The research team, led by Ying Xue and Yunpeng Gai of Beijing Forestry University together with colleagues including Guy Smagghe of the Vrije Universiteit Brussel, focused on a natural experiment that has fascinated evolutionary biologists for decades. Within M. sativa subsp. falcata, both diploid individuals with 16 chromosomes and tetraploid individuals with 32 chromosomes occur in nature. Whole-genome duplication, or polyploidization, is one of the most dramatic events that can happen to a plant lineage, reshaping nuclear gene dosage, silencing duplicate genes, and often triggering sweeping changes in cell size and physiology. Whether and how these nuclear upheavals ripple outward into the chloroplast genome, however, has remained poorly resolved, particularly within a single subspecies where cytotypes can be directly compared.

Chloroplasts carry their own small circular genomes, typically around 120,000 to 160,000 base pairs in flowering plants, descended from free-living cyanobacteria that were engulfed by an ancestral eukaryotic cell more than a billion years ago. Because plastid genomes are maternally inherited in most legumes, relatively compact, and present in thousands of copies per cell, they are workhorses of plant evolutionary biology, used to trace lineages, identify species, and reconstruct geographic histories. The team assembled the complete chloroplast genome of the diploid falcata cytotype at 125,934 base pairs and the tetraploid cytotype at 125,770 base pairs, a difference of just 164 base pairs. Both genomes showed nearly identical guanine-cytosine content of approximately 33.80 percent, and gene counts differed only marginally between the two ploidy levels.

That near-identity is itself a finding. If polyploidization imposed strong selective pressure on the plastid compartment, one might expect shifts in genome size, gene content, or structural organization between cytotypes. Instead, the comparison revealed that the chloroplast genomes of diploid and tetraploid falcata are essentially interchangeable in size, composition, and architecture. The authors conclude that polyploidization exerts limited influence on chloroplast genome size, gene content, and structural organization, and that strong functional constraints on the plastid genome are preserved across ploidy levels. In other words, whatever turbulence whole-genome duplication stirs up in the nucleus, the chloroplast keeps its house in order.

Beyond the ploidy comparison, the study dug into the fine-grained mechanics of chloroplast protein evolution through codon usage bias, the phenomenon by which genes preferentially use certain synonymous codons over others. The genetic code is degenerate, meaning most amino acids are specified by multiple three-letter codons, and organisms are far from random in choosing among them. In the Medicago chloroplast genomes, the analysis revealed a dominant preference for codons ending in adenine or uracil, the A/U-ending codons characteristic of AT-rich plastid genomes. Crucially, the statistical signatures indicated that this bias is shaped primarily by natural selection rather than by mutation pressure or random drift alone, suggesting that translational efficiency and accuracy continue to exert measurable selective forces on chloroplast coding sequences.

The team also examined the evolutionary forces acting on individual protein-coding genes by calculating the ratio of nonsynonymous substitution rates to synonymous substitution rates, abbreviated Ka/Ks. A ratio below one indicates purifying selection, the process by which natural selection removes harmful mutations and preserves protein function; a ratio near or above one would suggest neutral evolution or positive selection. Across the Medicago chloroplast gene set, 77.1 percent of genes showed Ka/Ks values below 0.5, a hallmark of strong purifying selection. The constraint was especially pronounced in genes involved in photosynthesis and energy metabolism, the core functions that the chloroplast exists to perform. This deep conservation underscores how little tolerance the photosynthetic machinery has for change, even across a genus spanning dozens of species and millions of years of divergence.

Not every region of the chloroplast genome is locked down, however, and that variability is precisely what makes the plastome useful for breeding and identification. The researchers mapped simple sequence repeats, short tandemly repeated DNA motifs that mutate rapidly and are abundant in the falcata chloroplast genomes. Most of these SSR loci were mononucleotide repeats, runs of a single base such as poly-A or poly-T tracts, and the vast majority sat in non-coding regions rather than within genes. Because these hypervariable stretches evolve quickly while flanking sequences remain conserved, they are ideal raw material for developing chloroplast-derived molecular markers that can distinguish accessions, assess genetic diversity in germplasm collections, and fingerprint cultivars without the complications of nuclear heterozygosity.

To place falcata in its evolutionary context, the team reconstructed phylogenetic relationships across 30 Medicago species and performed collinearity analysis, which compares the order and orientation of genes along the chloroplast genome. Both approaches demonstrated high structural conservation among closely related species, with gene order rearrangements rare and localized. In the resulting trees, the diploid and tetraploid falcata chloroplast genomes clustered together with other M. sativa accessions, confirming the subspecies-level relationships inferred from morphology and nuclear data and reinforcing the view of falcata as the wild gene pool most closely allied to cultivated alfalfa. The tight clustering also validates chloroplast genomes as reliable barcodes for untangling relationships within a genus famous for taxonomic complexity, driven in part by autopolyploidy and hybridization.

