpP, ndhB, petB, petD, rpl2, rpl16, rpoC1, rps12, rps16, and ycf3). Eight of the protein coding genes contain one intron, while three of them (clpP, rps12, and ycf3) contain two introns (Table S2).

The LSC/IRb boundary is relatively stable. While the LSC/IRb junction is on rpl22 in most species (76 of 81 samples), the LSC/IRb junction is on rps19 in P. concolor and P. wenshenanse × P. bellatulum, between rpl22 and rps19 in P. rhizomatosum, and between rps19 and trnH-GUG in P. hirsutissimum (Fig. 4). Compared to the LSC/IR boundaries, the IR/SSC boundaries of Paphiopedilum varied among species (Fig. 4). Substantial variation in the SSC/IR boundary was mainly in subg. Parvisepalum. In most other samples (56 of 81 samples), one end of the SSC/IR junction was located in the intergeneric spacer region trnL-ccsA, near trnL-UAG, whereas the other junction of SSC/IR was located on the ccsA gene (Fig. 2).

The contraction of the SSC region resulted in the typical SSC genes being transferred to the IR region. One to six genes from the SSC region were transferred to the IR region. For example, ycf1 was transferred to the IR region in all the sequenced samples, while ΨndhD, psaC, and rps15 were incorporated into the IR region in most species. The gene ccsA expanded in the IR region occasionally, and trnL-UAG was transferred to the IR region in P. delenatii, P. dianthum, and P. parishii (Fig. 2).

The genomic comparison demonstrates that the SSC region of Paphiopedilum differs greatly in gene content, gene order, and gene orientation (Figs. 2 and 4, S2). The SSC regions of most species contain trnL, rpl32, and partial ccsA, while the SSC regions of five species are on the brink of losing, P. appletonianum, P. barbigerum, P. emersonii, P. hirsutissimum, and P. villosum only contain trnL-UAG in this region (Fig. 2). In addition, the genes psaC and ΨnadD were preserved in the SSC region in six samples of subg. Parvisepalum (Fig. 2). In addition, there might be two copies of SSC with different directions in the same species [21]. Wang and Lanfear [22] used long-read sequencing to test the structural heteroplasmy in land plants and found the presence of chloroplast structural heteroplasmy in most land plant individuals, so the direction of the SSC region was not considered. Based on gene content and gene orientation, the SSC regions were classified into twelve types (Fig. S2), and type VIII is the dominant type (56 of 81 samples) (Fig. 2). Type I and type II are identical in gene content but differ in the gene direction of rpl32 and trnL-UAG (Fig. S2). Type IX and type X are also identical in gene content, but in type IX, the two genes run in opposite directions, while in type X, the two genes run in the same direction (Fig. S2). Type XI and type XII both have trnL-UAG, but one nucleotide in type XII has shifted to the IR region. Subg. Parvisepalum has six types, whereas sect. Cochlopetalum has only one type (type VIII) (Fig. 2). When the SSC types are plotted on the phylogenetic tree, the result shows that the SSC types are not lineage-specific and that even the closely related species have distinct SSC types, such as species in subg. Brachypetalum and subg. Parvisepalum (Fig. 2). Surprisingly, the gene content of SSC regions has intraspecies variation. For example, one sample of P. barbigerum contains trnL-UAG, while the other sample contains trnL-UAG and rpl32. The two samples of P. appletonianum also have different SSC types (Fig. 2).

Gene gain and loss in Paphiopedilum samples was also analysed. Some of the gene losses are shared throughout the genus (e.g., some ndh genes), while other gene losses are lineage specific (Table S3). Most of the ndh genes were pseudogenized (ΨndhD, ΨndhJ, and ΨndhK) or lost (ndhA, ndhC, ndhE, ndhF, ndhG, ndhH, and ndhI) from the Paphiopedilum species plastome, except for ndhB. Most of the samples (76 of 81) sequenced in this study retained an intact copy of ndhB. In addition, the complete open reading frame of ndhJ was preserved in 23 samples, including four samples of sect. Cochlopetalum and 19 samples of sect. Paphiopedilum (Table S3). The genes ndhC and ndhK were preserved as pseudogenes in subg. Parvisepalum and sect. Concoloria but lost in the other species (Table S3). In particular, in five species sequenced (P. barbatum, P. dayanum, P. platyphyllum, P. sugiyamanum, and P. tigrinum), all 11 of the ndh genes were lost or pseudogenized (Fig. 2, Table S3).

