- Research article
- Open Access
Complete plastome sequences of Equisetum arvense and Isoetes flaccida: implications for phylogeny and plastid genome evolution of early land plant lineages
© Karol et al; licensee BioMed Central Ltd. 2010
- Received: 18 February 2010
- Accepted: 23 October 2010
- Published: 23 October 2010
Despite considerable progress in our understanding of land plant phylogeny, several nodes in the green tree of life remain poorly resolved. Furthermore, the bulk of currently available data come from only a subset of major land plant clades. Here we examine early land plant evolution using complete plastome sequences including two previously unexamined and phylogenetically critical lineages. To better understand the evolution of land plants and their plastomes, we examined aligned nucleotide sequences, indels, gene and nucleotide composition, inversions, and gene order at the boundaries of the inverted repeats.
We present the plastome sequences of Equisetum arvense, a horsetail, and of Isoetes flaccida, a heterosporous lycophyte. Phylogenetic analysis of aligned nucleotides from 49 plastome genes from 43 taxa supported monophyly for the following clades: embryophytes (land plants), lycophytes, monilophytes (leptosporangiate ferns + Angiopteris evecta + Psilotum nudum + Equisetum arvense), and seed plants. Resolution among the four monilophyte lineages remained moderate, although nucleotide analyses suggested that P. nudum and E. arvense form a clade sister to A. evecta + leptosporangiate ferns. Results from phylogenetic analyses of nucleotides were consistent with the distribution of plastome gene rearrangements and with analysis of sequence gaps resulting from insertions and deletions (indels). We found one new indel and an inversion of a block of genes that unites the monilophytes.
Monophyly of monilophytes has been disputed on the basis of morphological and fossil evidence. In the context of a broad sampling of land plant data we find several new pieces of evidence for monilophyte monophyly. Results from this study demonstrate resolution among the four monilophytes lineages, albeit with moderate support; we posit a clade consisting of Equisetaceae and Psilotaceae that is sister to the "true ferns," including Marattiaceae.
- Land Plant
- Plastid Genome
- Equisetum Arvense
- Early Land Plant
- Leptosporangiate Fern
Patterns and processes of organic evolution are reflected in the structure and sequences of organisms' genomes. Although we are only starting to accumulate sufficient data to compare nuclear genomes of plants, more data are available for the smaller plastid genomes (plastomes). Comparative work on plastomes began in the early 1980's using restriction site mapping and hybridization with heterologous probes to generate phylogenetically informative data within small clades, generally below the family level [1, 2] as well as using comparative mapping to examine differences among more distantly related groups [3–6]. By the 1990's the emphasis shifted to nucleotide sequences from targeted regions (genes) with the result that studies with broad genomic sampling were few. However, a reduction in sequencing cost and concomitant development of analytical tools over the past 10 years has resulted in resurgence of comparative plastid genomics.
The plastome is highly conserved in overall structure and this provides the basis for comparative studies . However, there is still sufficient variation to identify rare genomic events that mark critical branches in land plant evolution, thereby elucidating early evolutionary modifications to the plastid genome [e.g., [5, 7]]. Plastomes of Marchantia polymorpha  and Nicotiana tabacum  were the first to be sequenced and within 19 years there were 45 complete plastome sequences for green plants (Viridiplantae) in GenBank . Acceleration of efforts yielded 146 plastome sequences by the end of 2009, with many others in progress.
Complete genome sequences offer several advantages over restriction site maps and nucleotide sequences of targeted regions. In addition to substantially increasing the number of gene sequences for comparison, plastome sequences provide vast structural and evolutionary information that includes gene order, genome rearrangements, patterns of base pair composition, codon usage, mechanisms of gene duplication and gene loss (e.g., pseudogenization), patterns of nucleotide insertion and deletion (indels), and the occurrence of noncoding regions (such as plastome microsatellites and introns). We now have sufficient sampling from most major land plant clades to begin comparative analyses of their plastomes in earnest.
Despite considerable progress over the last 15 years in our understanding of green plant phylogeny, several nodes remain poorly resolved. One example is the monilophytes, which include five major lineages: leptosporangiate ferns, horsetails, marattioid ferns, ophioglossoid ferns and psilophytes. The monilophytes seem to be well-supported as a group, and are generally accepted as sister to seed plants [11–14]. But relationships among the major monilophyte lineages remain unclear. Here we present the complete plastome for the horsetail Equisetum arvense L., representing one of the last major monilophyte lineages to have a representative complete plastome available. For further resolution of land plant relationships and plastome evolution we also sequenced the plastome of Isoetes flaccida Shuttlw. ex A. Braun, the last of the three major lycophyte lineages to be sampled. We use data for 49 genes from 43 green plant taxa to infer phylogenetic relationships of land plants and compile information on the distribution of gene translocations, genomic inversions, gene content and indels to augment the phylogenetic signal from gene variation. We also examine codon usage and base composition. Patterns of plastome architecture are compared across early land plant lineages that diverged 400 to 500 million years ago.
Our analyses included representatives of all major land plant lineages as well as sampling of charophycean green algae for appropriate phylogenetic context. We do not provide detailed comparative plastome analyses for charophycean algae and seed plants because these are presented elsewhere: charophycean green algae [15, 16], gymnosperms [17–20] and angiosperms [21–23].
Plastome structures and composition
List of plastomes analyzed in this study.
Plastome size (bp), G/C (%), ENc
154478, 40.3, 47.90
Sato et al. 1999 
158484, 40.5, 47.96
Ravi et al. 2007 
160928, 41.0, 48.64
Jansen et al. 2006 
155943, 41.0, 48.79
Shinozaki et al. 1986 
151104, 40.8, 48.56
Timme et al. 2007 
150725, 40.7, 48.25
Schmitz-Linneweber et al. 2001 
155129, 41.4, 49.68
Raubeson et al. 2007 
136584, 41.2, 49.76
Saski et al. 2007 
165955, 40.1, 46.69
Mardanov et al. 2008 
148964, 40.4, 47.83
Chang et al. 2006 
152609, 40.5, 47.73
Hansen et al. 2007 
153821, 41.7, 49.76
Goremykin et al. 2005 
160604, 41.6, 49.82
Cai et al. 2006 
159886, 41.8, 49.67
Cai et al. 2006 
148553, 42.1, 50.37
Hansen et al. 2007 
157772, 41.6, 49.65
Hansen et al. 2007 
159930, 42.2, 50.68
Goremykin et al. 2004 
162686, 41.7, 49.66
Goremykin et al. 2003 
119707, 41.4, 48.98
Wakasugi et al. 1994 
109518, 37.5, 44.35
Wu et al. 2009 
114914, 40.2, 53.80
Wu et al. 2009 
119726, 38.5, 45.79
Wu et al. 2009 
163403, 41.7, 48.58
Wu et al. 2007 
N. A., 41.8, 48.87
Leebens-Mack et al. 2005 
150568, 44.2, 55.12
Wolf et al. 2003 
156661, 43.6, 52.99
Gao et al. 2009 
153901, 39.2, 44.32
Roper et al. 2007 
138829, 39.3, 45.80
GenBank direct submission
133309, 37.2, 41.94
144170, 54.3, 58.30
Tsuji et al. 2007 
143780, 51.2, 57.73
GenBank direct submission
145303, 41.9, 48.84
154373, 40.3, 46.66
Wolf et al. 2005 
161162, 38.2, 42.21
Kugita et al. 2003 
122630, 34.5, 35.46
Oliver et al. (in press) 
122890, 35.1, 36.12
Sugiura et al. 2003 
121024, 34.0, 35.25
Ohyama et al. 1986 
157089, 37.1, 38.28
Turmel et al. 2005 
165372, 39.2, 42.45
Turmel et al. 2005 
131183, 35.0, 36.95
Turmel et al. 2002 
184933, 35.7, 39.57
Turmel et al. 2006 
152254, 39.1, 43.85
Lemieux et al. 2007 
118360, 34.9, 34.83
Lemieux et al. 2000 
Gene content of Isoetes and Equisetum
List of plastid genes and pseudogenes annotated for Equisetum arvense and Isoetes flaccid a.