The practical implications extend well beyond evolutionary theory. Yellow-flowered alfalfa is regarded as an essential genetic resource for forage improvement, biofuel production, and sustainable agriculture, and its superior cold tolerance and drought resistance make it suitable for cultivation in marginal and climate-stressed environments where common alfalfa fails. As global agriculture confronts hotter summers, colder snaps, and increasingly unreliable rainfall, the wild cytotypes of falcata represent a reservoir of adaptive alleles that breeders are eager to tap. The chloroplast genomic resources and candidate SSR loci delivered by this study can accelerate germplasm evaluation, phylogenetic inference, and marker-assisted breeding of Medicago forage cultivars, allowing breeders to track valuable cytoplasms through crossing programs and to certify the maternal lineage of elite hybrids.

The study also contributes to a broader scientific conversation about how organellar genomes respond to nuclear genome doubling. Because chloroplasts and mitochondria encode proteins that must interface with thousands of nuclear-encoded partners, polyploidization could in principle disrupt these co-adapted systems and force compensatory evolution in the organelle. The falcata comparison suggests that, at least for the chloroplast, such disruption is minimal: the plastome’s size, GC content, gene inventory, and codon preferences ride out whole-genome duplication essentially unchanged, buffered by intense purifying selection. For a crop genus that feeds livestock across the globe and anchors grassland agriculture from China to North America, that stability is good news. It means the chloroplast markers developed here should remain robust across ploidy levels, and it means the wild, cold-hardy cytoplasms of falcata can be introduced into breeding lines without unpredictable plastid side effects. As sequencing costs continue to fall, comparative plastomics of this kind is poised to become a routine tool in the effort to future-proof the world’s forage crops, one chloroplast genome at a time.

Subject of Research: Comparative chloroplast genome evolution and polyploidy in Medicago sativa subsp. falcata

Article Title: Comparative chloroplast genomics of Medicago: codon usage bias, purifying selection, and phylogenomics

Article References: Xue, Y., Liang, X., Zhao, W., Smagghe, G., Shen, L., Li, S., Wang, Z., Gao, F., Li, D., Zhang, T., & Gai, Y. (2026). Comparative chloroplast genomics of Medicago: codon usage bias, purifying selection, and phylogenomics. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10073-z

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10073-z

Keywords: Medicago, chloroplast genome, polyploidy, codon usage bias, purifying selection, simple sequence repeats, phylogenomics, alfalfa, forage breeding, plant molecular biology, genomics, Comparative

The chloroplast, the green organelle that powers photosynthesis, carries its own small circular genome that has become a workhorse of plant evolutionary biology. Because chloroplast DNA is abundant in leaf tissue, relatively easy to assemble, and evolves at a rate useful for resolving relationships among species and genera, complete plastomes have become the reference standard for plant phylogenetics. For commercially and ecologically dominant families such as Dipterocarpaceae, which anchor the lowland rainforests of Borneo, Sumatra and the Malay Peninsula, such genomic references have been strikingly incomplete. The new study closes one of those gaps.

The researchers worked from fresh leaf material collected at the Bogor Botanic Gardens in Indonesia, one of the oldest tropical botanical gardens in the world and an important ex situ refuge for dipterocarp diversity. DNA extracted from the leaves was sequenced on an Illumina platform, and the chloroplast genome was assembled de novo from the resulting reads. The team then annotated the genome, compared its structure with related dipterocarps, and placed it within a family-wide phylogenetic framework using modern maximum likelihood and Bayesian inference pipelines.

The resulting genome measures 152,067 base pairs and displays the classic quadripartite architecture seen across most flowering plants: a large single-copy region of 84,173 base pairs, a small single-copy region of 20,036 base pairs, and a pair of inverted repeats of 23,929 base pairs each that separate the two single-copy blocks. The overall guanine-cytosine content came in at 37.22 percent, a figure squarely within the range typical of angiosperm plastomes. In total, the team annotated 132 genes, comprising 87 protein-coding genes, 37 transfer RNA genes and 8 ribosomal RNA genes, a gene inventory consistent with the canonical plastid gene set of land plants.

Comparative analysis against other Dipterocarpaceae chloroplast genomes revealed a highly conserved genomic structure across the family. Gene order, gene content and the boundaries between the single-copy regions and the inverted repeats showed little variation, confirming that the plastome of P. lucida fits comfortably within the family’s established genomic blueprint. This structural conservatism is good news for comparative work: it means that variation among species is concentrated in sequence differences rather than large rearrangements, making the plastome a clean and interpretable signal for reconstructing evolutionary relationships.