In addition to the pseudogenes found in the ndh genes, we found that premature termination induced pseudogenization of other protein coding genes (cemA and ycf15). Most of the annotated copies (38 of 48) of cemA are preserved as pseudogenes, while all the annotated copies of ycf15 were retained as pseudogenes (Table S3). The pseudogenization of cemA is mainly due to the slippage of poly structure and the appearance of premature stop codons, while the pseudogenization of ycf15 is due to the appearance of more than one premature stop codon.

The plastomes of Paphiopedilum also show multiple structural rearrangements. We found widespread structural variation in the SSC regions, especially in subg. Parvisepalum, including the inversion and recombination of the SSC genes and the shift of the IR/SSC boundary (Figs. 2, 3 and 4). In particular, we found a 47 kb inversion spanning from petN to clpP in P. fairrieanum, which is absent in other species of Paphiopedilum (Figs. 3, S1).

Phylogenetic analyses

Phylogenetic analyses were performed on the three concatenated datasets. The results of ML and BI analyses based on different matrices are almost identical except for the difference in support values, so we used the result based on the whole plastomes directly. Relationships among the sections are consistent with previous studies (Figs. 1 and 2, Fig. S3) [14, 15]. The phylogeny analyses showed subg. Parvisepalum at the base of the tree, followed by subg. Brachypetalum, and then the five sections in subg. Paphiopedilum. The five sections grouped into two clades, with one clade formed by sect. Cochlopetalum-(sect. Coryopedilum, sect. Pardlopetalum) and the other clade formed by sect. Barbata and sect. Paphiopedilum, with just a few terminal nodes in the tree are still unresolved, especially species in sect. Paphiopedilum (Fig. S3 and S4).

Additionally, the four enigmatic species of Paphiopedilum all nested in subg. Paphiopedilum. P. fairrieanum clustered with the sect. Paphiopedilum-sect. Barbata clade with a high support value (BP = 100, PP = 1.00), and P. canhii clustered with the sect. Cochlopetalum -(sect. Coryopedilum, sect. Pardlopetalum) clade (BP = 94, PP = 1.00); meanwhile, P. rungsuriyanum and P. hirsutissimum formed a clade with a high support value (BP = 91, PP = 1.00), and the relationship with other branches is unresolved (Fig. S3). Removing the four taxa results in a tree with the same topology but with elevated support values for some of the branches (Fig. S4).

Nucleotide substitution rate analyses

Mean synonymous and nonsynonymous divergence was low (dN = 0.0471, dS = 0.1222) with the dS value almost three times that of dN (Fig. 5, Table S4). We detected signals of positive selection in clpP (dN/dS = 1.6561), psbH (dN/dS = 1.0318), and psbZ (dN/dS = 1.5768). Our analyses show that both dN and dS had significantly increased in psbM (dN = 0.3585, dS = 0.7519) (Fig. 5), which is induced by a frameshift mutation in P. niveum (KJ524105). Genes in the SC regions have higher substitution rates than genes in the IR regions. Unexpectedly, genes transferred to the IR region due to boundary shift did not show lower substitution rates (Fig. 5, Table S4).

Fig. 5
figure 5

Synonymous (dN) and nonsynonymous (dS) substitution rates of Paphiopedilum chloroplast genes grouped by complex and function. Gene names blackened are genes in the IR region