trnS-CGA in Ea
petB* b, e
petD* b, e
ndhD b, i
ndhJ i, e
rpl14b, i, e
rps2 ψ (If)
rps7x2 in If
rps12 x2 tr
rps16 ψ in If
infA ψ (If)
accD ψ (If), ss
tufA ψ in If
ycf66* ψ (Ea)
Duplicate gene fragments
ndhB ψ in If
ycf2 IR in If
ndhF IR in Ea
In our newly described plastomes, clpP contains only one intron in Equisetum arvense and two introns in Isoetes flaccida. The distribution of introns in clpP is variable across plastomes sampled to date: all bryophytes have two introns, lycophytes have either two introns (Huperzia lucidula and I. flaccida) or one intron (Selaginella moellendorffii and S. uncinata), and all monilophytes except E. arvense have two introns. Among gymnosperms, Cycas taitungensis has two introns and other gymnosperms have either lost both introns (Pinus thunbergii, Ephedra equisetina and Welwitschia mirabilis) or have lost clpP entirely (Gnetum parvifolium). Most flowering plant clpP genes have two introns, except the monocot Agrostis calamus. In the charophycean algae, Chara vulgaris and Chaetosphaeridium globosum each have one clpP intron, whereas other charophycean green algae have no introns in clpP. It appears that the second clpP intron is unique to land plants, although there have been several lineage-specific losses (e.g., Selaginella spp., E. arvense, most gymnosperms, and at least one lineage of monocots).
tufA in land plant plastomes
The functional tufA gene is encoded in the plastid genome of most green algae but has been transferred to the nuclear genome in land plants and certain charophycean green algae [24–26]. A tufA-like fragment was identified in Anthoceros formosae, cycads and Gingko biloba plastomes situated between psbE and petL . We found a 1,364-bp region between psbE and petL in Isoetes flaccida that shares 41% nucleotide similarity with the plastid encoded tufA gene found in Chara vulgaris. The I. flaccida tufA homologue lacks the expected start and stop codons and contains 29 internal stop codons within the reading frame. Furthermore, when this region was aligned with the C. vulgaris homologue, 38 indels were identified (Additional File 1). Taken together, the I. flaccida tufA-like region is most likely a pseudogene. These relictual fragments of a once functional plastid gene now identified in four distantly related land plant lineages (i.e., hornworts, lycophytes and two gymnosperms) offer an interesting opportunity to study patterns of plastome gene loss as well as the process of gene decay.
Putative RNA editing
Reliable inferences on RNA editing require comparisons of genomic sequence to that of mature mRNA or cDNA. However, a fraction of editing sites are in start or stop codons, or result in premature stop codons, thus providing a hint of RNA editing from genomic sequences. We found such evidence in two genes in Equisetum arvense and 28 genes in Isoetes flaccida (Table 2). The former had an undetermined start codon for rps11 and undetermined start and stop codons for accD. In I. flaccida we found 21 loci with undetermined start codons, two with undetermined stop codons, and seven internal stop codons. Based on the sequence of trnR-CCG, we also infer that the second position of the anticodon is RNA edited (U- >C).
The region between chlN and rps15 in plastomes presents an interesting challenge. In certain taxa this region contains two putative protein-coding genes, both read from the same DNA strand, but in different reading frames. Other taxa, including Equisetum arvense and Isoetes flaccida, contain a single large open reading frame (orf) commonly annotated as ycf1. Recent interpretation of the interruption of ycf1 has been either to recognize the entire region as a ycf1 pseudogene [e.g., ], or to recognize the larger of the two orfs as ycf1 while leaving the smaller unannotated [e.g., ]. The function of these orfs (or orf) is unknown and there are at least two processes that explain the differences in putative orfs in this region of the plastome. One possibility is that the large orf could yield two separate protein products through a post-transcription edit that adds a stop codon. Another possibility is that the two-orf pattern is a result of a frame shift that can be read through during translation, yielding a single protein product [e.g., ]. However, transcripts of this region have been determined for Anthoceros formosae with two-orfs and Adiantum capillus-veneris with a single large orf [30, 31]. These transcript data were consistent with the original DNA annotation. That is, the reading frame was not 'corrected' in Anthoceros formosae and an internal stop codon was not detected in the larger orf in Adiantum capillus-veneris. Therefore at both the DNA and the RNA transcript levels there appears to be two alternative patterns.
Previous reports of plastomes representing exemplar bryophytes (liverwort Marchantia polymorpha, mosses Physcomitrella patens and Syntrichia ruralis, hornwort Anthoceros formosae) have shown that plastome content and gene order are highly conserved [8, 30, 32, 33]. This is true even in Aneura mirabilis, the only non-photosynthetic bryophyte . One exception was reported in P. patens, involving a large inversion (~71 kb) encompassing nearly the entire LSC relative to a sample of four other mosses . Goffinet et al. surveyed the phylogenetic extent of the P. patens inversion and found it was restricted to the moss order Funariales. Oliver et al. found that the moss S. ruralis does not have this large inversion but instead has the ancestral land plant plastome gene order.
The plastomes of Isoetes flaccida and Huperzia lucidula (lycophytes) share similar gene order with bryophytes. However, we identified two unique structural changes in I. flaccida (Fig. 2). First, ycf2 in I. flaccida, which normally resides in the LSC in most land plant plastomes, has been translocated to the SSC with the 5' end now incorporated into the IR. Second, the chlL/chlN gene cluster has been inverted in I. flaccida and now resides adjacent to ycf2 rather than ycf1 as in H. lucidula. Neither the ycf2 translocation nor the chlL/chlN inversion occurs in either of the Selaginella plastomes.
