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Introduction

Hagenia is a monotypic genus under the Rosaceae family which is one of the largest and most economically important families with over 100 genera and more than 3,100 species (Potter et al., 2007). Many genera in this family include species that have been domesticated for fruit production, medicinal values and for ornamental purposes. Hagenia abyssinica (Bruce) J.F. Gmel. is a dioecious tree species, endemic to the isolated Afromontane forests of Africa at elevations between 2,300 and 3,400 m above sea level (Hedberg, 1969). The species is characterized by large reddish female and whitish male inflorescences, and its pollen and seeds are dispersed by wind (Negash, 1995). H. abyssinica has traditionally been used by the African communities as a source of herbal medicine for the treatment of gastrointestinal ailments in both man and animals (Assefa, Glatzel & Buchmann, 2010; Nibret & Wink, 2010; Scantlebury et al., 2013; Feyssa et al., 2015). Over the past few decades, there has been a vast reduction of natural populations of this species resulting from overharvesting, selective logging and habitat destruction. Consequently, H. abyssinica is now listed in the Red List of endangered species in Ethiopia and other regions where assessment has been done in eastern Africa (Negash, 1995; Vivero, Kelbessa & Demissew, 2005; Seburanga, Nsanzurwimo & Folega, 2014).

Several studies employing both traditional (morphology and chromosome number) and molecular techniques have been conducted aiming to assess the relationships within the family Rosaceae (Rosales). Molecular studies have analysed both the nuclear and plastid DNA. One of the early molecular phylogenetic studies in Rosaceae used chloroplast sequences of a single gene- rbcL- to assess the traditional subfamilial classification and to shed light on some problematic taxa within this family (Morgan, Soltis & Robertson, 1994). Further molecular phylogenetic analyses have been conducted in Rosaceae utilizing various coding and non-coding sequences, from the nuclear and/or the chloroplast genomes (Evans, 1999; Evans et al., 2000; Potter et al., 2002). In these studies, some of the traditional groupings were validated e.g., sub-dividing the family into Rosoideae, Maloideae, Spiraeoideae and Amygdaloideae by Schulze-Menz (1964). However, major contraditions between traditional and molecular-based studies were noted and significant differences were also observed between the molecular studies probably due to the use of different but limited number of partial DNA sequences. Additional clarifications in the phylogeny and classification of Rosaceae were made in Potter et al. (2007), where three sub-families (Rosoideae, Dryadoideae and Spiraeoideae) were supported. These studies have greatly boosted our understanding of phylogenetic relationships in Rosaceae. However, certain clades, as discussed in Potter et al. (2007), remain ambiguously classified while others are weakly supported.

The first complete sequences of cpDNA were reported three decades ago in Marchantia polymorpha (Ohyama et al., 1986) and in Nicotiana tabacum (Shinozaki et al., 1986), and since then there had been gradual increase in the number of sequenced complete chloroplast genomes. However, the advent of next-generation DNA sequencing technologies significantly reduced the cost and time involved in DNA sequencing (Shendure & Ji, 2008; Daniell et al., 2016. Consequently, the number of species with complete sequenced nuclear and organellar genomes has rapidly increased. The chloroplast genome is circular and it is characterized by a quadripartite structure with two inverted repeats (IRa and IRb) that are separated by one Large Single Copy region (LSC) and one Small Single Copy region (SSC). The size of complete chloroplast genome sequences range between 107 and 217 kb. Genome size fluctuations could be attributed to; duplication of genes and occurrence of small repeats (Xu et al., 2015), gene loss and/or transfer to other genomes (Stegemann et al., 2003) and the contraction/expansion of the inverted repeats at the four IR/SC junctions (Downie & Jansen, 2015).

In angiosperms, one of the key traits of the organellar DNA is uniparental inheritance; thus, it is well conserved and allows for the development of informative universal markers. These attributes make the chloroplast genome more valuable for application in various molecular studies in plants e.g., DNA barcoding, outlining species evolutionary histories, molecular phylogenetics and population genetics. Recently, complete chloroplast genomes have extensively been used in plant identification and resolution of phylogenetic relatioships at different taxonomic levels (Jansen et al., 2007; Yang et al., 2013; Zhang et al., 2016).