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Discussion

The phylogeny of Paphiopedilum

Our phylogeny based on the whole plastome resolves the relationship among sections, which is largely consistent with the most recent phylogenies obtained from partial cytoplasmic DNA markers (Fig. S3) [14, 15]. The phylogeny resolved the monophyly of sect. Cochlopetalum with high bootstrap support (BP = 98, PP = 1.00), while this clade was poorly resolved in previous studies [14, 15]. The four enigmatic species all fall outside the established sections. P. fairrieanum clusters with sect. Paphiopedilum-sect. Barbata clade with a high support value (BP = 100, PP = 1.00), P. canhii clusters with sect. Cochlopetalum-(sect. Coryopedilum, sect. Pardlopetalum) clade with a medium support value (BP = 94, PP = 1.00) (Fig. S3). However, the phylogenetic position of the other two enigmatic taxa (P. hirsutissimum and P. rungiyasanum) clustered together with a high support value (BP = 99, PP = 1.00) and formed parallel relationships with P. fairrieanum, sect. Paphiopedilum, and sect. Barbata (Fig. S3). Notably, the support values of several clades slightly increased after the removal of these species. For example the support value of sect. Cochlopetalum increased to 99 (Fig. S4). The uncertain phylogenetic positions of the enigmatic taxa might be related to the heteroplasmy found in the chloroplast genomes (unpublished data). The short branch lengths in sect. Paphiopedilum suggest their recent radiation, which means species in this section are in their preliminary stage of speciation. On the other hand, the long branch length represents the differentiation from the closely related species. For example, the long branch of P. druryi might be due to its isolated distribution in south India.

The chloroplast genome evolution of Paphiopedilum

The largest (P. emersonii, 164,092 bp) and smallest (P. tigrinum, 152,130 bp) plastome sizes differ by ~ 12 kb, and the gene number ranges from 127 to 134 genes (Table S1). The genome size and gene number variation are mainly due to the IR expansion and the loss of the ndh genes, and the genome size is larger than the average plastome size (151 kb) [6]. Compared with other sequenced chloroplast genomes of photoautotrophic orchid, such as Dendrobium [23] and Holcoglossum [24], the plastomes of Paphiopedilum contain more size variation and structure variation at the genus level (Fig. 1, Table S1). In addition, the GC content of the genus (34.7 − 36.3%) is also more variable and lower at the genus level than in other sequenced genera in Orchidaceae, e.g., Cymbidium (36.8 − 37%) [25], Dendrobium (37.31 − 37.68%) [23], and Holcoglossum (35.3 − 35.5%) [24]. The lower GC content might relate to the extremely lengthy AT-repeat regions in the genus. We found AT repeats more than 100 bp long in some species, especially in the 11 samples with gaps in the LSC regions (Table S1).

The LSC/IRb boundary of Paphiopedilum is relatively stable; LSC/IRb resided on rpl22 in most species, while the IR/SSC boundary of the genus is different from those in other species of orchids and variable at the genus level (Fig. 4). Owing to the instability of the IR/SSC boundaries, the chloroplast genome of Paphiopedilum experienced IR expansion and SSC contraction. The typical land plant IR is 25 kb [6, 7], while the IR region of Paphiopedilum is 32 – 37 kb, and the IRs account for approximately 40 – 45% of the plastomes. The increased length in the IR regions results from the IR/SSC boundary shifts.

A large IR expansion (several kilobases) has also been reported in other angiosperm lineages, including Acacia and Inga [26], Erodium [27], Passiflora [28, 29], and Pelargonium [8, 30]. In Pelargonium, IR expanded towards both the LSC and the SSC regions. However, the IR expansions in Paphiopedilum mainly included former SSC genes. IR expansion into the SSC region has also been reported in other species, e.g., Musa acuminate [31], Pedicularis ishidoyana [32], and Vanilla [11, 13, 33].

The large IRs of the plastomes are hypothesized to contribute to plastome stabilization because their absence often coincides with severe changes in gene order [34]. IRs are thought to stabilize the plastome through homologous recombination-induced repair mechanisms [35]. However, large IRs have no impact on the plastome stability of the genus. With the shifts in the IR/SSC boundaries, the SSC regions in Paphiopedilum have shortened and variable SSC lengths (524 – 5913 bp). The tightening of the SSC regions is associated with the loss of ndh genes and the transfer of former SSC genes to the IR region. For example, the 524 bp SSC region of P. hirsutissimum is smaller than other sequenced plastomes in Orchidaceae. The strong shrinking of SSC regions was also documented in other unlinked photoautotrophic plants, such as Annona cherimola (2966 bp) [36], Asarum (0 – 14 bp) [37], Lamprocapnos spectabilis (1645 bp) [38], P. ishidoyana (27 bp) [32], Pelargonium (c. 6.7 kb) [8], and Vanilla (c. 2 kb) [11, 13, 33]. Interestingly, the other genera that experienced SSC contraction have a relatively stable SSC length at the genus level, e.g., all the sequenced plastomes of Pelargonium shared a 6.7 kb SSC region [8], while the length of the SSC region of Paphiopedilum is more variable. The SSC-contracted plastomes of Paphiopedilum have unique features compared with other tightened SSC lineages, including gene content variation, length variation, and gene rearrangement (Figs. 2, 3 and 4), according to the sequenced chloroplast genomes of other genera of slipper orchids [10, 11, 39], suggesting that SSC contraction occurred after Paphiopedilum split from its sister clade.