Both Selaginella plastomes are considerably different in gene order from typical land plant plastomes (Fig. 2) [, Banks et al. personal communications]. A large region (~14-kb) has been translocated from the LSC to the IR/SSC in both plastomes, although the genes included in this translocation differ slightly. In both Selaginella plastomes, rps4 marks one endpoint of this translocated segment and this gene now resides in the IR. The other endpoint resides in the SSC and is marked by psbD in S. moellendorffii and by three additional genes (trnE-UUC, trnY-GUA and trnD-GUC) in S. uncinata. These tRNA genes remain in the LSC adjacent to ycf2 in S. moellendorffii, as is the case in most land plant plastomes. Several unique features were identified in the plastome of S. uncinata including a ~20-kb LSC inversion (psbI to rpoB-trnC-GAC), duplication of psbK and trnQ-UUG, and translocation of petN from the LSC to the SSC .
Raubeson and Jansen  identified a ~30-kb LSC inversion marking the euphyllophytes as a monophyletic group to the exclusion of the lycophytes. This inversion is confirmed in all monilophyte (including Equisetum arvense, Fig. 2) and seed plant plastomes sequenced and does not occur thus far in bryophyte or lycophyte plastomes (including Isoetes flaccida, Fig. 2).
The inverted repeat boundaries
Phylogenetic relationships among the major bryophyte lineages have been controversial with nearly every possible topology being reported, often with strong support. However, recent convergence in results supports hornworts as sister to vascular plants (as seen here) and liverworts sister to all other land plants [[7, 42] and references therein]. Monophyly of the bryophytes has also been proposed using plastome gene data. However this relationship was only found when inferred amino acid sequences or protein plus DNA sequences corrected for A-T bias were analyzed [[15, 43, 44], Additional file 3]. A sister relationship between mosses and liverworts was proposed previously based on morphological and molecular data. Renzaglia and Garbary  concluded that characters related to sperm cell development were compelling evidence for the monophyly of mosses plus liverworts, a clade they referred to as Setaphytes. In a recent analysis that included seven genes from all three DNA compartments (plastid, mitochondria and nuclear) for 192 land plant and green algal taxa, the liverworts were found with strong support as sister to all remaining land plants .
In the nucleotide analyses presented here, regardless of taxon set or analytical method, monophyly of land plants was confirmed, as was the sister relationship between the hornwort Anthoceros formosae and tracheophytes (e.g., lycophytes, monilophytes, gymnosperms and angiosperms; Fig. 5). More problematic is the position of liverworts among bryophytes as our results show two taxon-dependent placements of the liverwort Marchantia polymorpha. In ML and BI analyses that included all taxa or excluded Selaginella spp., with or without gnetophytes, M. polymorpha was sister to the mosses Physcomitrella patens and Syntrichia ruralis (Fig. 5A, 5B). The alternative topology with liverworts sister to the remaining land plants was supported in the present study only when the gnetophytes were excluded (Fig. 5C). ML bootstrap proportions were generally weak for this part of the tree regardless of taxon exclusion and BI posterior probabilities were low with the analysis that included all taxa and the analysis that only excluded the gnetophytes. Increased taxon sampling is critical in resolving the pattern of these ancient divergences.
The lycophytes (Huperzia lucidula, Isoetes flaccida, with or without Selaginella spp.) and monilophytes (Adiantum capillus-veneris, Alsophila spinulosa, Angiopteris evecta, Equisetum arvense and Psilotum nudum) were each strongly supported monophyletic groups (Fig. 5). However, the relationships of these two clades to each other and to seed plants varied across our analyses. The monilophytes were sister to the seed plants only when Selaginella spp. were excluded, with or without gnetophytes (Fig. 5B). Alternatively, the lycophytes were supported (weakly in most analyses) as sister to seed plants when Selaginella spp. were included (Fig. 5A, 5C). Strong support was recovered for the monilophyte + seed plant relationship by Qiu et al. and this relationship is further supported by the large plastid LSC inversion identified in monilophytes and seed plants and not found in lycophytes (Fig. 2 discussed above) [5, 39]. Within the lycophyte clade, the homosporous H. lucidula was consistently sister to the heterosporous taxa I. flaccida and Selaginella spp. (or sister to just I. flaccida in analyses where Selaginella spp. were excluded).
Strongly supported resolution among the major monilophyte lineages has been elusive, and it was not clear if lack of support was a function of insufficient phylogenetic signal, rapid radiation that is indistinguishable from a true polytomy, or ancient extinctions resulting in long-branch taxa that make phylogenetic reconstruction challenging. Our analyses that used 43,491 nucleotide characters (and several subpartitions of these) revealed a moderately supported sister relationship between Psilotum nudum and Equisetum arvense. Although these groups are usually not found as sister lineages, this topology is reasonable given the lack of sampling of sister taxa to both genera. More typically, P. nudum is found in a lineage that is sister to ophioglossoid ferns [12, 13, 46], a lineage not sampled in this study, and these two lineages are found sister to the remaining monilophytes (marattioid ferns, horsetails and leptosporangiate ferns). However, a sister relationship between Psilotaceae and Equisetaceae was recently reported in a phylogenetic analysis of morphological characters across vascular plants where five unambiguous character state changes for the two groups were identified . These were primarily characters associated with leaves and organization of the shoot system. Schneider et al. did not find the same relationship from an analysis of the same taxa using nucleotide sequences, and they cautioned that the morphological evidence for this sister relationship could well be a function of homoplasy and structural simplification reverting to a pleisiomorphic state, thus causing "concern for a possible misleading bias." Failure of so many studies to resolve relationships among the main lineages of monilophytes is consistent with a rapid radiation (i.e., very short internal branches) rather than lack of phylogenetic signal. Additional data from Tmesipteris (a close relative of Psilotum), ophioglossoid ferns, and additional Equisetum species may help to resolve these nodes.
Some of the deep branching events in the land plant tree are still poorly resolved even with full plastome sequence data now available from most of the major clades. It is likely this is because the intervals of shared history among these major clades were relatively short as compared to their subsequent separate histories, giving rise to extreme asymmetry in branch lengths which causes known problems with inferring the correct tree - particularly using data with a small number of character states such as nucleic acid sequence data . Rare genome structural changes such as gene-order rearrangements have the potential for resolving such short internal branches . Indeed a phylogenetic analysis of plastome gene-order data found liverworts sister to all other land plants and monilophytes sister to the seed plants . However, in that study the lycophytes were not resolved as a monophyletic group as they are here. This topological convergence suggests that a future combined analysis with nucleotide, structural and morphological data may be able to resolve some of the short internal branches.
Many genes contained three base-pair indels with apparent phylogenetic signal. We examined all aligned gene sequences for potential indels across the entire taxon sample, scoring characters as 1 for presence or 0 for absence of a sequence stretch. We scored 152 indels (see Additional files 4 and 5); 40 support the phylogenetic tree (synapomorphic), 99 are unique to a terminal taxon (autapomorphic) and 13 contradict the tree (homoplastic). The synapomorphic and autapomorphic indels are presented in Fig. 5A; the 13 homoplastic indels are not mapped. Indels alone, analyzed by MP, did not provide a well-resolved tree (not shown). However, several nodes were supported by uncontradicted indels; land plants (2 indels), lycophytes (2 indels), Selaginella spp. + Isoetes flaccida (1 indel), Selaginella spp. (12 indels), Angiopteris evecta + Adiantum capillus-veneris + Alsophila spinulosa (1 indel), seed plants (1 indel), gnetophytes (11 indels) and angiosperms (1 indel). Angiosperm-specific indels are shown in Additional file 2. Interpretation of indels is dependent on taxon sampling. When subsets of the data were examined more closely we could score additional indels that were difficult to interpret at the broader level. Several such indels were identified; focusing on the monilophytes we found two indels (in atpA and rps4) uniting the monilophytes; the rps4 indel has been reported previously . Five indels in psbT, petG, atpA, rpl16 and rpoB uniting Adiantum capillus-veneris + Alsophila spinulosa, and one in rpl16 uniting Equisetum arvense and Psilotum nudum were also identified (Fig. 5A).