Currently, whole chloroplast genomes of several species from the Rosaceae family representing nine genera have been studied and deposited at the GenBank database (NCBI; http://www.ncbi.nlm.nih.gov/). However, only a few of these species, such as Fragaria chiloensis (Salamone et al., 2013) and Potentilla micrantha (Ferrarini et al., 2013), are from the sub-family Rosoideae whose whole cpDNA have been sequenced. At present, none from the Agrimoniinae clade has been sequenced and the closest studied genus- to Hagenia- is Rosa (Yang, Li & Li, 2014). Therefore, the objectives of this study were to establish and characterize the organization of the complete chloroplast genome sequence of H. abyssinica and to compare its structure, gene arrangement and IR boarders to other members of the Rosaceae family. Because this is the first whole chloroplast genome presented from the Sanguisorbeae tribe, it will act as a reference chloroplast genome within the tribe.

Materials and Methods

DNA extraction and sequencing

Young leaf samples were collected from natural populations of Hagenia abyssinica in Mt. Kenya (Kenya; 00°09′35.29″S/037°26′56.40″E). A voucher specimen (SAJIT_001956) was deposited at the Herbaria of Wuhan Botanical Garden, Chinese Academy of Sciences (HIB). Total genomic DNA was extracted from 100–150 mg of leaves using the MagicMag Genomic DNA Micro Kit (Sangon Biotech Co., Shanghai, China) following the manufacturer’s instructions. The quality of the extracted DNA was checked by gel electrophoresis and confirmed using Qubit DNA Assay kit in Qubit 2.0 Fluorometer (Life Technologies, San Diego, CA, USA). Paired-end library was constructed using an Illumina TruSeq Library preparation kit (Illumina, San Diego, CA, USA) following the manufacturer’s protocol. Genomic DNA was sequenced using the Illumina Hiseq 2500 platform (Illimina Inc.), yielding 41.2 million 150-bp paired-end reads from a library of ∼350  bp DNA  fragment.

Genome assembly and annotation

We used a reference-guided strategy to assemble the chloroplast genome. Firstly, whole clean data were identified using BLAST (http://blast.ncbi.nlm.nih.gov/) with default parameters, by searching against the plastome sequences of Fragaria chiloensis (JN884816). The generated contigs were sorted, and the chloroplast genome reads were extracted by mapping the contigs against already available chloroplast sequences of Fragaria chiloensis (JN884816; Salamone et al., 2013) using the Basic Local Alignment Search Tool (BLAST; http://blast.ncbi.nlm.nih.gov/) with default parameters. The retained high quality reads were then assembled into non- redundant contigs using Velvet 1.2.10 (Zerbino & Birney, 2008) with K-mer length of 95–107. Five contigs whose size ranged between 1,960 and 47,845 bp were then blasted against Fragaria chiloensis and Pyrus pyrifolia (AP012207; Terakami et al., 2012). Specific primers were designed using PRIMER 5.0 (PREMIER Biosoft International, CA, USA) and used in Polymerase Chain Reaction to fill gaps between the contigs and to validate the joints between the IR/LSC and IR/SSC, based on the Sanger sequencing technique. The primer sequences used in filling the gaps and validating the IR/SC junctions are listed in File S1.

The assembled chloroplast genome was annotated using an online-based program: the Dual OrganellarGenomMe Annotator (DOGMA; http://dogma.ccbb.utexas.edu/, Wyman, Jansen & Boore, 2004) followed by manual corrections of the start, stop codons and the boundaries between the introns and exons based on homologous genes from other sequenced chloroplast genomes. Protein coding, transfer RNA (tRNA) and ribosomal RNA (rRNA) genes were also predicted in DOGMA with default parameters. The tRNA genes were further verified using tRNAscan-SE 1.23 program (http://lowelab.ucsc.edu/tRNAscan-SE/; Schattner, Brooks & Lowe, 2005). Finally, a circular gene map was constructed using the OrganellarGenomeDRAW software (OGDRAW; http://ogdraw.mpimp-golm.mpg.de). The complete chloroplast genome sequence of H. abyssinica can be found in GenBank under the accession number KX008604.