In addition to size variation, the SSC region of Paphiopedilum experienced extensive gene rearrangement (Fig. 3). The SSC regions of most species in sect. Concoloria, sect. Cochlopetalum, sect. Coryopedilum, sect. Paphiopedilum, and sect. Barbata retain trnL-UAG, rpl32 and a truncated ccsA fragment (trnL-UAG( +)-rpl32( +)-ccsA-P( +), type VIII, approximately 1.8 kb). In the remaining species, the gene content, gene order, and length of the region are highly variable (524 – 5913 bp) (Figs. 1 and 2, S2; Table S1). In other angiosperm, there are two gene clusters in the SSC region, namely, rpl32( +)-trnL-UAG( +)-ccsA( +) and ndhD( −)-psaC( −)-rps15( −). In other orchids, such as Apostasioideae [40], Cypripedium [9, 39], Cymbidium [25], and Phalaenopsis aphrodite [41], the genes on the two clusters are in opposite orientations. However, the original rpl32( +)-trnL-UAG( +)-ccsA ( +) linkage was only preserved in P. armeniacum [10]. In other species of the genus, the above gene linkages were broken and changed to trnL-UAG( +)-rpl32( +)-ccsA( +) and trnL-UAG( −)-rpl32( −)-ccsA( −) in most cases, which means that there is inversion and recombination in the SSC region. Gene arrangement brought rpl32 and ccsA (which are normally separated in the plastome) next to each other (Figs. 2 and 4). In addition, we found a 47 kb inversion in the LSC region of P. fairieanum (Fig. 3, S1c). A 61 kb inversion in the LSC region was previously reported in Cypripedium formosanum [11]. But the mechanisms for these inversions are unknown.

Complete gene duplications (of rpl32, trnL-UAG, ccsA, ΨndhD, psaC, rps15, and ycf1) were documented due to the expansion of the IR region. Additionally, gene duplications outside of the IR regions was also documented. We found that trnG-GCC was duplicated in the LSC in two species, while trnQ-UUG was duplicated in the LSC in three species, and the two duplication events both occurred in sect. Paphiopedilum. The second copies of trnG-GCC and trnQ-UUG are identical to sequences from other orchids, indicating that the two copies might be horizontally transferred, especially trnQ-UUG. The two copies of the gene have eight nucleotide variations. The duplication of trnQ-UUG has also been documented in other studies [42, 43].

Most of the ndh genes were pseudogenized or lost from the genus, but the functional ndhJ and ndhB were preserved in some lineages (Table S3). Considering that ndhA, ndhE, ndhF, ndhG, ndhH, and ndhI were also lost in P. longifolium [10] whereas they were preserved in C. formosanum [11] and C. japonicum [39], we infer that the loss of these genes occurred in the ancestors of the conduplicate-leaved genera. The pseudo copies of ndhK and ndhC were preserved in most samples of subg. Parvisepalum and subg. Brachypetalum (Table S3) and lost in subg. Paphiopeilum, suggesting that ndhK and ndhC were lost in the ancestor of subg. Paphiopeilum. Considering the retention of ndhJ and ndhB in the genus (Table S3), we infer that the loss/pseudogenization of ndhB and ndhJ probably occurred rather recently because they are repeatedly retained in the terminal taxa.