Nucleotide composition and codon usage
Codon usage bias is often correlated with genomic nucleotide composition bias but may also be correlated with gene expression, selection and drift [51, 52]. The overall G/C content of the genes used in our phylogenetic analyses ranged from 34% in Marchantia polymorpha to 54% in Selaginella uncinata. Both S. uncinata and S. moellendorffii were remarkable for their high (greater than 50%) G/C content. Among land plants included in this study, only M. polymorpha and the mosses contain less than 35% G/C. The nucleotide composition bias found in many plastomes may result in codon usage biases as well.
The effective number of codons (ENc) is often used as a measure of the amount of codon bias within a gene or genome  with values ranging from 20 (very biased) to 61 (very unbiased). Most vascular plants in our study had a value between 46 and 50 (Table 1). Less bias was detected in Adiantum capillus-veneris (ENc = 55.12), Alsophila spinulosa (52.99), Selaginella uncinata (58.30) and S. moellendorffii (57.73). More bias was found in bryophytes and charophycean algae, notably Marchantia polymorpha (35.25), Physcomitrella patens (36.12) and Syntrichia ruralis (35.46). These differences may have ramifications for the estimation of phylogenetic relationships among these groups.
This study illustrates the advantage of whole plastome sequences for studies of plastome architecture and land plant evolution. Nucleotide sequence data from targeted regions have contributed greatly to our understanding of plant phylogeny. However short sequence data can have limitations because they can miss important aspects of genome structure, and nucleotide substitution rates are not always applicable to solving specific evolutionary questions. Restriction site mapping also is limiting because it usually requires a series of cross comparisons every time a taxon is added. Complete plastome sequences provide data that can be used for nucleotide-based analyses, but also can add to a growing database of structural genomic data, including inversions, gene content, and indels. New taxa can be added relatively easily without additional experimental comparisons. Also, changes that might first appear to characterize a large clade might turn out to be more restricted once additional taxa are sampled. For example, some of the unique structural attributes first identified in Selaginella uncinata plastome  appear to be unique (so far), whereas others are shared with a second plastome sequence of S. moellendorffii (Banks et al. personal communications). Another important aspect of this type of data is that changes that first appear autapomorphic in small datasets might later emerge as key synapomorphies as additional taxa are sampled. This is illustrated by the series of large inversions in the IR first identified in Adiantum capillus-veneris [36, 37] but later shown to be shared with the distantly related tree fern Alsophila spinulosa , and by inference shared among more than 90% of leptosporangiate fern species.
Most of the currently available plastome sequences are from seed plants, with very few available from the presumed sister clade, the monilophytes. Here we have added an additional monilophyte plastome sequence as well as a lycophyte plastome sequence that represent two critical lineages that were not previously sampled (i.e., Equisetaceae and Isoetaceae). We examine multiple aspects of these plastomes to reveal patterns of evolution across land plants. We demonstrate that comparative plastome analysis can provide valuable information about evolutionary processes at the nucleotide, gene and genome scale in early land plants. These data are phylogenetically informative at many levels. Gene rearrangement and indels are informative at deep nodes. Plastome sequences also provide valuable nucleotide data that can be used in studies of ancient and more recent divergences.
Now that plastome sequence data are available for most major lineages of early land plants additional sampling can focus on resolution of poorly supported nodes by increased taxon sampling within these lineages. Although we have one or two representatives of each major group, these often represent lineages containing hundreds or thousands of species. Additional sampling within species-rich lineages may resolve some areas of phylogenetic conflict. As DNA sequencing costs continue to decrease it will become possible to improve sampling and gain further insight into early land plant evolution as well as the patterns and processes that shape the evolution of plastomes.
We compared and analyzed complete plastid genomes of 43 taxa, including 37 land plants and six charophycean algae (Table 1). Two of the land plant plastomes are new to this study (herbarium vouchers at WTU): Equisetum arvense and Isoetes flaccida. For E. arvense and I. flaccida, tissue preparation, plastid isolation, DNA extraction, cloning, sequencing, and assembly followed Wolf et al.. Annotation was carried out with DOGMA  and tRNAscan .
Gene maps for representative plastomes were drawn with OGDraw and compared for gene content, gene order and IR boundaries . Codon usage and nucleotide frequencies were determined using CodonW .
Nucleotide sequences for the 49 protein-coding genes found in all study taxa were extracted from DOGMA-annotated plastomes or from those found in GenBank (Table 1). The hornwort Anthoceros formosae and the fern Adiantum capillus-veneris both have extensive RNA editing [30, 36, 37]. Therefore, we used the cDNA sequences rather than genomic sequences for phylogenetic analysis. Each gene was aligned with MacClade v4  and the resulting individual gene alignments were assembled into a single data matrix. A Nexus block was written that identified all unalignable DNA regions, overlapping gene regions, and inferred RNA edited stop codons for exclusion in subsequent phylogenetic analyses. Indels were scored in a separate data matrix as binary characters.
The resulting 49-gene alignment including all codon positions (see Additional file 5) was partitioned into four data sets, 1) one that included all 43 taxa and 30,018 characters, 2) one that excluded both Selaginella species (41 taxa and 29,961 characters), 3) one that excluded the gnetophytes Ephedra equisetina, Gnetum parvifolium and Welwitschia mirabilis (40 taxa and 29,871 characters), and 4) one that excluded both Selaginella species and all gnetophytes (38 taxa and 29,814 characters). Altering the taxon set changes the number of characters because of lineage specific indels. Nucleotide alignments were translated into amino acids using MacClade assuming the universal code . Ambiguous amino acids resulting from polymorphic nucleotides were treated as missing.
Maximum Likelihood (ML) and Bayesian Inference (BI) analyses were performed on each of the four nucleotide data sets using PAUP* v4.0b10  and MrBayes v3.1.2 [60–62], respectively. MrModeltest v2  was used to determine the best fitting model of DNA substitution. With the best fitting model (GTR+I+Γ with four rate catagories) an iterative procedure described in Swofford et al. was used to converge on the best fitting model parameters. The resulting model parameters were then fixed for ML analyses and a heuristic search was performed with random taxon addition using TBR branch swapping. Two hundred bootstrap replicates  were performed for each data set, each replicate with a single random taxon addition and NNI branch swapping. For BI analyses (also using GTR+I+Γ) two runs each with four markov chains running five million generations were performed. The heated markov chain was sampled every 100th generation. The -ln likelihood scores were plotted against generation number and all samples collected prior to the markov chain reaching stationarity were discarded as burn-in. The remaining samples were summarized using the sumt command in MrBayes. The insertion/deletion matrix was analyzed using the maximum parsimony (MP) criterion in PAUP* as unordered characters with 10 random taxon additions and 200 bootstrap replicates. BI analyses of amino acid data sets were executed in a fashion similar to the nucleotide analyses. We used a fixed model, cpREV+I+Γ, estimated specifically for phylogenetic estimation of plastid-encoded proteins . ML bootstrap analyses of amino acid data were performed in RAxML also using cpREV+I+Γ .