Microsatellite discovery and comparative analyses

The Perl script based Microsatellite identification tool (MiSa) (Thiel et al., 2003) was used to detect microsatellites with minimal iterations of eight repeat motifs for mononucleotides, five for dinucleotides, four for trinucleotides and three for Tetra-, Penta- and hexa-nucleotides. The location and size of the repeating sequences (forward, reverse, complementary and palindromic) were visualized in REputer (Kurtz & Schleiermacher, 1999) with minimal repeat size set at ≥15 and Hamming distance at 3.

To highlight structural differences and similarities between H. abyssinica and other already sequenced chloroplast genomes in Rosaceae family, we retrieved 20 currently available complete chloroplast genomes from the NCBI (Table 1) and conducted comparative analyses. Special attention was paid to the sizes of the entire complete genomes and inverted repeats, the location of the IR/SC junctions and arrangement of genes adjacent the IR/SC boarders.

Table 1:
Comparison of complete chloroplast genomes in 21 taxa of Rosaceae; size, contraction/expansion of the inverted repeats and gene arrangement around the four IR/SC junctions.
IRa/LSC IRa/SSC IRb/SSC IRb/LSC
GenBank No. Species Genome size LSC length SSC Length IR length Sub-family rps19 (bp) rpl2 (bp) Ψycf1 (bp) ndhF (bp) ycf1 (bp) rpl2 (bp) Ψrps19 (bp) trnH-GUG (bp)
KU851961 Malus prunifolia 160,041 88,119 19,204 26,359 Spiraeoideae 119 9 11 1,073 −190 129 −38
AP012207 Pyrus pyrifolia 159,922 87,901 19,237 26,392 21 −92 −90 110 975 −289 149 −3
HG737342 Pyrus spinosa 159,161 87,694 19,205 26,396 8 −79 −114 113 493 −520 141 −91
KC571835 Prinsepia utilis 159,328 85,239 18,485 26,302 178 −107 −110 −32 978 −3,398 179 −91
KP760072 Prunus padus 158,955 87,667 18,872 26,208 38 −109 5 19 1,035 −109 * −22
KP760073 Prunus serrulata var. spontanea 157,882 85,969 19,121 26,396 177 −248 13 −2 1,045 −248 162 −24
KP760070 Prunus yedoensis 157,859 85,978 19,121 26,380 179 −250 18 −21 1,040 −250 185 −46
KP760071 Prunus maximowiczii 157,852 85,848 19,134 26,435 216 −287 13 −2 1,045 −287 221 −21
KP760075 Prunus surbhirtela 157,833 85,952 19,121 26,381 179 −250 17 −21 1,040 −250 185 −46
HQ336405 Prunus persica 157,790 85,968 19,060 26,381 95 −167 −81 96 946 −338 182 −3
KF990036 Prunus kansuensis 157,736 85,755 19,209 26,386 181 −252 5 9 1,050 −338 182 −79
KF765450 Prunus mume 157,712 85,830 19,094 26,394 196 −267 −102 −17 1,018 −298 206 −2
KF753637 Rosa odorata var. gigantea 156,634 85,767 18,761 26,053 Rosoideae −14 −55 57 −44 1,105 −54 * −4
JQ041763 Pentactina rupicola 156,612 84,970 18,942 26,350 152 −223 0 40 1,057 −222 151 −35
JF345175 Fragaria vesca var. vesca 155,691 85,606 18,175 25,555 −10 −55 31 −93 1,091 −54 * −35
JN884817 Fragaria virginiana 155,621 85,587 18,146 25,944 −13 −54 12 −33 1,091 −54 * −34
JN884816 Fragaria chiloensis 155,603 85,568 18,147 25,944 −13 −54 12 −33 1,091 −54 * −34
KC507760 Fragaria mandshurica 155,596 85,515 18,171 25,955 −13 −54 12 59 1,091 −54 * −34
KC507759 Fragaria iinumae 155,554 85,569 18,059 25,963 −13 −55 21 −50 1,091 −54 * −34
KX008604 Hagenia abyssinica 154,961 84,320 18,696 25,971 −130 −57 53 12 1,082 −57 * −3
HG931056 Potentilla micrantha 154,959 85,137 18,762 25,530 −1,016 −489 −476 400 1,040 −60 * −3
DOI: 10.7717/peerj.2846/table-1

Notes:

SSC

small single copy

LSC

large single copy

IR

inverted repeat (a/b)

bp

base pairs

Ψ

pseudogene;

*

missing]

The negative (−) numbers indicate the size of the gap between the IR/SC junction and the gene involved. Except for Ψrps19, the other numbers shows the size of the gene that is located in the IR.

