Chloroplast genomes of raspberries and blackberries reveal family secrets and new ID markers

Raspberries and blackberries are among the most beloved fruits in the world, yet the family tree behind the supermarket punnet has long been tangled. The genus Rubus, a sprawling branch of the rose family, contains hundreds of species, countless hybrids, and a breeding history so interwoven that even specialists sometimes struggle to say exactly which plant gave rise to which cultivar. A new study published in Plant Cell Reports has now sequenced and compared dozens of complete chloroplast genomes from wild and cultivated Rubus, confirming the taxonomic placement of major cultivated groups and delivering a practical toolkit of DNA markers that can tell black raspberries, red raspberries, and raspberry-blackberry hybrids apart with a simple laboratory test.

The research team, led by Lichao Hu and Yaqiong Wu of Nanjing Forestry University and the Nanjing Botanical Garden, together with colleagues including Zefu Wang and Weilin Li, assembled 81 newly sequenced Rubus plastomes, drawing on 22 wild species and 59 cultivars. Chloroplast genomes, often called plastomes, are the circular DNA chromosomes housed inside the chloroplasts, the green organelles that perform photosynthesis. Because chloroplasts are inherited predominantly through the maternal line in most flowering plants, plastomes act like a genetic surname, allowing researchers to trace maternal ancestry even when centuries of hybridization have scrambled the nuclear genome.

Technically, the assembly pipeline followed the now-standard playbook for organelle genomics. High-quality reads were mapped and assembled de novo using tools such as GetOrganelle, structural verification was performed with visualization software including Bandage and CPGView, and annotations were checked against curated references. Every one of the 81 genomes displayed the canonical quadripartite architecture seen across most angiosperms: a large single-copy region and a small single-copy region separated by a pair of inverted repeats. Sizes ranged narrowly from 155,385 to 157,012 base pairs, and each plastome carried 130 annotated genes, corresponding to 113 unique functional genes once duplicates within the inverted repeats are counted once. That tight range is itself a finding, demonstrating that despite the morphological diversity of Rubus, from arching blackberry canes to upright red raspberry primocanes, the chloroplast chromosome has remained remarkably conservative in structure and gene order.

From the full set, the researchers distilled 16 representative cultivars whose plastomes captured every unique plastid sequence in the collection, and subjected these to a systematic comparative analysis. They catalogued repeat sequences, including simple sequence repeats and longer dispersed repeats, using tools such as MISA-web and REPuter. They quantified codon usage bias, the non-random preference for synonymous codons that reflects mutational pressures and translational selection in the plastid. And they scanned whole-genome alignments produced with MAFFT to locate hypervariable regions, the mutational hotspots where nucleotide diversity concentrates. Such hotspots, often found in intergenic spacers and in genes like matK that have long served as plant DNA barcodes, are the raw material from which species-discriminating markers are mined.

The phylogenetic analysis, performed with maximum-likelihood methods under frameworks such as RAxML-NG and IQ-TREE, placed the modern cultivars into three principal subgenera: Rubus, Idaeobatus, and Melanobatus. Red raspberries, as expected, fell within Idaeobatus, the subgenus of erect-caned raspberries, while blackberries clustered in subgenus Rubus. The more surprising result concerned black raspberry, the glossy, dark-fruited crop prized for its intense anthocyanin content: phylogenetically, it sits within Melanobatus rather than alongside its red namesake. For growers and breeders, this is more than a taxonomic footnote. It means that red and black raspberries, despite their culinary similarity, carry chloroplast lineages from distinct evolutionary branches, a divergence that shapes how their traits can be combined in breeding programs.