That phylogenetic signal delivered a clear verdict. In the team’s analyses, Parashorea lucida fell squarely within the Parashorea clade, cleanly separated from Shorea, the largest and taxonomically most notoriously difficult genus in the family. The distinction matters because dipterocarp classification has been contested for decades, with morphological characters such as fruit calyx and flower anatomy producing conflicting groupings. Recent taxonomic revisions have begun to dissect the tribe Shoreae more rigorously, and complete plastomes like this one provide the kind of whole-genome evidence needed to test whether traditional genus boundaries reflect true evolutionary lineages.

Beyond the tree of life, the study delivered a practical bonus for conservation genetics. The team identified 80 simple sequence repeat loci, short tandemly repeated DNA motifs that are highly variable and easily typed, along with three highly variable intergenic regions. These markers give researchers a ready-made set of tools for probing population structure, gene flow and genetic diversity in wild and cultivated P. lucida populations. For a species listed as Near Threatened on the IUCN Red List, whose lowland forest habitat continues to be cleared and degraded across its native range, such markers can inform everything from seed collection strategies for restoration to the design of genetically representative protected areas.

The broader context gives the work its urgency. Dipterocarps dominate the canopy of Southeast Asian lowland rainforests and are famous for their irregular, mass-flowering and masting reproductive cycles, which make seed availability unpredictable and regeneration vulnerable to disturbance. Many family members are prized timber trees, and logging, land conversion and fire have pushed a large fraction of the roughly 500 species toward extinction risk. Recent genomic assessments of dipterocarps at the margins of Asian rainforests have documented small population sizes and substantial genomic erosion, underscoring how quickly genetic diversity can be lost even before a species disappears entirely. Reference genomes and molecular markers are the raw material for detecting and countering that erosion.

The study also fits into a wider effort led from Indonesia’s National Research and Innovation Agency, BRIN, which funded the work through its life sciences research organization. The project’s stated theme is the complete chloroplast genome sequencing of dipterocarp collections held in Indonesian botanic gardens that appear on the IUCN Red List, with the explicit aim of supporting conservation and sustainable use. By sequencing living collections that are already curated, documented and propagated, the researchers are effectively converting botanical garden accessions into genomic reference points, a strategy that multiplies the conservation value of ex situ collections. The assembled genome has been deposited in GenBank under accession number PX673255, with raw reads archived in the Sequence Read Archive, making the resource immediately usable by other laboratories.

What makes the achievement notable is how much conservation leverage a single 152,067-base-pair circle can provide. The genome settles the species’ phylogenetic home, confirms the structural stability of the family’s plastome, and hands practitioners a panel of variable markers, all from one sequencing run on garden-grown material. As the costs of sequencing continue to fall, the approach demonstrated here, pairing botanic garden collections with genome skimming and comparative plastomics, offers a scalable template for building a genomic safety net for the entire family. For Parashorea lucida and the rainforests it helps to build, the first complete chloroplast genome is less an endpoint than an opening move: the beginning of a genetic record that may prove essential to keeping the species, and the forests it defines, alive through the coming decades.

Subject of Research: Complete chloroplast genome sequencing and phylogenetic analysis of the Near Threatened dipterocarp tree Parashorea lucida

Article Title: First Complete Chloroplast Genome of Parashorea lucida (Miq.) Kurz (Dipterocarpaceae): Phylogenetic Resolution and Genomic Insights into a Native Species and Near Threatened Dipterocarp of Indonesian Lowland Rainforests

Article References: Febria, R., Priyadi, A., Pratiwi, R. A., Zulkarnaen, R. N., Andila, P. S., Prihatini, I., Rahayu, S., & Cao, Y. (2026). First Complete Chloroplast Genome of Parashorea lucida (Miq.) Kurz (Dipterocarpaceae): Phylogenetic Resolution and Genomic Insights into a Native Species and Near Threatened Dipterocarp of Indonesian Lowland Rainforests. Indian Journal of Genetics and Plant Breeding. https://doi.org/10.1007/s44489-026-00048-7

Image Credits: AI Generated

DOI: 10.1007/s44489-026-00048-7

Keywords: chloroplast genome, Parashorea lucida, Dipterocarpaceae, conservation genomics, plastome, phylogenetics, simple sequence repeats, Indonesia, lowland rainforest, IUCN Red List, genome assembly, botanic gardens

Mitochondria, the energy-producing organelles in every plant cell, carry their own small genomes, separate from the DNA in the nucleus and the chloroplast. In animals, these genomes are compact and predictable, but in plants they are famously unruly: enormous in size, structurally fluid, and prone to recombination that can reshape them from one generation to the next. That instability is precisely why plant mitochondrial genomes fascinate evolutionary biologists, and why they matter commercially, since they carry genes involved in fertility and other agronomically critical traits. Yet for all the attention paid to konjac chloroplasts and nuclear markers, the mitochondrial genomes of A. muelleri and A. krausei had never been sequenced and characterized, leaving a conspicuous gap in the genetic resources available to breeders and biologists working with the genus.