To gain insight into the relationship of members of Rosaceae, a Maximum Likelihood (ML) phylogenetic tree was reconstructed. We used 71 protein-coding genes common in all the 21 species of Rosaceae. Two taxa; Morus indica (Moraceae) and Eleagnus macrophylla (Eleagnaceae), from the clade Rosales, were used as outgroups. All the PCGs were aligned in MUSCLE (Edgar, 2004) with default settings and appropriately edited manually. The jModelTest 2.1.7 program (Darriba et al., 2012) was used to select the best fitting substitution model based on the Akaike information criterion (Posada & Buckley, 2004). The best-fitting substitution model GTR + I + G model of all genes was used. The GTR + I + G model was used for ML analyses implemented in RAxML 8.0.20 following instructions from the manual (Stamatakis, 2014). A bootstrap analysis was performed with 1,000 replications.

Results and Discussion

Genome content and organization

The complete chloroplast genome of H. abyssinica exhibited a double- stranded circular DNA molecule, with a total length of 154,961 bp (Fig. 1). It also displayed a quadripartite structure, typical to chloroplast genomes of most terrestrial plants. The chloroplast genome possesses a pair of inverted repeats (IRa and IRb) of 25,971 bp each. The IRs are separated by a large single copy (LSC) and a small single copy (SSC) with 84,320 bp and 18,696 bp respectively (Fig. 1). The total GC content for this chloroplast genome is 37.1%, which is consistent with those from other species in Rosaceae. The chloroplast genome of H. abyssinica encodes 129 genes (excluding the ORFs and the the hypothetical genes; ycf68 and ycf15), comprising 78 unique protein—coding genes (PCGs), 30 unique tRNA and 4 rRNA genes (Table 2). In total there were 17 duplicated genes, 7 of which code for protein in the IRs including rpl2, rpl23, ycf2, ndhB, rps7, rps12, and ycf1, 6 tRNA and 4 rRNA were also among the duplicates in the IRs. The gene order in the SSC region begins with ndhF, followed by rpl32, trnL, ccsA, ndhD, PsaC, ndhE, ndhG, ndhI, ndhA, ndhH and rps15 and ends with ycf1. Six protein coding genes contained either one intron (rps16, rpl2, rpl23, rpoC1, ndhA and ndhB) or two introns (clpP). The hypothetical gene ycf3, contained two introns (Table 2). The rps12 gene is trans-spliced with the 3′ exon being duplicated in the IR, while the 5′ end is located at the LSC region.

A gene map of Hagenia abyssinica chloroplast genome.

Figure 1: A gene map of Hagenia abyssinica chloroplast genome.

The GC content is represented by the dark shading on the inner side of the small circle, whereas the light shading represents the AT content. The genes are color-coded based on different functional group.
Table 2:
List of genes in the chloroplast genome of Hagenia abyssinica.
Category Gene type Gene
Self-replication Ribosomal RNA rrn16 rrn23 rrn4.5 rrn5
Transfer RNA trnA-UGC* trnfM-CAU trnI-GAU* trnM-CAU trnR-ACG trnS-UGA
trnC-GCA trnG-GCC* trnK-UUU* trnN-GUU trnW-CCA trnT-GGU
trnD-GUC trnG-UCC trnL-CAA trnY-GUA trnR-UCU trnT-UGU
trnE-UUC trnH-GUG trnL-UAA* trnP-UGG trnS-GCU trnV-GAC
trnF-GAA trnI-CAU trnL-UAG trnQ-UUG trnS-GGA trnV-UAC*
Small ribosomal units rps11 rps12 rps14 rps15 rps16* rps18
rps19 rps2 rps3 rps4 rps7 rps8
Large ribosomal units rpl14 rpl16 rpl2* rpl20 rpl22 rpl23 rpl32
rpl33 rpl36
RNA polymerase sub-units rpoA rpoB rpoC1* rpoC2
Photosynthesis genes NADH dehydrogenase ndhA* NdhB* ndhC ndhD ndhE ndhF
ndhG ndhH ndhI ndhJ ndhK
Photosystem I psaA psaB psaC psaI psaJ ycf3** ycf4
Photosystem II psbA psbB psbC psbD psbE psbF psbH
psbI psbJ psbK psbL psbM psbN psbT
lbhA
Cytochrome b/f complex petA petB petD petG petL petN
ATP synthase atpA atpB atpE atpF atpH atpI
Large subunit of rubisco rbcL
Other genes Maturase matK
Protease clpP**
Acetyl-CoA-carboxylase sub-unit accD
Envelope membrane protein cemA
Component of TIC complex ycf1
c-type cytochrome synthesis ccsA
Unknown hypothetical genes reading frames ycf2
DOI: 10.7717/peerj.2846/table-2

