Scientists Decode the Full Chloroplast Genome of a Chinese Rhododendron, Exposing Hidden Hotspots of Evolution

A deciduous shrub with brilliant springtime flowers and a quiet foothold in the woodlands of central China has finally given up its full plastid blueprint. Researchers have assembled and annotated the complete chloroplast genome of Rhododendron mariesii, a species prized by horticulturists for its ornamental and medicinal qualities and valued by ecologists for its role in the forest flora of the Dabie Mountains. The work, published in Comparative Cytogenetics, delivers not only a new reference genome but also a comparative survey of related Rhododendron species that clarifies where this lineage sits within the heath family, Ericaceae, and pinpoints the DNA segments most likely to drive future barcoding and conservation studies.

The genome was built from wild-collected material. Young, disease-free leaves were sampled in the Dabie Mountains at an elevation of roughly 1,005 meters, dried in silica, and preserved for laboratory work. Specimens were identified by a doctoral-level botanist and deposited in the Huanggang Normal University Herbarium in Hubei province. High-molecular-weight DNA was extracted, quality-checked on agarose gels, and quantified spectrophotometrically before the team constructed paired-end Illumina libraries and sequenced them on a NovaSeq6000 platform. After filtering with SOAPnuke, the pipeline retained more than 19.3 million clean reads totaling about 2.89 gigabases, with a GC content of 39.52 percent and quality scores comfortably above standard thresholds. De novo assembly with GetOrganelle, followed by validation with BOWTIE2 mapping, produced a closed circular plastome that was annotated with DOGMA and CPGAVAS tools and manually verified in Geneious before submission to GenBank under accession OM161981.

The assembled molecule measures 203,480 base pairs and displays the canonical quadripartite architecture familiar from most angiosperm plastomes: a large single-copy region of 113,715 base pairs, a small single-copy region of 7,953 base pairs, and a pair of identical inverted repeat regions of 40,918 base pairs each that separate the two single-copy segments. Annotation yielded 151 unique genes, of which 98 encode proteins, 45 are transfer RNAs and eight are ribosomal RNAs. The protein-coding set includes a full complement of 55 photosynthesis-related genes, spanning the ATP synthase, photosystem I, photosystem II, NADH-dehydrogenase and cytochrome b/f complexes together with the rubisco large subunit rbcL, alongside genes for self-replication functions such as ribosomal proteins, RNA polymerase subunits and the translational initiation factor infA. A single copy of the accD gene, which encodes a subunit of acetyl-CoA carboxylase and has been implicated in adaptation to specific ecological niches during dicot radiation, is also present, along with conserved open reading frames ycf3 and ycf4. Sixteen genes carry introns, and all but ycf3 and accD follow the conventional two-exon, one-intron arrangement.

Codon usage patterns in the protein-coding sequences reveal the same asymmetries seen in many other plastomes. Across 40,013 codons, leucine is the most abundant amino acid at 10.477 percent, followed by isoleucine and glycine, while cysteine and tryptophan are the rarest. Relative synonymous codon usage analysis flagged 30 codons with clear preference, and 29 of those end in A or U rather than C or G. This A/U-ending bias, an outcome of mutation pressure and translational selection, matches the pattern previously reported for other flowering plant chloroplast genomes and provides useful context for anyone optimizing heterologous gene expression or reconstructing selective dynamics in Ericaceae plastids.

Diversity analysis across eleven fully sequenced Rhododendron plastomes identified the mutational hotspots that matter most for species-level discrimination. Nucleotide diversity values ranged from zero to 0.06896, with the small single-copy region the most variable compartment, the large single-copy region intermediate, and the inverted repeats the most conserved by a wide margin. Six gene regions exceeded the practical hotspot threshold of Pi greater than 0.02: trnI-GAU, trnG-UCC, rps3, rps12, trnV-UAC and trnK-UUU. Several of these, including matK relatives and rbcL-adjacent loci, echo the genes long used as DNA barcodes, but the newly flagged trn and ribosomal protein loci offer fresh candidates for markers tailored specifically to Rhododendron and its relatives.

Microsatellite scanning adds another layer of marker resources. The R. mariesii plastome harbors 70 simple sequence repeats, five of them in compound arrangements. The overwhelming majority, 65 repeats or roughly 93 percent, are mononucleotide motifs, and A/T repeats dominate at 91.4 percent, with C/G repeats nearly absent. Dinucleotide, trinucleotide and hexanucleotide motifs are present only as singletons or pairs, including one (AAGGGT/ACCCTT)5 hexamer. Most repeats sit in intergenic spacers, though 28 were found within the rpl16 gene and the remainder scattered across genes such as ccsA, cemA, ndhA, rpoA, rpoC2, rps7, rps8 and trnL-UAA. Compared with plastomes of other woody plants, the count is higher than in mango or Spondias bahiensis but far lower than in Syringa pinnatifolia, reinforcing that microsatellite density varies widely even among conserved plastid genomes.