The team, led by corresponding authors Lifang Li and Lei Yu of the Yunnan Key Laboratory of Konjac Biology at Kunming University, sequenced the genomes, assembled them, and annotated their full complement of genes, then placed the two species in comparative and phylogenetic context against close relatives in the arum family. The scale of the assembled molecules alone is striking. The mitochondrial genome of A. muelleri measures 592,591 base pairs, while that of A. krausei reaches 593,535 base pairs, both arranged as circular molecules. To put those numbers in perspective, the entire human mitochondrial genome fits into fewer than 17,000 base pairs; the konjac mitochondrial genomes are roughly thirty-five times larger, a vivid illustration of how extravagantly plant mitochondrial DNA can expand over evolutionary time.

Size, however, is only the headline figure. The GC content, the proportion of the genome composed of the bases guanine and cytosine, came out at 45.90 percent for A. muelleri and 46.05 percent for A. krausei, values that sit within the range typical of flowering plant mitochondria and hint at broadly similar compositional pressures across the two species. More revealing is the gene inventory. A. muelleri carries 59 genes and A. krausei carries 62, with 55 genes shared between them. That core of 55 conserved genes represents the essential functional machinery of the mitochondrion, including the components of the respiratory chain and the ribosomal and transfer RNAs needed to build its proteins, while the differences between the two species illustrate how gene content can drift even between closely related plants.

Beneath the gene counts, the researchers dug into the fine texture of the two genomes, and it is here that the study becomes a treasure map for future work. They identified 28 high-frequency codons, codons used more often than expected, in both species, a signature of the shared translational preferences of the two mitochondrial systems. They also catalogued repetitive elements, the raw material of mitochondrial genome rearrangement: 91 dispersed repeats in A. muelleri against 125 in A. krausei, and 141 versus 148 simple sequence repeats, short tandemly repeated motifs that serve as convenient molecular markers for population studies and cultivar identification. Perhaps most intriguingly, the two species differ dramatically in one particular respect: the team found 43 RNA editing sites in A. muelleri but 220 in A. krausei, a fivefold difference that suggests the two species manage the post-transcriptional correction of their mitochondrial transcripts in notably different ways.

RNA editing is one of the stranger quirks of plant mitochondria. After a gene is transcribed from DNA into RNA, specific letters in the transcript are chemically altered, often converting a C to a U, so that the final protein differs from what the gene sequence alone would predict. Many of these edits are essential, restoring start codons or fixing amino acids that would otherwise render a protein nonfunctional. The stark contrast between 43 editing sites in one species and 220 in its close relative raises questions the study leaves open for future investigation: whether the difference reflects genuinely divergent editing machineries, different analytical sensitivities, or evolutionary turnover in the editing landscape itself. Either way, the catalogued sites provide a concrete dataset for anyone probing how konjac mitochondria maintain the integrity of their proteins.

To understand how these genomes fit into the broader evolutionary picture, the researchers compared the coding sequences of mitochondrial genomes from eight species across the Araceae family, identifying 33 shared sequences that could be aligned across the group. They then applied a classic evolutionary yardstick, the ratio of non-synonymous to synonymous substitution rates, known as Ka/Ks. Non-synonymous substitutions change the amino acid sequence of a protein, while synonymous ones do not; when the ratio of the former to the latter falls below 1, it signals that natural selection is actively weeding out protein-altering changes, preserving function. For most of the shared protein-coding genes, the Ka/Ks ratios were indeed below 1, indicating that these genes have been under purifying selection and that their protein functions have remained remarkably conserved across the family’s evolution. In other words, even as the mitochondrial genomes ballooned in size and shuffled their structures, the proteins they encode have stayed stubbornly faithful to their ancient roles.

The phylogenetic analysis, built on this comparative data, delivered a clear verdict on relationships within the genus: A. muelleri is closely related to both A. krausei and A. albus. That placement matters for more than taxonomy. Konjac breeders seeking to improve glucomannan yield, disease resistance, or tuber traits rely on knowing which species are genetically compatible and which traits can be moved between them. A well-resolved tree anchored in organellar genome data gives breeders and conservationists alike a firmer framework for choosing parents in crossing programs and for prioritizing which wild relatives to preserve as genetic resources.