Notes:

Genes with a single intron.
Genes with two introns.

Discovery of SSRs

Microsatellite markers are considered ideal for plant molecular studies due to their high mutation rates, multi- allelism and locus- specificity (Varshney, Graner & Sorrells, 2005; Govindaraj, Vetriventhan & Srinivasan, 2015) and thus highly informative. Recently, seventeen species-specific nuclear SSR markers have been reported for this species (Gichira et al., 2016). In a previous study, three concensus chroloplast microsatellite markers had been used to study genetic diversity of H. abyssinica (Ayele et al., 2009). Chloroplast-derived microsatellite markers have generated great impact on population genetics, plant evolutionary studies and phylogenetics (Provan, Powell & Hollingsworth, 2001). In this study, a total of 172 SSR repeat motifs were discovered (Table 3).

Table 3:
Characterization of simple sequence repeats discovered in the chloroplast genome of Hagenia abyssinica.
Microsatellite sequences Number of repeats Total
3 4 5 6 7 8 9 10 11 12 13 14 15
A 25 14 10 5 6 3 1 1 65
C 6 3 1 10
G 3 1 4
T 30 20 14 6 2 1 1 1 75
AT 2 2 4
TA 5 5
TC 1 1
AAAT 2 2
AATA 1 1
ATGT 1 1
TAAA 1 1
TAAT 1 1
TTTA 2 2
Total 172
DOI: 10.7717/peerj.2846/table-3

Mononucleotides had the highest number of repeats (88%), most of which had the A/T repeat type which is in line with the findings of a previous study that polyA and polyT repeats dominate in chloroplast microsatellites (Cai et al., 2008). A total of 5.8% represented dinucleotides while the rest were tetranucleotides, nine of the dinucleotides had the AT/TA repeat motif while AAAT/TTTA motifs dominated among the tetranucleotides. There were no trinucleotide repeats detected in H. abyssinica’s chloroplast genome. Repeat motifs are potential molecular tools for studying recombination and rearrangement in genomes (Smith, 2002). In addition to SSRs, a total of 49 repeat sequences with at least 21 bp were identified by REPuter. The repeat units had a sequence identity of ≥90% and their sizes ranged from 21 to 69 bp. The 49 repeats constituted 22 palindrome (inverted) repeats, 19 direct (forward) repeats, seven reverse repeats and one complementary repeat (Table 4). The majority of the identified repeats were located in the non-coding regions of the genome which is in line with observations made in other chloroplast genomes of angiosperms (Provan, Powell & Hollingsworth, 2001; George et al., 2015). This trend of cpSSR distribution, has been observed in other chloroplast genomes species in Rosaceae suggesting that they may be suitable for conducting population genetic diversity, phylogenetic and evolutionary studies in species under this family.