We thank Jo Ann Banks, Robert Jansen, James Leebens-Mack and Linda Raubeson for access to unpublished data. We also thank Andrew G. Murdock and Melvin J. Oliver for invaluable assistance in various aspects of this project and anonymous reviewers for helpful suggestions to improve the manuscript. This research was supported in part by the U.S. National Science Foundation grant DEB-0228660 to RGO, DEB-0228432 to PGW, DEB-0228679 to KSR, DEB-0228729 to BDM and JLB and by the U.S. National Institutes of Health Interdisciplinary Training in Genomic Sciences Grant T32-HG00035 to KGK. This work was also partly conducted by the U.S. Department of Energy Joint Genome Institute supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
- Olmstead RG, Palmer JD: Chloroplast DNA systematics--a review of methods and data analysis. Am J Bot. 1994, 81: 1205-1224. 10.2307/2445483.View ArticleGoogle Scholar
- Palmer JD, Zamir D: Chloroplast DNA evolution and phylogenetic relationships in Lycopersicon. Proc Natl Acad Sci USA. 1982, 79: 5006-5010. 10.1073/pnas.79.16.5006.PubMed CentralView ArticlePubMedGoogle Scholar
- Downie SR, Palmer JD: The use of chloroplast DNA rearrangements in reconstructing plant phylogeny. Molecular systematics of plants. Edited by: Soltis PS, Soltis DE, Doyle JJ. 1992, New York, NY: Chapman and Hall, 14-35.View ArticleGoogle Scholar
- Palmer JD, Stein DB: Chloroplast DNA from the fern Osmunda cinnamomea: Physical organization, gene localization and comparison to angiosperm chloroplast DNA. Curr Genet. 1982, 5: 165-170. 10.1007/BF00391801.View ArticlePubMedGoogle Scholar
- Raubeson LA, Jansen RK: Chloroplast DNA evidence on the ancient evolutionary split in vascular land plants. Science. 1992, 255: 1697-2699. 10.1126/science.255.5052.1697.View ArticlePubMedGoogle Scholar
- Raubeson LA, Jansen RK: A rare chloroplast-DNA structural mutation is shared by all conifers. Biochem Syst Ecol. 1992, 20: 17-24. 10.1016/0305-1978(92)90067-N.View ArticleGoogle Scholar
- Kelch DG, Driskell A, Mishler BD: Inferring phylogeny using genomic characters: A case study using land plant plastomes. Molecular Systematics of Bryophytes (Monographs in Systematic Botany 98). Edited by: Goffinet B, Hollowell V, Magill R. St. 2006, Louis: Missouri Botanical Garden Press, 3-12.Google Scholar
- Ohyama K, Fukuzawa H, Kohchi T, Shirai H, Sano T, Sano S, Umesono K, Shiki Y, Takeuchi M, Chang Z, Aota S, Inokuchi H, Ozeki H: Chloroplast gene organization deduced from complete sequence of liverwort Marchantia polymorpha chloroplast DNA. Nature. 1986, 322: 572-574. 10.1038/322572a0.View ArticleGoogle Scholar
- Shinozaki K, Ohme M, Tanaka M, Wakasugi T, Hayashida N, Matsubayashi T, Zaita N, Chunwongse J, Obokata J, Yamaguchi-Shinozaki K, Ohto C, Torazawa K, Meng BY, Sugita M, Deno H, Kamogashira T, Yamada K, Kusuda J, Takaiwa F, Kato A, Tohdoh N, Shimada H, Sugiura M: The complete nucleotide sequence of tobacco chloroplast genome: Its gene organization and expression. EMBO J. 1986, 5: 2043-2049.PubMed CentralPubMedGoogle Scholar
- Jansen RK, Raubeson LA, Boore JL, dePamphilis CW, Chumley TW, Haberle RC, Wyman SK, Alverson AJ, Peery R, Herman SJ: Methods for obtaining and analyzing whole chloroplast genome sequences. Method Enzymol. 2005, 395: 348-384. full_text.View ArticleGoogle Scholar
- Pryer KM, Schneider H, Smith AR, Cranfill R, Wolf PG, Hunt JS, Sipes SD: Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature. 2001, 409: 618-622. 10.1038/35054555.View ArticlePubMedGoogle Scholar
- Qiu YL, Li LB, Wang B, Chen ZD, Dombrovska O, Lee J, Kent L, Li RQ, Jobson RW, Hendry TA, Taylor DW, Testa CM, Ambros M: A nonflowering land plant phylogeny inferred from nucleotide sequences of seven chloroplast, mitochondrial, and nuclear genes. Int J Plt Sci. 2007, 168: 691-708. 10.1086/513474.View ArticleGoogle Scholar
- Qiu YL, Li LB, Wang B, Chen ZD, Knoop V, Groth-Malonek M, Dombrovska O, Lee J, Kent L, Rest J, Estabrook GF, Hendry TA, Taylor DW, Testa CM, Ambros M, Crandall-Stotler B, Duff RJ, Stech M, Frey W, Quandt D, Davis CC: The deepest divergences in land plants inferred from phylogenomic evidence. Proc Natl Acad Sci USA. 2006, 103: 15511-15516. 10.1073/pnas.0603335103.PubMed CentralView ArticlePubMedGoogle Scholar
- Wikstrom N, Pryer KM: Incongruence between primary sequence data and the distribution of a mitochondrial atp1 group II intron among ferns and horsetails. Mol Phylo Evol. 2005, 36: 484-493. 10.1016/j.ympev.2005.04.008.View ArticleGoogle Scholar
- Lemieux C, Otis C, Turmel M: A clade uniting the green algae Mesostigma viride and Chlorokybus atmophyticus represents the deepest branch of the Streptophyta in chloroplast genome-based phylogenies. BMC Biol. 2007, 5: 2-10.1186/1741-7007-5-2.PubMed CentralView ArticlePubMedGoogle Scholar
- Turmel M, Pombert JÄo, Charlebois P, Otis C, Lemieux C: The green algal ancestry of land plants as revealed by the chloroplast genome. Int J Plt Sci. 2007, 168: 679-689. 10.1086/513470.View ArticleGoogle Scholar
- Hirao T, Watanabe A, Kurita M, Kondo T, Takata K: Complete nucleotide sequence of the Cryptomeria japonica D. Don. chloroplast genome and comparative chloroplast genomics: Diversified genomic structure of coniferous species. BMC Plt Biol. 2008, 8: 70-10.1186/1471-2229-8-70.View ArticleGoogle Scholar
- McCoy SR, Kuehl JV, Boore JL, Raubeson LA: The complete plastid genome sequence of Welwitschia mirabilis: An unusually compact plastome with accelerated divergence rates. BMC Evol Biol. 2008, 8: 130-10.1186/1471-2148-8-130.PubMed CentralView ArticlePubMedGoogle Scholar