That divergence also created the opportunity for the study’s most applied contribution: molecular markers for species delimitation. By scanning the aligned plastomes for insertion-deletion polymorphisms, or InDels, stretches of DNA that are present in one lineage and absent in another, the team identified candidate sites that cleanly separate the major cultivated groups. From these candidates they developed and validated four InDel markers capable of distinguishing black raspberry, red raspberry, and raspberry-blackberry hybrids. In practice, such markers work like a genetic litmus test: a polymerase chain reaction assay produces fragments of diagnostic size depending on which plastid lineage a sample carries, and the pattern on a gel or capillary sequencer assigns the plant to its maternal group. Because InDels are codominant, easy to score, and highly reproducible across laboratories, they are among the most user-friendly marker types available for routine identification.

The practical value of this toolkit is hard to overstate. The nursery trade has wrestled with cultivar mislabeling for decades; as early as 1992, isoenzyme analysis was being used to detect mislabeled raspberry plants, and DNA fingerprinting later became the standard for verifying blackberry breeding lines. Misidentified stock propagates through nurseries, contaminates research collections, and undermines breeding decisions, particularly for raspberry-blackberry hybrids, whose intermediate morphology can fool even experienced eyes. A cheap, chloroplast-based assay that confirms whether a plant’s maternal parent was a black raspberry, a red raspberry, or a blackberry gives propagators, breeders, and gene bank curators a fast quality-control check that requires nothing more than a few milligrams of leaf tissue.

The study also matters for the future of hybrid breeding. Raspberry-blackberry hybrids, such as the loganberry and boysenberry lineages and modern primocane-fruiting hybrids, combine traits from across the genus, and breeders increasingly want to move disease resistance, fruit quality, and machine-harvestable architecture between subgenera. Knowing the maternal ancestry of every elite line, and having markers that verify it, helps breeders plan crosses that exploit cytoplasmic effects, including maternally inherited variation in photosynthesis and stress tolerance that plastid genomes can confer. The expanded plastome resource, with raw reads deposited in the Genome Sequence Archive under accession CRA043307 and assembled genomes available in GenBank under accession numbers PV528353 through PV528434, gives the global Rubus research community a reference panel that future studies of hybrid identification, cytoplasmic diversity, and domestication history can build upon immediately.

There is a broader evolutionary story here as well. Chloroplast genomes evolve slowly and mostly without recombination, which makes them ideal for reconstructing deep relationships, but it also means that a single plastome tree tells only the maternal side of a reticulate history. Rubus is notorious for apomixis, polyploidy, and interspecific hybridization, especially in the blackberries, where pseudogamous reproduction allows hybrid lineages to breed true while accumulating enormous nuclear diversity. The Melanobatus placement of black raspberry, and the clear separation of the three cultivated plastid lineages, provide a maternal scaffold onto which nuclear data can be layered, helping to disentangle which wild species contributed which genomes to modern cultivars. Comparative plastomics of this scale, following similar efforts in cotton, pea, and other crop genera, is rapidly turning the chloroplast into a workhorse for crop genealogy.

For a genus that most people encounter only as a summer treat, Rubus has now yielded a dataset of unusual completeness: 81 new plastomes, a resolved maternal framework for the major cultivated groups, a catalogue of repeats, codon preferences, and hypervariable regions, and four validated markers ready for deployment in nurseries and breeding stations. The work confirms that the existing taxonomic classification of cultivated Rubus holds up under genomic scrutiny, while giving breeders the molecular tools to keep that classification honest at the level of individual plants. As demand for berry crops grows and hybrid breeding accelerates, the humble chloroplast, inherited quietly from mother plant to seedling across generations of cultivation, has proven to be the most reliable witness to where these fruits really came from.

Subject of Research: Comparative chloroplast genome analysis of Rubus cultivars and development of InDel molecular markers for species delimitation

Article Title: Comparative analysis of plastomes of Rubus cultivars and development of molecular markers for species delimitation

Article References: Comparative analysis of plastomes of Rubus cultivars and development of molecular markers for species delimitation. (n.d.). https://doi.org/10.1007/s00299-026-04013-2

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04013-2

Keywords: Rubus, plastome, chloroplast genome, raspberry, blackberry, phylogenetic analysis, molecular markers, InDel, species delimitation, hybrid breeding, Rosaceae, Plant Cell Reports