Table 4:
List and location of long repeat sequences in the chloroplast genome of Hagenia abyssinica.
Repeat size (bp) Repeat 1 start Repeat 2 start Repeat type Location 1 Location 2
69 26,722 26,745 F IGS (rpoB-trnC-GCA) IGS (rpoB-trnC-GCA)
67 52,510 52,510 P IGS (trnM-CAU-atpE) IGS (trnM-CAU-atpE)
59 52,514 52,514 P IGS (trnM-CAU-atpE) IGS (trnM-CAU-atpE)
56 10,134 10,134 P IGS (trnR-UCU-atpA) IGS (trnR-UCU-atpA)
46 26,722 26,768 F IGS (rpoB-trnC-GCA) IGS (rpoB-trnC-GCA)
40 98,746 12,0719 F IGS (rps7-trnV-GAC) IGS (ndhA-ndhA)
40 12,0719 14,0493 P IGS (ndhA-ndhA) IGS (trnV-GAC-rps7)
39 44,079 98,748 F ycf3 IGS (rps7-trnV-GAC)
39 44,079 14,0492 P ycf3 IGS (trnV-GAC-rps7)
38 44,079 12,0721 F ycf3 IGS (ndhA-ndhA)
37 12,859 12,859 P IGS (atpF-atpH) IGS (atpF-atpH)
34 8342 45,240 P IGS (psbI-trnS-GCU) trnS-GGA
30 8,346 45,240 P IGS (psbI-trnS-GCU) trnS-GGA
30 10,7688 10,7720 F IGS (rrn4.5-rrn5) IGS (rrn4.5-rrn5)
30 10,7688 13,1529 P IGS (rrn4.5-rrn5) IGS (rrn5-rrn4.5)
30 10,7720 13,1561 P IGS (rrn4.5-rrn5) IGS (rrn5-rrn4.5)
30 13,1529 13,1561 F IGS (rrn5-rrn4.5) IGS (rrn5-rrn4.5)
29 35,992 36,014 F IGS (trnS-UGA-lbhA) IGS (trnS-UGA-lbhA)
28 67,251 67,275 F IGS (psaJ-rpl33) IGS (psaJ-rpl33)
30 47,381 47,381 P IGS (trnT-UGU-trnL-UAA) IGS (trnT-UGU-trnL-UAA)
24 36,841 36,841 P IGS (trnG-UCC-trnfM-CAU IGS (trnG-UCC-trnfM-CAU)
24 67,255 67,279 F IGS (psaJ-rpl33) IGS (psaJ-rpl33)
27 9,748 36,800 F IGS (trnS-GCU-trnG-GCC) trnG-UCC
29 7,294 12,5722 R IGS (trnQ-UUG-psbK) ycf1
29 8,344 35,778 F IGS (psbI-trnS-GCU) trnS-UGA
23 26,722 26,791 F IGS (rpoB-trnC-GCA) IGS (rpoB-trnC-GCA)
31 96,104 96,104 P IGS (ndhB-ndhB) IGS (ndhB-ndhB)
31 96,104 14,3144 F IGS (ndhB-ndhB) IGS (ndhB-ndhB)
31 14,3144 14,3144 P IGS (ndhB-ndhB) IGS (ndhB-ndhB)
28 10,275 10,275 P IGS (trnR-UCU-atpA) IGS (trnR-UCU-atpA)
28 59,119 59,119 P IGS (accD-psaI) IGS (accD-psaI)
22 35,852 45,182 P IGS (rpoB-trnS-UGA) IGS (ycf3-trnS-GGA)
22 56,966 56,966 R IGS (rbcL-accD) IGS (rbcL-accD)
22 80,656 80,656 P IGS (rps8-rpl14) IGS (rps8-rpl14)
25 8,348 35,782 F trnS-GCU trnS-UGA
25 35,782 45,243 P trnS-UGA trnS-GGA
30 7,017 7,021 R IGS (rps16-trnQ-UUG) IGS (rps16-trnQ-UUG)
30 28,761 98,934 R IGS (petN-psbM) IGS (rps7-trnV-GAC)
30 28,761 14,0315 C IGS (petN-psbM) IGS (trnV-GAC-rps7)
30 39,041 41,265 F psaB psaA
30 81,696 12,0708 F IGS (rpl16-rps3) IGS (ndhA-ndhA)
27 10,259 36,722 P IGS (trnR-UCU-atpA) IGS (lbhA-trnG-UCC)
27 56,961 56,966 R IGS (rbcL-accD) IGS (rbcL-accD)
21 8,352 35,786 F trnS-GCU trnS-UGA
21 12,792 68,281 F IGS (atpF-atpH) rps18
21 30,092 30,092 R IGS (psbM-trnD-GUC) IGS (psbM-trnD-GUC)
21 35,786 45,243 P trnS-UGA trnS-GGA
21 63,682 63,682 R psbJ psbJ
29 32,026 32,026 P IGS (trnT-GGU-psbD) IGS (trnT-GGU-psbD)
DOI: 10.7717/peerj.2846/table-4