- Wu CS, Lai YT, Lin CP, Wang YN, Chaw SM: Evolution of reduced and compact chloroplast genomes (cpDNAs) in gnetophytes: Selection toward a lower-cost strategy. Mol Phylo Evol. 2009, 52: 115-124. 10.1016/j.ympev.2008.12.026.View ArticleGoogle Scholar
- Wu CS, Wang YN, Liu SM, Chaw SM: Chloroplast genome (cpDNA) of Cycas taitungensis and 56 cp protein-coding genes of Gnetum parvifolium: Insights into cpDNA evolution and phylogeny of extant seed plants. Mol Biol Evol. 2007, 24: 1366-1379. 10.1093/molbev/msm059.View ArticlePubMedGoogle Scholar
- Hansen DR, Dastidar SG, Cai ZQ, Penaflor C, Kuehl JV, Boore JL, Jansen RK: Phylogenetic and evolutionary implications of complete chloroplast genome sequences of four early-diverging angiosperms: Buxus (Buxaceae), Chloranthus (Chloranthaceae), Dioscorea (Dioscoreaceae), and Illicium (Schisandraceae). Mol Phylo Evol. 2007, 45: 547-563. 10.1016/j.ympev.2007.06.004.View ArticleGoogle Scholar
- Jansen RK, Cai Z, Raubeson LA, Daniell H, dePamphilis CW, Leebens-Mack J, Muller KF, Guisinger-Bellian M, Haberle RC, Hansen AK, Chumley TW, Lee SB, Peery R, McNeal JR, Kuehl JV, Boore JL: Analysis of 81 genes from 64 plastid genomes resolves relationships in angiosperms and identifies genome-scale evolutionary patterns. Proc Natl Acad Sci USA. 2007, 104: 19369-19374. 10.1073/pnas.0709121104.PubMed CentralView ArticlePubMedGoogle Scholar
- Raubeson LA, Peery R, Chumley TW, Dziubek C, Fourcade HM, Boore JL, Jansen RK: Comparative chloroplast genomics: analyses including new sequences from the angiosperms Nuphar advena and Ranunculus macranthus. BMC Genomics. 2007, 8: 174-10.1186/1471-2164-8-174.PubMed CentralView ArticlePubMedGoogle Scholar
- Baldauf SL, Manhart JR, Palmer JD: Different fates of the chloroplast tufA gene following its transfer to the nucleus in green algae. Proc Natl Acad Sci USA. 1990, 87: 5317-5321. 10.1073/pnas.87.14.5317.PubMed CentralView ArticlePubMedGoogle Scholar
- Baldauf SL, Palmer JD: Evolutionary transfer of the chloroplast tufA gene to the nucleus. Nature. 1990, 344: 262-265. 10.1038/344262a0.View ArticlePubMedGoogle Scholar
- Sugita M, Murayama Y, Sugiura M: Structure and differential expression of two distinct genes encoding the chloroplast elongation factor Tu in tobacco. Curr Genet. 1994, 25: 164-168. 10.1007/BF00309543.View ArticlePubMedGoogle Scholar
- Roper JM, Hansen SK, Wolf PG, Karol KG, Mandoli DF, Everett KDE, Kuehl J, Boore JL: The complete plastid genome sequence of Angiopteris evecta (G. Forst.) Hoffm. (Marattiaceae). Am Fern J. 2007, 97: 95-106. 10.1640/0002-8444(2007)97[95:TCPGSO]2.0.CO;2.View ArticleGoogle Scholar
- Wickett NJ, Zhang Y, Hansen SK, Roper JM, Kuehl JV, Plock SA, Wolf PG, dePamphilis CW, Boore JL, Goffinet B: Functional gene losses occur with minimal size reduction in the plastid genome of the parasitic liverwort Aneura mirabilis. Mol Biol Evol. 2008, 25: 393-401. 10.1093/molbev/msm267.View ArticlePubMedGoogle Scholar
- Mindell DP, Sorenson MD, Dimcheff DE: An extra nucleotide is not translated in mitochondrial ND3 of some birds and turtles. Mol Biol Evol. 1998, 15: 1568-1571.View ArticlePubMedGoogle Scholar
- Kugita M, Kaneko A, Yamamoto Y, Takeya Y, Matsumoto T, Yoshinaga K: The complete nucleotide sequence of the hornwort (Anthoceros formosae) chloroplast genome: Insight into the earliest land plants. Nucl Acids Res. 2003, 31: 716-721. 10.1093/nar/gkg155.PubMed CentralView ArticlePubMedGoogle Scholar
- Wolf PG, Rowe CA, Hasebe M: High levels of RNA editing in a vascular plant chloroplast genome: analysis of transcripts from the fern Adiantum capillus-veneris. Gene. 2004, 339: 89-97. 10.1016/j.gene.2004.06.018.View ArticlePubMedGoogle Scholar
- Oliver MJ, Murdock AG, Mishler BD, Kuehl JV, Boore JL, Mandoli DF, Everett KDE, Wolf PG, Duffy AM, Karol KG: Chloroplast genome sequence of the moss Tortula ruralis: gene content, polymorphism, and structural arrangement relative to other green plant chloroplast genomes. BMC Genomics. 2010, Google Scholar
- Sugiura C, Kobayashi Y, Aoki S, Sugita C, Sugita M: Complete chloroplast DNA sequence of the moss Physcomitrella patens: Evidence for the loss and relocation of rpoA from chloroplast to the nucleus. Nucl Acids Res. 2003, 31: 5324-5331. 10.1093/nar/gkg726.PubMed CentralView ArticlePubMedGoogle Scholar
- Goffinet B, Wickett NJ, Werner O, Ros RM, Shaw AJ, Cox CJ: Distribution and phylogenetic significance of the 71-kb inversion in the plastid genome in Funariidae (Bryophyta). Ann Bot. 2007, 99: 747-753. 10.1093/aob/mcm010.PubMed CentralView ArticlePubMedGoogle Scholar
- Tsuji S, Ueda K, Nishiyama T, Hasebe M, Yoshikawa S, Konagaya A, Nishiuchi T, Yamaguchi K: The chloroplast genome from a lycophyte (microphyllophyte), Selaginella uncinata, has a unique inversion, transpositions and many gene losses. J Plt Res. 2007, 120: 281-290. 10.1007/s10265-006-0055-y.View ArticleGoogle Scholar
- Hasebe M, Iwatsuki K: Gene localization on the chloroplast DNA of the maiden hair fern: Adiantum capillis-veneris. Bot Mag Tokyo. 1992, 105: 413-419. 10.1007/BF02497656.View ArticleGoogle Scholar
- Wolf PG, Rowe CA, Sinclair RB, Hasebe M: Complete nucleotide sequence of the chloroplast genome from a leptosporangiate fern, Adiantum capillus-veneris L. DNA Res. 2003, 10: 59-65. 10.1093/dnares/10.2.59.View ArticlePubMedGoogle Scholar
- Gao L, Yi X, Yang YX, Su YJ, Wang T: Complete chloroplast genome sequence of a tree fern Alsophila spinulosa: Insights into evolutionary changes in fern chloroplast genomes. BMC Evol Biol. 2009, 9: 130-10.1186/1471-2148-9-130.PubMed CentralView ArticlePubMedGoogle Scholar