The practical significance of the work extends beyond the laboratory. Amorphophallus tubers are rich in glucomannan, the soluble dietary fiber that gives konjac foods their characteristic texture and that is increasingly studied for its effects on satiety, blood sugar, and cholesterol. Yunnan Province, where the research team is based, is a center of konjac cultivation and of the wild diversity of the genus, and the study’s funding, which includes support from the Yunnan Provincial Science and Technology Department and the National Natural Science Foundation of China, reflects the region’s investment in konjac biology. Complete mitochondrial genomes add a new class of molecular markers, from the simple sequence repeats to the species-specific gene complements, that can be used to authenticate cultivars, trace germplasm, and monitor genetic diversity in breeding programs, all of which help protect a crop whose supply chains depend on a relatively narrow genetic base.

For the wider scientific community, the study is a reminder of how much fundamental biology remains hidden inside familiar crops. Two species grown and processed for centuries have only now had one of their three genomes read in full, and the reading has already surfaced surprises: genomes approaching six hundred thousand base pairs, a fivefold difference in RNA editing between close relatives, and dozens of repeats poised to drive future rearrangements. The newly assembled mitochondrial blueprints of A. muelleri and A. krausei now stand as a foundation for research into the molecular diversity, genetic evolution, and cultivation of the konjac plants, and they set the stage for the next round of questions, from the mechanisms behind their RNA editing disparity to the structural dynamics that shape the largest genomes in their cells. For a genus that quietly underpins a global food and health industry, that is a significant step out of the genomic dark.

Subject of Research: Complete mitochondrial genome assembly and comparative analysis of two Amorphophallus species

Article Title: Characterization and comparative analysis of the complete mitochondrial genomes of two Amorphophallus species (Araceae)

Article References: Shi, H., Yang, M., Qi, Y., Gao, P., Zhao, Y., Guo, J., Huang, F., Liu, J., Zhao, J., Li, L., & Yu, L. (2026). Characterization and comparative analysis of the complete mitochondrial genomes of two Amorphophallus species (Araceae). BMC Genomics. https://doi.org/10.1186/s12864-026-13344-z

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13344-z

Keywords: Amorphophallus, mitochondrial genome, konjac, Araceae, genomics, RNA editing, purifying selection, phylogenetics, simple sequence repeats, glucomannan, genome assembly, plant evolution

Siberian elm, Ulmus pumila, is a botanical workhorse. It tolerates cold, drought, and salt-alkali soils that would kill most other trees, which makes it a prime candidate for ecological restoration in arid and degraded landscapes. Its wood is hard, fine-grained, and wear-resistant, prized for furniture and construction, while its bark and leaves contain flavonoids and other compounds with documented medicinal and industrial potential—the phloem even functions as a natural plant adhesive. Because the species is so useful, breeders have generated a wealth of cultivars selected from seedling progenies, each with subtle differences in crown shape, branch angle, leaf texture, and bark color. The problem is that these morphological differences are so subtle that traditional identification is prone to confusion, and the standard universal DNA barcodes lack the resolution to separate cultivated varieties that share such a recent common origin.

Chloroplast genomes have long been the molecular detective’s favorite tool in plant science. Unlike the nuclear genome, plastid DNA is typically inherited from only one parent—in elms, the mother—mutates at a moderate rate, and exists in a highly conserved structural framework that makes sequencing and comparison straightforward. Most land plant chloroplast genomes adopt a quadripartite architecture: a large single-copy region, a small single-copy region, and a pair of inverted repeats that mirror each other and bracket the single-copy sections. This architecture is so stable across species that deviations from it often signal meaningful evolutionary events. For the five elm cultivars, the team used a dual-sequencing strategy, combining short Illumina reads with long PacBio Sequel II reads, assembling the genomes with GetOrganelle, and verifying long-read alignments with BLASR before annotating genes with GeSeq.

The results revealed genomes that are both reassuringly familiar and quietly surprising. Total lengths ranged from 159,391 base pairs in ‘Yangguang Nühai’ to 159,769 base pairs in ‘Yangguang Nanhai,’ with guanine-cytosine content hovering between 35.47 and 35.56 percent—figures squarely within the norm for flowering plants. Each genome contained the expected LSC region of roughly 88,000 base pairs, an SSC region of about 18,800 base pairs, and inverted repeats of approximately 26,300 base pairs. But here came the twist: the number of annotated genes differed among cultivars, ranging from 131 to 133. The culprit was not some dramatic genomic upheaval but the pseudogenization or partial deletion of a single transfer RNA gene in the large single-copy region—a known form of intraspecific structural polymorphism in plant plastomes, yet one that had never been documented at this fine a scale within Ulmus pumila.