Notes:

F

forward

R

reverse

P

palindromic

C

complementary

Comparative analysis and phylogenetics

The number of species from the Rosaceae family with completely sequenced cholorplast genomes is rapidly increasing. Currently, the complete chloroplast genomes of 20 species from eight genera in two sub-families of Rosaceae family have been sequenced and deposited at GenBank (http://www.ncbi.nlm.nih.gov/). Out of the 20 species, 12 belong to the Spiraeaideae sub-family while the rest fall under the Rosoideae sub-family (Potter et al., 2007; Hummer & Janick, 2009). We compared the structure of Hagenia’s chloroplast genome to those available from the eight genera. The list of the species used for comparision and their accession numbers are shown in (Table 1). Characteristically, there are four junctions in the chloroplast genomes of angiosperms, due to the presence of two identical copies of the inverted repeats. However, the loss of one inverted repeat has been reported in some flowering plants e.g., in legumes (Palmer et al., 1987b). All chloroplast genomes appeared to be structurally similar with a typical quadripartite structure of two IRs separated by a LSC and a SSC. The whole genome sizes ranged from 154,959 (Potentilla micrantha) to 160,041 (Malus prunifolia) and there was a clear distinction of the sub-families based on genome sizes. Species from the Maloideae sub- family have a larger chloroplast genome compared to those from the Rosoideae. The size of H. abyssinica’s chloroplast genome (154,961 bp) is only 2 bp larger than that of the smallest chloroplast genome of P. micrantha (154,959 bp; Ferrarini et al., 2013).

Size variations of the chloroplast genome may be attributed to the expansion/contraction of the IR, with small variations (<100 bp) being common even among species under the same genus (Goulding et al., 1996). The expansion and/or contraction of the IRs is regarded as a significant evolutionary event and can be a source of polymorphic genetic markers for species identification and for analyzing phyologenetic studies in plants (Wang et al., 2008). In this study, sizes of the IRs varied from 26,435 bp in Prunus maximowiczii to 25,530 in P. micrantha. Although certain genes near the IR/SC boarders appeared to be conserved in all the species, key variations were noted in gene arrangement along the IR/SC junctions (Table 1). Two genes (rps19 and rpl2) are adjacent the IRa/LSC boarder at varying positions, while the IRb/LSC junction is flanked between genes rpl2 and trnH-GUG and in some cases a pseudogene (Ψ) of rps19 gene is included in this region. This is a common feature in angiosperms, excluding monocots whose trnH-GUG gene is located in the IR between the genes rpl2 and rps19 (Goulding et al., 1996; Wang et al., 2008).

In all species from the Spiraeoideae subfamily and in one Rosoideae species—Pentactina rupicola—the IRa/LSC junction occurs within the coding region of the rps19 gene resulting into the presence of Ψrps19 gene of various length in the IRb. This event has also been reported in the chloroplast genomes of other species e.g., Arabidopsis thaliana (Sato et al., 1999) and Coffea arabica (Samson et al., 2007). However in the other species, including H. abyssinica the entire rps19 gene is located in the LSC region, leaving a gap of varying length between the 5′ end of the gene and the IRa/LSC junction, this is similar to other dicots such as Nicotiana tabacum (Shinozaki et al., 1986). The largest gap was 1,016 bp in P. micrantha followed by 130 bp in H. abyssinica. The rpl2 gene is entirely located in both IRs region in all species, consequently leaving a gap of non-coding region between the IR/LSC junction and rpl2 gene. The IRb/LSC junction is situated in the down-stream of non-coding region of the trnH-GUG gene in all analysed species. Those species with Ψrps19, the pseudogene was located within the IR, between the rpl2 and the trnH-GUG. In some dicots e.g., Actinidia chinensis (Yao et al., 2015), trnH-GUG and a section of the psbA occur in the inverted repeat due to expansion on the IRs into the LSC region.