- Wolf PG, Karol KG, Mandoli DF, Kuehl J, Arumuganathan K, Ellis MW, Mishler BD, Kelch DG, Olmstead RG, Boore JL: The first complete chloroplast genome sequence of a lycophyte, Huperzia lucidula (Lycopodiaceae). Gene. 2005, 350: 117-128. 10.1016/j.gene.2005.01.018.View ArticlePubMedGoogle Scholar
- Palmer JD, Stein DB: Conservation of chloroplast genome structure among vascular plants. Curr Genet. 1986, 10: 823-833. 10.1007/BF00418529.View ArticleGoogle Scholar
- Goulding SE, Olmstead RG, Morden CW, Wolfe KH: Ebb and flow of the chloroplast inverted repeat. Mol Gen Genet. 1996, 252: 195-206. 10.1007/BF02173220.View ArticlePubMedGoogle Scholar
- Qiu YL: Phylogeny and evolution of charophytic algae and land plants. J Syst Evol. 2008, 46: 287-306.Google Scholar
- Goremykin VV, Holland B, Hirsch-Ernst KI, Hellwig FH: Analysis of Acorus calamus chloroplast genome and its phylogenetic implications. Mol Biol Evol. 2005, 22: 1813-1822. 10.1093/molbev/msi173.View ArticlePubMedGoogle Scholar
- Nishiyama T, Wolf PG, Kugita M, Sinclair RB, Sugita M, Sugiura C, Wakasugi T, Yamada K, Yoshinaga K, Yamaguchi K, Ueda K, Hasebe M: Chloroplast phylogeny indicates that bryophytes are monophyletic. Mol Biol Evol. 2004, 21: 1813-1819. 10.1093/molbev/msh203.View ArticlePubMedGoogle Scholar
- Renzaglia KS, Garbary DJ: Motile gametes of land plants: Diversity, development, and evolution. Cr Rev Plt Sci. 2001, 20: 107-213.View ArticleGoogle Scholar
- Pryer KM, Schuettpelz E, Wolf PG, Schneider H, Smith AR, Cranfill R: Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. Am J Bot. 2004, 91: 1582-1598. 10.3732/ajb.91.10.1582.View ArticlePubMedGoogle Scholar
- Schneider H, Smith AR, Pryer KM: Is morphology really at odds with molecules in estimating fern phylogeny?. Syst Bot. 2009, 34: 455-475. 10.1600/036364409789271209.View ArticleGoogle Scholar
- Mishler BD: Deep phylogenetic relationships among "plants" and their implications for classification. Taxon. 2000, 49: 661-683. 10.2307/1223970.View ArticleGoogle Scholar
- Mishler BD: The logic of the data matrix in phylogenetic analysis. Parsimony, Phylogeny, and Genomics. Edited by: Albert VA. 2005, Oxford: Oxford University Press, 57-70.Google Scholar
- Mishler BD, Kelch DG: Phylogenomics and early land plant evolution. Bryophyte Biology: Second Edition. Edited by: Shaw AJ, Goffinet B. 2009, Cambridge: Cambridge University Press, 173-197.Google Scholar
- Liu Q, Xue Q: Comparative studies on codon usage pattern of chloroplasts and their host nuclear genes in four plant species. J Genet. 2005, 84: 55-62. 10.1007/BF02715890.View ArticlePubMedGoogle Scholar
- Wall D, Herbeck J: Evolutionary patterns of codon usage in the chloroplast gene rbcL. J Mol Evol. 2003, 56: 673-688. 10.1007/s00239-002-2436-8.View ArticlePubMedGoogle Scholar
- Wright F: The 'effective number of codons' used in a gene. Gene. 1990, 87: 23-29. 10.1016/0378-1119(90)90491-9.View ArticlePubMedGoogle Scholar
- Wyman SK, Jansen RK, Boore JL: Automatic annotation of organellar genomes with DOGMA. Bioinformatics. 2004, 20: 3252-3255. 10.1093/bioinformatics/bth352.View ArticlePubMedGoogle Scholar
- Lowe TM, Eddy SR: tRNAscan-SE: A program for improved detection of transfer RNA genes in genomic sequence. Nucl Acids Res. 1997, 25: 955-964. 10.1093/nar/25.5.955.PubMed CentralView ArticlePubMedGoogle Scholar
- Lohse M, Drechse O, Bock R: OrganellarGenomeDRAW (OGDRAW): A tool for the easy generation of high-quality custom graphical maps of plastid and mitochondrial genomes. Curr Genet. 2007, 52: 267-274. 10.1007/s00294-007-0161-y.View ArticlePubMedGoogle Scholar
- Peden JF: Correspondence Analysis of Codon Usage. 1999, Nottingham: University of Nottingham, [http://codonw.sourceforge.net]Google Scholar
- Maddison DR, Maddison WP: MacClade 4: Analysis of phylogeny and character evolution. Version 4.06. 2003, Sunderland, Massachusetts: Sinauer Associates, IncGoogle Scholar
- Swofford DL: PAUP*: Phylogenetic Analysis Using Parsimony (*and other methods). Version 4.0b10. 2003, Sunderland, Massachusetts: Sinauer Associates, IncGoogle Scholar
- Altekar G, Dwarkadas S, Huelsenbeck JP, Ronquist F: Parallel Metropolis coupled Markov chain Monte Carlo for Bayesian phylogenetic inference. Bioinformatics. 2004, 20: 407-415. 10.1093/bioinformatics/btg427.View ArticlePubMedGoogle Scholar
- Huelsenbeck JP, Ronquist F: MRBAYES: Bayesian inference of phylogenetic trees. Bioinformatics. 2001, 17: 754-755. 10.1093/bioinformatics/17.8.754.View ArticlePubMedGoogle Scholar
- Ronquist F, Huelsenbeck JP: MrBayes 3: Bayesian phylogenetic inference under mixed models. Version 3.1.2. Bioinformatics. 2003, 19: 1572-1574. 10.1093/bioinformatics/btg180.View ArticlePubMedGoogle Scholar
- Nylander JAA: MrModeltest. 2004, Evolutionary Biology Centre, Uppsala University: Program distributed by the authorGoogle Scholar
- Swofford DL, Olsen GJ, Waddell PJ, Hillis DM: Phylogeny reconstruction. Molecular Systematics. Edited by: Hillis DM, Moritz C, Mable BK. 1996, Sunderland, Massachusetts: Sinauer Associates, Inc, 407-514. 2Google Scholar
- Felsenstein J: Confidence limits on phylogenies: An approach using the bootstrap. Evolution. 1985, 39: 783-791. 10.2307/2408678.View ArticleGoogle Scholar
- Adachi J, Waddell PJ, Martin W, Hasegawa M: Plastid genomephylogeny and a model of amino acid substitution for proteins encoded by chloroplast DNA. J Mol Evol. 2000, 50: 348-358.PubMedGoogle Scholar