Beyond gene counts, the team cataloged an arsenal of repetitive elements that could serve as genetic fingerprints. Between 154 and 164 simple sequence repeats—short tandem motifs of DNA—were identified across the six cultivars analyzed, including a reference cultivar named ‘Jinmi’ retrieved from public databases. Mononucleotide repeats dominated overwhelmingly, with adenine and thymine bases showing a clear preference, and the number of repeats declined steadily as the repeat unit length increased. Some repeats were strikingly cultivar-specific: the pentanucleotide motif TTATT appeared only in ‘Yangguang Nühai’ and ‘Xu Ri,’ while the hexanucleotide TGGTAG was confined to the same two cultivars. Long repeats of thirty base pairs or more, detected with the REPuter software, ranged from 96 to 135 per cultivar within U. pumila, with forward and palindromic repeats predominating—sequences known to drive genome rearrangement and nucleotide variation over evolutionary time.

Comparative analysis across twenty-two Ulmus chloroplast genomes, visualized with mVISTA, confirmed that the genus maintains a deeply conserved overall structure, with the four ribosomal RNA genes showing near-perfect conservation. Variable sites clustered in the single-copy regions, and non-coding stretches proved far more variable than coding sequences. Nucleotide diversity analysis with DnaSP quantified this pattern: within U. pumila cultivars, overall diversity values stayed below 0.01, reflecting their shared maternal origin, yet several intergenic spacers—most notably trnH-GUG_psbA and ndhF_rpl32—showed polymorphism levels exceeding those of the universal DNA barcodes that have long frustrated elm taxonomists. At the interspecific level, diversity climbed as high as 0.05, with hotspots concentrated in the LSC and SSC regions while the inverted repeats remained quiet. The ycf1 gene, straddling the IR-SSC boundary with a diversity value of 0.035, emerged as a particularly promising core barcode for distinguishing elm species and cultivars at low taxonomic levels.

The boundary regions between the single-copy sections and the inverted repeats told their own evolutionary story. Across the broader Ulmaceae family, whose chloroplast genomes range from 157,356 base pairs in Trema levigatum to 160,388 in Ulmus lamellosa, genes such as rps19, rpl2, ycf1, and ndhF sit at different distances from the junctions, reflecting ancient expansion and contraction events that reshaped plastid architecture over millions of years. Within the five U. pumila cultivars, however, boundary shifts were confined to fluctuations of less than fifty base pairs—evidence of intraspecific genetic polymorphism rather than major evolutionary restructuring. These subtle variations, the authors note, carry limited evolutionary weight but can still function as auxiliary molecular features for cultivar differentiation.

Codon usage analysis added another layer of conservation to the picture. Across all twenty-two Ulmus taxa examined, leucine codons were used most frequently and cysteine codons least often, with methionine and tryptophan locked into single synonymous options. Preferential codons—those with relative synonymous codon usage values above 1.00—overwhelmingly ended in adenine or uracil, a hallmark of land plant plastid genomes. Effective number of codons values fell in the medium-high range, indicating weak overall codon bias, which the researchers interpret as evidence of strong purifying selection shaping these genomes. That interpretation was reinforced by substitution rate analysis: every shared protein-coding gene showed a ratio of non-synonymous to synonymous substitutions below 1, with values concentrated between 0 and 0.05, confirming that elm plastid genes are under intense selective constraint and evolve slowly enough to serve as reliable phylogenetic markers.

Phylogenetic trees built from complete chloroplast genomes of twenty-eight Ulmaceae species, using both maximum likelihood and Bayesian methods, produced strikingly concordant topologies. Ulmus emerged as a monophyletic group divided into three main clades, with Ulmus elongata occupying a basal, early-diverging position. The five cultivars clustered tightly together within the U. pumila clade, clearly separated from other elm species and only subtly differentiated from wild accessions—a pattern consistent with their shared maternal genetic background and their derivation from seedling selections of the same species. Collinearity analysis showed dense homologous gene connections across genomes, with only a few local rearrangements attributable to inversions or microdeletions, underscoring the structural stability of the elm plastome.