Comparative alignment across the eleven Rhododendron plastomes showed high overall similarity with no rearrangements or inversions, confirming that structural conservatism is the rule in this genus. Coding regions were consistently more conserved than non-coding ones, and the most divergent coding loci, including rpoB, rpoC2, rps8, petD, rpl23, rpl22 and ndhF, owed their variability largely to intron content. The most variable stretches lie in intergenic spacers such as trnK-pafI, atpE-rpoB, trnT-rpl14, rpoA-psbJ, rpl20-trnE, ndhI-rps19 and rpl16-ndhI, regions that, under near-neutral evolution, accumulate substitutions with little interference from natural selection and therefore carry a legible record of lineage history.

The most striking structural finding concerns the inverted repeat boundaries. Even though overall gene order is stable, the edges of the IR expand and contract dramatically across the genus, and the authors argue this boundary drift is the main driver of plastome size differences among related Ericaceae. Rhododendron mariesii’s plastome, at 203,480 base pairs, is larger than those of R. pulchrum, R. simsii, R. molle, R. delavayi and R. platypodum, yet smaller than those of R. kawakamii, R. micranthum, R. henanense, R. griersonianum, R. concinnum and R. datiandingense. Junction mapping shows the LSC-IR border landing between different gene pairs in different species, with genes such as trnV, ycf15 and rps12 variously stretched across or excluded from the repeat, while the SSC-side boundary shifts the position of ndhF and rps15 relative to the repeats. These differences in gene content and placement at the junctions are precisely the kind of comparative signal that phylogeneticists exploit when reconstructing deep relationships.

Phylogenetic reconstruction pulled together 22 complete Ericaceae plastomes, including twelve Rhododendron species plus representatives of Vaccinium, Arbutus, Gaultheria and several mycoheterotrophic lineages, with Pyrola rotundifolia as an outgroup. Maximum-likelihood analysis under 1,000 bootstrap replicates placed all Ericaceae taxa in a single clade split into two subclades, a topology consistent with previously published work, and nestled R. mariesii closest to R. simsii and R. pulchrum. The species of Arbutus, Hemitomes, Allotropa, Monotropa, Pityopus, Vaccinium and Gaultheria branched as distant relatives, underscoring how much phylogenetic resolution a single well-assembled plastome can add at the genus and family level.

The practical stakes extend beyond systematics. Rhododendron mariesii occupies a critical place in the woodland ecology of the Dabie Mountains, but habitat fragmentation, booming Rhododendron-themed tourism and human activity are squeezing wild populations, and the species’ limited genomic resources have long hindered population genetic and conservation work. A complete, well-annotated plastome with a catalog of hotspots and microsatellites gives conservation geneticists immediately usable markers for phylogeography and population monitoring, and gives breeders a reference for tracing maternally inherited lineages in hybrid programs. More broadly, the study reinforces a now-familiar lesson of plastid genomics: chloroplast genomes evolve slowly and conservatively in structure, yet it is precisely their small set of labile boundaries and hotspot loci that carry the information needed to tell closely related species apart. For the heath family, this new reference is a foundation on which future barcoding, phylogenetic and ecological research across Ericaceae can be built.

Subject of Research: Complete chloroplast genome sequencing and comparative genomics of Rhododendron mariesii and related Ericaceae species

Article Title: Complete chloroplast genome sequence of Rhododendron mariesii and comparative genomics of related species in the family Ericaeae

Article References: Li, Z., Huang, Z., Wan, X., Yu, J., Dong, H., Zhang, J., Zhang, C., & Wang, S. (2023). Complete chloroplast genome sequence of Rhododendron mariesii and comparative genomics of related species in the family Ericaeae. Comparative Cytogenetics, 17, 163-180. https://doi.org/10.3897/compcytogen.17.101427

Image Credits: AI Generated

DOI: 10.3897/compcytogen.17.101427

Keywords: chloroplast genome, Rhododendron mariesii, Ericaceae, plastome, phylogenetics, DNA barcoding, microsatellites, nucleotide diversity, inverted repeats, codon usage, conservation genetics, Dabie Mountains