- Stamatakis A, Hoover P, Rougemont J: A rapid bootstrap algorithm for the RAxML web servers. Syst Biol. 2008, 57: 758-771. 10.1080/10635150802429642.View ArticlePubMedGoogle Scholar
- Sato S, Nakamura Y, Kaneko T, Asamizu E, Tabata S: Complete structure of the chloroplast genome of Arabidopsis thaliana. DNA Res. 1999, 6: 283-290. 10.1093/dnares/6.5.283.View ArticlePubMedGoogle Scholar
- Ravi V, Khurana JP, Tyagi AK, Khurana P: The chloroplast genome of mulberry: Complete nucleotide sequence, gene organization and comparative analysis. Tree Genet Genomes. 2007, 3: 49-59. 10.1007/s11295-006-0051-3.View ArticleGoogle Scholar
- Jansen RK, Kaittanis C, Saski C, Lee SB, Tomkins J, Alverson AJ, Daniell H: Phylogenetic analyses of Vitis (Vitaceae) based on complete chloroplast genome sequences: Effects of taxon sampling and phylogenetic methods on resolving relationships among rosids. BMC Evol Biol. 2006, 6: 32-10.1186/1471-2148-6-32.PubMed CentralView ArticlePubMedGoogle Scholar
- Timme RE, Kuehl JV, Boore JL, Jansen RK: A comparative analysis of the Lactuca and Helianthus (Asteraceae) plastid genomes: Identification of divergent regions and categorization of shared repeats. Am J Bot. 2007, 94: 302-312. 10.3732/ajb.94.3.302.View ArticlePubMedGoogle Scholar
- Schmitz-Linneweber C, Maier RM, Alcaraz JP, Cottet A, Herrmann RG, Mache R: The plastid chromosome of spinach (Spinacia oleracea): Complete nucleotide sequence and gene organization. Plt Mol Biol. 2001, 45: 307-315. 10.1023/A:1006478403810.View ArticleGoogle Scholar
- Saski C, Lee SB, Fjellheim S, Guda C, Jansen RK, Luo H, Tomkins J, Rognli OA, Daniell H, Clarke JL: Complete chloroplast genome sequences of Hordeum vulgare, Sorghum bicolor and Agrostis stolonifera, and comparative analyses with other grass genomes. Theor Appl Genet. 2007, 115: 571-590. 10.1007/s00122-007-0567-4.PubMed CentralView ArticlePubMedGoogle Scholar
- Mardanov AV, Ravin NV, Kuznetsov BB, Samigullin TH, Antonov AS, Kolganova TV, Skyabin KG: Complete sequence of the duckweed (Lemna minor) chloroplast genome: Structural organization and phylogenetic relationships to other angiosperms. J Mol Evol. 2008, 66: 555-564. 10.1007/s00239-008-9091-7.View ArticlePubMedGoogle Scholar
- Chang CC, Lin HC, Lin IP, Chow TY, Chen HH, Chen WH, Cheng CH, Lin CY, Liu SM, Chaw SM: The chloroplast genome of Phalaenopsis aphrodite (Orchidaceae): Comparative analysis of evolutionary rate with that of grasses and its phylogenetic implications. Mol Biol Evol. 2006, 23: 279-291. 10.1093/molbev/msj029.View ArticlePubMedGoogle Scholar
- Cai ZQ, Penaflor C, Kuehl JV, Leebens-Mack J, Carlson JE, dePamphilis CW, Boore JL, Jansen RK: Complete plastid genome sequences of Drimys, Liriodendron, and Piper: implications for the phylogenetic relationships of magnoliids. BMC Evol Biol. 2006, 6: 77-10.1186/1471-2148-6-77.PubMed CentralView ArticlePubMedGoogle Scholar
- Goremykin VV, Hirsch-Ernst KI, Wolfl S, Hellwig FH: The chloroplast genome of Nymphaea alba: Whole-genome analyses and the problem of identifying the most basal angiosperm. Mol Biol Evol. 2004, 21: 1445-1454. 10.1093/molbev/msh147.View ArticlePubMedGoogle Scholar
- Goremykin VV, Hirsch-Ernst KI, Wolfl S, Hellwig FH: Analysis of the Amborella trichopoda chloroplast genome sequence suggests that Amborella is not a basal angiosperm. Mol Biol Evol. 2003, 20: 1499-1505. 10.1093/molbev/msg159.View ArticlePubMedGoogle Scholar
- Wakasugi T, Tsudzuki J, Ito S, Nakashima K, Tsudzuki T, Sugiura M: Loss of all ndh genes as determined by sequencing the entire chloroplast genome of the black pine Pinus thunbergii. Proc Natl Acad Sci USA. 1994, 91: 9794-9798. 10.1073/pnas.91.21.9794.PubMed CentralView ArticlePubMedGoogle Scholar
- Leebens-Mack J, Raubeson LA, Cui LY, Kuehl JV, Fourcade MH, Chumley TW, Boore JL, Jansen RK, dePamphilis CW: Identifying the basal angiosperm node in chloroplast genome phylogenies: Sampling one's way out of the Felsenstein zone. Mol Biol Evol. 2005, 22: 1948-1963. 10.1093/molbev/msi191.View ArticlePubMedGoogle Scholar
- Turmel M, Otis C, Lemieux C: The complete chloroplast DNA sequences of the charophycean green algae Staurastrum and Zygnema reveal that the chloroplast genome underwent extensive changes during the evolution of the Zygnematales. BMC Biol. 2005, 3: 22-10.1186/1741-7007-3-22.PubMed CentralView ArticlePubMedGoogle Scholar
- Turmel M, Otis C, Lemieux C: The chloroplast and mitochondrial genome sequences of the charophyte Chaetosphaeridium globosum: Insights into the timing of the events that restructured organelle DNAs within the green algal lineage that led to land plants. Proc Natl Acad Sci USA. 2002, 99: 11275-11280. 10.1073/pnas.162203299.PubMed CentralView ArticlePubMedGoogle Scholar
- Turmel M, Otis C, Lemieux C: The chloroplast genome sequence of Chara vulgaris sheds new light into the closest green algal relatives of land plants. Mol Biol Evol. 2006, 23: 1324-1338. 10.1093/molbev/msk018.View ArticlePubMedGoogle Scholar
- Lemieux C, Otis C, Turmel M: Ancestral chloroplast genome in Mesostigma viride reveals an early branch of green plant evolution. Nature. 2000, 403: 649-652. 10.1038/35001059.View ArticlePubMedGoogle Scholar
- Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA, McWilliam H, Valentin F, Wallace IM, Wilm A, Lopez R, Thompson JD, Gibson TJ, Higgins DG: Clustal W and Clustal X version 2.0. Bioinformatics. 2007, 23: 2947-2948. 10.1093/bioinformatics/btm404.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.