The practical implications extend well beyond evolutionary curiosity. Because chloroplast DNA is maternally inherited, the hypervariable markers identified here can trace maternal lineages and verify germplasm purity, but they cannot detect nuclear hybridization or introgression—events that are common in elm breeding programs. The authors therefore recommend pairing these plastid markers with nuclear tools such as genomic SSRs or SNPs for comprehensive cultivar authentication. Even with that caveat, the study delivers something elm science has lacked: cultivar-level molecular markers that work regardless of the tree’s growth stage, independent of the subjective judgment of crown shape or bark texture. For a species destined to anchor ecological restoration projects across arid and saline-alkali landscapes, and for breeders racing to develop stress-tolerant varieties, the ability to fingerprint germplasm with a handful of hypervariable chloroplast regions transforms both cultivar protection and molecular-assisted breeding. Five nearly identical trees, it turns out, were hiding five distinguishable genomic stories all along.

Subject of Research: Comparative chloroplast genomics and germplasm authentication of five Ulmus pumila cultivars

Article Title: Complete Chloroplast Genomes of Five Ulmus pumila Cultivars: Comparative Analysis, Evolutionary Dynamics, and Germplasm Authentication Implications

Article References: Wang, N., Yu, A., Ye, X., Zhang, L., Feng, S., Jiang, Z., Liu, C., Li, Y., & Huang, Y. (2026). Complete Chloroplast Genomes of Five Ulmus pumila Cultivars: Comparative Analysis, Evolutionary Dynamics, and Germplasm Authentication Implications. Ecology and Evolution, 16(10), Article e74352. https://doi.org/10.1002/ece3.74352

Image Credits: AI Generated

DOI: 10.1002/ece3.74352

Keywords: chloroplast genome, Ulmus pumila, Siberian elm, cultivar authentication, DNA barcoding, simple sequence repeats, nucleotide diversity, phylogenetics, purifying selection, inverted repeats, germplasm, molecular markers

Chloroplasts, the organelles that convert sunlight into the chemical energy sustaining nearly all life on Earth, descend from a cyanobacterial ancestor engulfed roughly one to one and a half billion years ago. Their genomes retain a semi-autonomous, circular architecture with a large single-copy region, a small single-copy region, and two inverted repeats that help stabilize the molecule through homologous recombination. Within these genomes, codon usage bias, the nonrandom use of synonymous codons, reflects the interplay of natural selection, mutation pressure, and genetic drift. Because synonymous codons encode the same amino acid, their frequencies offer a sensitive record of the evolutionary forces operating on a genome, from translational efficiency demands to directional mutational biases. Studying codon usage in a basal lycophyte like Isoetes therefore provides a rare window into how these forces have balanced each other across hundreds of millions of years of vascular plant evolution.

The research team assembled complete chloroplast genome sequences from 60 Isoetes species drawn from public databases, reannotating each sequence with reference-guided pipelines and manually verifying gene structures to ensure comparability. Despite the genus spanning continents and ecological niches, the plastomes proved remarkably uniform in size, ranging from 142,880 base pairs in Isoetes harleyi to 145,535 base pairs in Isoetes malinverniana. Overall GC content hovered near 37.95 percent with minimal interspecific variation, confirming the strongly AT-rich character of these genomes. Positional analysis of codons sharpened the picture: GC content was highest at first codon positions, intermediate at second positions, and dramatically lowest at third positions, the sites most sensitive to synonymous substitution. The effective number of codons averaged 47.67, approaching the theoretical maximum of 61 and indicating that codon usage bias across the genus is, in genome-wide terms, relatively weak.

The relative synonymous codon usage analysis added a strikingly consistent signature. Across all 60 species, every codon with an elevated frequency, indicating preferential use, terminated in either A or U, mirroring the AT-rich genomic background. The leucine codon UUA stood out as strongly enriched, while the alanine codon GCU showed elevated use despite containing guanine and cytosine at its first two positions, hinting that factors beyond simple third-position nucleotide bias, such as translational considerations or functional constraints, may influence preference for certain amino acids. The fact that these high-frequency codons were conserved across the entire genus suggests that synonymous codon choice is evolutionarily stable in Isoetes, a stability that underscores how deeply conserved the compositional fabric of these plastomes has remained since the lineage’s ancient origin.

Simple sequence repeats, the short tandemly repeated motifs scattered through genomes, told a complementary story. The team identified 5,084 SSR loci across the 60 plastomes, with mononucleotide repeats dominating at nearly 45 percent of all loci and dinucleotide repeats adding another fifth. This prevalence of short, A/T-rich motifs aligns with the AT-biased mutational character of the genome and points to replication slippage as a plausible engine of fine-scale variation. Crucially, most SSRs clustered in noncoding regions rather than within protein-coding sequences, a distribution consistent with weaker functional tolerance for repeat length changes in coding regions, where an insertion or deletion could disrupt a reading frame. The coexistence of broadly shared repeat motifs with species-specific ones offers a practical dividend: these variable loci can serve as molecular markers for distinguishing closely related quillwort species, tracing population structure, and guiding conservation of endangered taxa.