Furthermore, four of the five species with all 11 ndh genes pseudogenized or lost are distributed on islands, and the other one (P. tigrinum) is distributed on the mainland. The five species are all photoautotrophic plants. In addition, the five species belong to three sections (Fig. 2) and are nested in different clades, suggesting that there are four independent ndh loss/pseudogenization events at the genus level. The above evidence provided clues that the ndh complex is undergoing degradation in this genus, and the degradation of the ndh genes is not lineage specific, suggesting a very recent pseudogenization of some ndh genes. The independent degradation of ndh genes was also found in Cymbidium [44]. In addition, the loss of the full set of plastid ndh genes in other photoautotrophic orchids, such as Erycina pusilla [45], Holcoglossum [24], P. aphrodite [41], and Vanilla planifolia [11], has also been reported in previous studies. Lin et al. [46] even proposed that the loss of ndh complexes probably increased the transition of the life history from photoautotrophic to heterotrophic.

Kim et al. [8] proposed that the ndhF gene strongly correlated with the IR/SSC junction stability, whereas the result of Niu et al.’s [13] study indicated that the ndh genes strongly related to the IR/SSC junction stability. Though the IR/SSC boundary is different from other orchids but relatively stable in most Paphiopedilum species, we infer that ndhF or ndh is not correlated with the IR/SSC junction stability at the genus level. The ndhF gene was lost in all the sequenced species of Paphiopedilum, and the five species that lost all 11 functional ndh genes have stable IR/SSC junctions (Fig. 2). We infer that there are other underlying mechanisms related to the structure variations in the genus.

The substitution rates of the chloroplast genes

The substitution rate of the genus is low (dN = 0.0471, dS = 0.1222) and varied among genes, with the dS value almost three times that of dN (Fig. 5, Table S4). We detected signals of positive selection in clpP (dN/dS = 1.6561), psbH (dN/dS = 1.0318), and psbZ (dN/dS = 1.5768). In several angiosperm lineages, such as Acacia and Inga [26], Passiflora [29], and Silene [47], positive selection of clpP, which is involved in protein metabolism, has been identified. Two other positively selected genes are involved in photosynthesis: psbH was under positive selection in the shade tolerant Oryza [48], and psbZ showed positive selection in Rhododendron pulchrum [49].

Previous studies have revealed that genes in the IR region have lower substitution rates than genes in the SC region [30, 50, 51]. Genes in the IR region also have relatively lower substitution rates compared to SC region genes in orchids [25]. Unexpectedly, ycf1 moved to the IR region but still had a high substitution rate (dN = 0.0821, dS = 0.1140), and its substitution rate is higher than that the other genes in the IR region. Even psaC (dN = 0, dS = 0.0825) and rps15 (dN = 0.0388, dS = 0.1183), which transferred to the IR region, have higher divergence than the other genes in the IR region. Furthermore, ccsA (dN = 0.0932, dS = 0.2543) and rpl32 (dN = 0.1222, dS = 0.3193) were mostly retained in the SSC region and have higher divergence than other genes in the SSC region (Fig. 5). Weng et al. [8] found similar phenomena in Pelargonium, where the expanded IR genes in plastid genomes did not have lower substitution rates. Interestingly, the divergence of photosynthesis-related genes is relatively lower than the divergence of other genes (Fig. 5).

Conclusion

Analysing the chloroplast genome is a key way to study the molecular evolution of orchids. However, previously published orchid studies mainly focused on heterotrophic species and the subfamily Epidendroideae. We used comparative chloroplast genome sequences to reveal the evolutionary history of Paphiopedilum. The genome size of Paphiopedilum is slightly larger than that of other angiosperm, and all the species in Paphiopedilum experienced IR expansion and SSC contraction. The expansion of the IR region ranged from ~ 7 kb to ~ 12 kb, and the IR expansion in Paphiopedilum is associated with the transfer of former SSC genes to the IR region. The comparison of plastomes showed that IR expansion and ndh loss contributed to the size variation in the genus. Because of the variation in the IR/SSC boundaries, the SSC region of Paphiopedilum chloroplast genomes showed massive variation in length and gene content. Paphiopedilum provides the best opportunity to study the dynamics of chloroplast genome evolution and to test the rate of heterogeneity related to the variation in the IR/SSC boundaries. Though genome rearrangement is mainly reported in the mitochondrial genome (e.g. [52]), this study provides evidence of highly active genome rearrangements in the SSC regions of Paphiopedilum with dense sampling. The ndh genes of Paphiopedilum experienced varying degrees of degradation, and five species have lost/pseudogenized all 11 ndh genes. However, the mechanisms underlying the loss/pseudogenization of these genes are unknown. This study sheds light on the tempo and mode of evolutionary changes that occurred in the chloroplast genomes across Paphiopedilum and provides a good example of how to study the chloroplast genome evolution of Orchidaceae.