In all the studied species, the IRb/SSC junction is located within the coding region of the ycf1 gene. Consequently, the ycf1 gene extends into the IRb at varying lengths ranging from 946 bp in Prunus persica to 1,091 bp in all species of genus Fragaria. As a result, the IRa/SSC junction is bordered by Ψycf1 and gene ndhF, which is a general structure among the dicots e.g., tobacco and Arabidopsis. In Hagenia, the ycf1 gene has an extension of 1,040 bp into the IRb and therefore, its Ψycf1 of 1,151 bp overlaps with ndhF (2,234 bp) at 65 bp. The chloroplast genome of Annona cherimola, which is one of the largest plastid genomes with 201,723 bp, has an extremely reduced SSC (2,966 bp) due to major expansions of the IRs and most genes including the ycf genes have been incorporated in the IRs (Blazier et al., 2016).

Chloroplast DNA is reported to have evolved from free-living Cyanobacteria through endosymbiosis with a history of more than 1.2 billion years and since then a number of genes, initially found in the chloroplast genomes have relocated to the nuclear genome (Timmis et al., 2004), e.g., in Arabidopsis 18.1% of its functional nuclear genes originated from the plastid genome (Martin et al., 2002). Further studies presented more evidence on independent gene transfers from the chloroplast to the nuclear genome in rosids (Millena et al., 2001), these includes the successful transfers of rpl22 gene in Castanea, Quercus and Passiflora (Jansen et al., 2011),  infA gene in Arabidopsis (Sato et al., 1999) and in Elaeagnus (Choi, Son & Park, 2015). These transfers occurred in the initial stages of plastid evolution, though a high relocation rate of non-coding DNA happens continuously (Martin et al., 2002; Timmis et al., 2004). Generally, loss and/or transfer of genes from the chloroplast genomes to the nuclear or mitochondria genomes is as a result of evolutionary events, allowing chloroplast genomes to act as valuable molecular tools in phylogenetic and evolutionary studies. Further comparative analyses revealed that the initiation factor 1 (infA) gene which was observed in other species of Rosaceae, is conspicuously missing from the Hagenia chloroplast genome. The loss/transfer of the infA gene, which is an essential gene in Escherichia coli (Cummings & Hershey, 1994), is common among the angiosperms and it is regarded as a highly mobile gene (Millena et al., 2001; Daniell et al., 2016). Therefore, besides the expansion/contraction of the IRs, gene loss provides crucial information that is essential for evolutionary studies and resolution of phylogenetic relationships among plant species.

Complete chloroplast genome sequences provide essential genetic data for precise systematics and phylogenetic resolutions in plants. The ML phylogenetic tree that was constructed using 71 PCGs, common in all 21 taxa from Rosaceae and in two outgroups, clearly placed the Rosaceae species into two clades. The two main clades concurred with two sub-families: Spiraeoideae and Rosoideae (Fig. 2). This classification was in agreement with the phylogeny of Rosaceae (Potter et al., 2007). Previously, Hagenia had been classified in sub- family Rosoideae under Agrimoniinae, a subtribe in the tribe Sanguisorbeae, alongside the genera Aremonia, Agrimonia, Leucosidea and Spenceria  (Eriksson et al., 2003; Potter et al., 2007).

Phylogenetic relationship of 21 species of Rosaceae based on maximum likelihood analysis of 71 protein coding genes.

Figure 2: Phylogenetic relationship of 21 species of Rosaceae based on maximum likelihood analysis of 71 protein coding genes.

Conclusion

This study provides the complete chloroplast sequences of H. abyssinica; an endemic species to the isolated mountains of Africa and the only species under the genus Hagenia. Comparative analysis revealed significant similarity in the structural organization of the chloroplast genomes in the Rosaceae family, with slight variations in size attributed to the expansion/contraction of the inverted repeats. The lost infA gene in the Hagenia chloroplast genome may have been shifted to the nuclear genome. This is the first chloroplast genome to be sequenced in the Sanguisorbeae tribe, and therefore provides valuable information for phylogenetic studies. Additionally, the data generated here provide valuable molecular markers as tools for further population genetic studies needed to support formulation of appropriate conservation measures for this endangered medicinal plant.

Supplemental Information

List of primer pairs used to fill the chloroplast gaps

Note: F, forward, R, reverse

DOI: 10.7717/peerj.2846/supp-1