The inverted repeat boundaries, long recognized as dynamic elements of plastome evolution, proved strikingly conserved within Isoetes. Pseudogenes ycf1 and ycf2 were consistently retained near the small single-copy junction, with only minor boundary shifts observed in a handful of species such as Isoetes yunguiensis and Isoetes japonica. By contrast, the closely related spike-moss Selaginella showed far more dramatic boundary dynamics, with substantial expansions and gene duplications. This contrast suggests fundamentally different strategies of chloroplast genome organization between the two lycophyte lineages, with Isoetes adhering to an exceptionally conservative architectural template even as its relatives restructured their plastomes more freely.

The most analytically revealing results came from comparing observed codon usage against theoretical expectations. Using the effective number of codons plotted against GC content at synonymous third positions, the researchers evaluated 4,523 gene records across the genus. More than 61 percent fell below Wright’s expected curve, and 22.91 percent deviated markedly, meaning their codon usage was more biased than mutational composition alone could explain. These marked deviations were not randomly distributed. Photosystem II genes accounted for nearly half of all marked deviations, followed by large ribosomal protein genes, photosystem I genes, and small ribosomal protein genes. In contrast, NADH dehydrogenase genes, RNA polymerase genes, and the RuBisCO large subunit showed no marked deviations. Parity rule 2 analysis reinforced the mutational signal, revealing systematic T-over-A and G-over-C preferences at third positions, while neutrality plots showed regression slopes near zero, indicating that third-position composition varies almost independently of the functionally constrained first and second positions.

Selection pressure analysis at the protein level confirmed pervasive purifying selection across the plastome. Nearly all chloroplast genes displayed ratios of nonsynonymous to synonymous substitution rates well below one, demonstrating that deleterious mutations are being efficiently weeded out. Core photosynthetic genes such as psaB, psbC, and psbD showed extremely low ratios, generally between 0.08 and 0.09, reflecting the intense functional conservation of the photosynthetic machinery, while rbcL, encoding the carbon-fixing enzyme RuBisCO, showed a median ratio of just 0.15. Yet a handful of NADH dehydrogenase complex genes, most notably ndhC, displayed elevated ratios, with a mean of 1.81 and more than half of comparisons exceeding one. The authors interpret these signals cautiously, noting that elevated values may reflect relaxed selective constraints, lineage-specific rate acceleration, or functional divergence rather than definitive adaptive evolution, but the pattern marks the NDH complex as the most dynamic corner of an otherwise deeply conservative genome.

The broader implications reach beyond evolutionary theory into urgent conservation practice. Many Isoetes species are threatened worldwide by habitat destruction, eutrophication, and pollution; in China, every known species enjoys first-grade national protection. The hypervariable loci identified here, including atpF, ycf1, ccsA, matK, and ndhF, together with species-specific SSR markers, provide a toolkit for phylogeographic reconstruction and population monitoring of these critically endangered plants. The study’s hypothesis-driven framework also clarifies a conceptual model for basal vascular plants: genome-wide mutational bias sets the compositional stage, gene-specific constraints concentrate in essential functional modules such as the photosystems and ribosomes, and localized sequence divergence accumulates in noncoding hotspots without disturbing the overall architecture. Whether these codon usage patterns carry adaptive significance in the fluctuating wetland environments quillworts inhabit, some of which demand remarkable physiological feats like the crassulacean acid metabolism evolved by Isoetes howellii, remains an open question. Testing it will require integrating ecological data, transcriptomics, and plastid translational profiles with the genomic baseline this study has now established for one of Earth’s oldest living plant lineages.

Subject of Research: Codon usage bias and chloroplast genome evolution in the ancient aquatic lycophyte genus Isoetes

Article Title: Mutation Pressure and Gene‐Specific Selection Shape Codon Usage Bias in the Chloroplast Genomes of Isoetes, an Ancient Aquatic Vascular Plant Lineage

Article References: Li, B., Xue, Y., Zhang, D., Smagghe, G., Liang, S., Wang, S., Ge, Y., Aimaitijiang, Z., & Gai, Y. (2026). Mutation Pressure and Gene‐Specific Selection Shape Codon Usage Bias in the Chloroplast Genomes of Isoetes , an Ancient Aquatic Vascular Plant Lineage. Ecology and Evolution, 16(9), Article e74216. https://doi.org/10.1002/ece3.74216

Image Credits: AI Generated

DOI: 10.1002/ece3.74216

Keywords: Isoetes, chloroplast genome, codon usage bias, mutation pressure, lycophyte, purifying selection, plastome evolution, simple sequence repeats, photosystem genes, conservation genomics, living fossil, wetland plants