Methods

Taxon sampling and library construction

Plant materials sequenced in this study were collected from the National Orchid Conservation & Research Center of Shenzhen (NOCC) (Table S1), including four species downloaded from GenBank. A total of 81 samples representing 63 species and covering seven major clades of Paphiopedilum, four natural hybrids and one man-made hybrid were also included. Based on previous phylogenetic studies, P. longifolium was chosen as the outgroup.

Total genomic DNA was extracted from fresh leaves using the CTAB method [53]. Paired-end libraries with 350 bp inserts were constructed for this study, and sequencing was performed at Novogene (Beijing, China) on the Hiseq platform (150 bp). And we obtained 20 G of raw data for each sample.

Sequence assembling and annotation

High-quality filtered paired-end reads were filtered in CLC Genomics Workbench v.10.1.1 (https://www.qiagenbioinformatics.com/) reference to published plastomes, and then de novo assembled using CLC Genomics Workbench v.10.1.1 (with different word sizes and bubble sizes) or NOVOPlasty3.5 (K-mer = 39, rbcL from P. armeniacum was used as the seed sequence) [54]. The assembled contigs were merged in Geneious v.11.1.2 (Biomatters, Inc.) to build draft plastomes. We then mapped reads to draft plastomes in Geneious v.11.1.2 to check the ambiguous regions. Annotation of the plastomes was performed in Geneious v.11.1.2 (Biomatters, Inc.) coupled with manual corrections.

Plastome maps were generated with OrganellarGenomeDRAW [55]. The boundaries of IR and SC regions were defined by REPuter [56] with default settings. We calculated GC content in Geneious v.11.1.2. Then, changes in genome structure were visualized using progressive Mauve [57] with IRa removed. Protein coding regions (CDS), rRNA, tRNA, intergenic spacers (IGS), and intron sequences were extracted and aligned to generate original CDS, rRNA, tRNA, IGS, and intron alignments.

Phylogenetic analysis

For phylogenetic analyses, 68 protein-coding gene sequences (ndhB included), four rRNA gene sequences, 30 tRNA gene sequences, and 87 intergenic spacer regions (Tables S2, S5) were aligned with Muscle v3.8.1551 [58] with default settings and refined manually. The poorly aligned regions of IGS and introns were removed with Gblocks v 0.91b [59] with default settings, except with gaps allowed in up to half of sequences. Then, they were concatenated into different datasets. We obtained three concatenated datasets: 1) protein-coding sequences (CDS) and rDNA; 2) intergenic spacer (IGS), intron, and tRNA; and 3) whole plastomes (CDS + IGS + intron + rRNA + tRNA).

Maximum likelihood (ML) trees were inferred from RAXml 8.2.4 [60] under the GTRGAMMA model with 1000 bootstrap replicates. Bayesian inferences (BI) were performed with MrBayes [61]. PartitionFinder v2.1.1 [62] was used to determine the optimal partitioning schemes with pre-defined partitioning by genes/intergenic-spacers under corrected Akaike Information Criterion (AICc) and greedy search. The partitioning schemes were used in the following analyses. For the Bayesian inference, one cold and three incrementally heated Markov chain Monte Carlo (MCMC) chains were run for 10,000,000 cycles and repeated twice to avoid spurious results. One tree per 1000 generations was sampled, with a burn-in of the first 2500 samples for each run.

Nucleotide substitution rate analyses

We used the CODEML programme in PAML v. 4.9 (model = 0) [63] to calculate the average nonsynonymous substitution rate (dN) and synonymous substitution rate (dS) for 67 protein coding genes by the F3X4 codon model. Gapped regions were excluded for rate estimation (cleandata = 1). The ML tree inferred from whole plastome was used as the input tree.

Correlation analysis

Correlation between IR length and SCC length were calculated in RStudio version 1.4.1106 [64]. The cor.test function was used for assessing p-values with Pearson’s correlation test.

Availability of data and materials

All plastomes generated in this study are deposited in NCBI database (https://www.ncbi.nlm.nih.gov/) (GenBank accession Nos. MN587749 – MN587825, see Table S1), and all the datasets supported the conclusion are available at Dryad Digital Repository. These data will remain private until the related manuscript has been accepted. All other data generated in this manuscript are available from the corresponding author upon reasonable request.

Abbreviations

AIC:

Akaike information criterion

AT:

Adenosine Thymine

BI:

Bayesian inference

BP:

Bootstrap probability

bp:

Base pair

BS:

Branch support

CDS:

Protein-coding sequences

cp:

Chloroplast

CTAB:

Cetyltrimethylammonium bromide

dN:

Non-synonymous

dS:

Synonymous mutation

GC:

Guanine-cytosine

GTR:

General time reversible

IGS:

Intergenic sequences

IR:

Inverted repeat

IRa :

Inverted repeat A

IRb :

Inverted repeat B

IRs:

Inverted repeat regions

kb:

Kilobase

LSC:

Large single copy

ML:

Maximum likelihood

rRNAs:

Ribosomal RNAs

SC:

Single copy

SSC:

Small single copy

tRNAs:

Transfer RNAs

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Acknowledgements

The authors thank Mr Wen-Hui Rao, Mr Jie Huang and Miss Xin-Yi Wu for sample collection.

Funding

This work was supported by grants from National Natural Science Foundation of China (No. U1804117); National Key R&D Program of China (Nos. 2019YFD1000400, 2018YFD1000401, and 2019YFD1001000); and Key scientific research projects of Henan Province (No. 17A180023). The funders had no role in the design of the study, analysis of the data nor in writing the manuscript.

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Authors and Affiliations

  1. College of Plant Protection, Henan Agricultural University, Zhengzhou, 450002, China

    Yan-Yan Guo, Jia-Xing Yang & Ming-Zhu Bai

  2. Key Laboratory of National Forestry and Grassland Administration for Orchid Conservation and Utilization, Shenzhen Key Laboratory for Orchid Conservation and Utilization, The National Orchid Conservation Center of China, The Orchid Conservation and Research Center of Shenzhen, Shenzhen, 518114, China

    Guo-Qiang Zhang

  3. Key Laboratory of National Forestry and Grassland Administration for Orchid Conservation and Utilization At College of Landscape Architecture, Fujian Agriculture and Forestry University, Fuzhou, 350002, China

    Zhong-Jian Liu

  4. College of Forestry, Fujian Colleges and Universities Engineering Research Institute of Conservation and Utilization of Natural Bioresources, Fujian Agriculture and Forestry University, Fuzhou, 350002, China

    Zhong-Jian Liu

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Contributions

YYG, ZJL conceived and designed the study. YYG, JXY, MZB, and GQZ analyzed the data. YYG wrote the paper. All authors have read and approved the final manuscript.

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Supplementary Information

Additional file 1: Figure S1.

Chloroplast genome structure of Paphiopedilum. a) Paphiopedilum charlesworthii, b) P. emersonii, c) P. fairrieanum, and d) P. vietnamense. Figure S2. The twelve SSC types found in Paphiopedilum. Figure S3. Phylogenetic tree (ML) of Paphiopedilum based on whole plastomes. The number above the branches are the bootstrap values ≥ 70 and Bayesian posterior probabilities ≥ 0.90. The branched in bold are the four unstable species. Figure S4. Phylogenetic tree (ML) of Paphiopedilum based on whole plastomes with four unstable species excluded. The number above the branches are the bootstrap values ≥ 70 and Bayesian posterior probabilities ≥ 0.90.

Additional file 2: Table S1.

Details of accessions included in this study. Table S2. List of genes identified in the chloroplast genomes of Paphiopedilum. Table S3. Gene losses and pseudogenes in Paphiopedilum. Table S4. Non-synonymous substitution rate (dN), synonymous substitution rate (dS), and dN/dS for each gene. Table S5. List of intergenic spacer regions used in this study.

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Guo, YY., Yang, JX., Bai, MZ. et al. The chloroplast genome evolution of Venus slipper (Paphiopedilum): IR expansion, SSC contraction, and highly rearranged SSC regions. BMC Plant Biol 21, 248 (2021). https://doi.org/10.1186/s12870-021-03053-y

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