Dioctophyme renale holds a special place among parasitic worms. Commonly called the giant kidney worm, it is the largest nematode known to parasitize humans and other mammals, including dogs, minks, and various wild carnivores. Its life cycle involves aquatic annelid worms as first intermediate hosts, fish and amphibians as paratenic hosts that carry the larvae forward, and ultimately mammals, where adult worms lodge in the kidneys and cause dioctophymiasis, a disease in which the parasite can consume the entire renal parenchyma, leaving a nonfunctioning organ. Despite its considerable medical, veterinary, and biological importance, genomic resources for this species have been strikingly limited. Before this study, researchers had access only to partial sequences of mitochondrial genes and the nuclear 18S ribosomal RNA gene, which was enough to place D. renale within Clade I of the nematode tree of life, alongside medically significant parasites such as Trichuris, the whipworm, and Trichinella, the cause of trichinellosis, but far too little to support deeper studies of its evolution, population structure, or molecular biology.

Mitochondrial genomes have long been workhorses in evolutionary biology. Because they are compact, maternally inherited, and present in many copies per cell, they are comparatively easy to sequence and are widely used for phylogenetic reconstruction, biogeographic analysis, and population genetics. In nematodes, the standard mitochondrial genome architecture is a single circular chromosome, typically encoding twelve protein-coding genes, twenty-two transfer RNAs, and two ribosomal RNAs. Deviations from this arrangement exist: some nematodes have lost atp8 or rearranged their gene orders, and in other groups of animals, particularly certain plants, fungi, and a scattering of metazoans, mitochondrial genomes are multipartite, distributed across multiple small chromosomes sometimes called minichromosomes. Until now, however, no vertebrate-parasitic nematode had been shown to carry such a fragmented configuration, making the new finding a genuine anomaly within a well-studied phylum.

The research team, led by corresponding author Laura Kamenetzky and including co-first authors Natalia Macchiaroli and Agustin Baricalla, set out to close the genomic gap by sequencing the D. renale mitogenome using genome skimming, an approach in which a whole-genome sequencing library is generated and the low-cost, shallow sequencing output is computationally mined for the organellar genomes embedded within it. Crucially, the researchers paired this with Oxford Nanopore long-read sequencing, which produces reads long enough to span entire mitochondrial chromosomes rather than just fragments of them. This combination proved decisive. Short reads alone could have assembled the mitochondrial sequences, but they would have left open the possibility that the fragmented appearance of the genome was an assembly artifact, with pieces of a single circular chromosome appearing disconnected because reads failed to bridge intervening regions. Long reads, assembled independently from separate library constructions, allowed the team to confirm that each circular molecule was real and intact.

What emerged from the assembly was striking. The mitochondrial genome of Dioctophyme renale is organized into eleven circular molecules, and each of these carries exactly one protein-coding gene together with an associated non-coding region. This one-gene-per-chromosome arrangement is a hallmark of multipartite mitogenomes found in certain other animals, such as some clams and related bivalve lineages and certain parasitic lice, but it had never before been demonstrated in a nematode that parasitizes vertebrates. The fact that the same configuration appeared repeatedly, from independent libraries and with long-read support, indicates that this multipartite architecture is not a rare variant or a sequencing anomaly but the dominant, presumably functional, organization of the mitochondrial genome in this species.

With the structure established, the team turned to comparative and phylogenetic analyses to ask how D. renale fits into the broader evolutionary picture. When they examined the sequences of the mitochondrial protein-coding genes, the picture was conventional: D. renale clustered confidently with other vertebrate-parasitic nematodes within Clade I, consistent with the earlier placement based on 18S rRNA. The fragmented genome, however, did not fit into any tidy pattern of shared ancestry. In other words, the genes say the species belongs where taxonomists thought it did, but the architecture of its mitochondrial genome appears to have evolved independently, a case of structural convergence or novel innovation rather than an inherited trait from a multipartite ancestor. This separation between sequence-level phylogeny and genome-level structure is one of the most intriguing aspects of the finding, because it suggests that the forces driving mitochondrial genome fragmentation operate independently of the lineage-level relationships that the genes themselves record.

Why would a parasite’s mitochondrial genome shatter into eleven pieces? At present, the answer is unknown, and the authors are appropriately cautious about overinterpreting their result. In their conclusions, they note that the functional implications of a multipartite mitogenome in D. renale remain to be determined, and they propose that future work should examine whether specific life-history traits or selective pressures associated with parasitism may have contributed to its emergence. It is a question with rich possibilities. The giant kidney worm passes through multiple hosts across its life cycle, endures aquatic environmental stages, and reaches an extraordinary adult size, any of which could impose unusual metabolic or replicative demands on its mitochondria. Multipartite genomes also change the dynamics of mitochondrial inheritance and recombination, and in some other organisms fragmentation has been linked to altered mutation rates, gene expression patterns, or tolerance of genomic rearrangement. Testing these ideas in D. renale will require functional studies that genome sequencing alone cannot deliver.

The study’s significance extends beyond one remarkable worm. Vast swaths of the parasitic nematode tree of life remain genomic dark matter, with no mitogenome sequences available for entire families and orders, particularly among parasites of wildlife and rare or difficult-to-collect species. Obtaining fresh tissue, extracting high molecular weight DNA, and sequencing entire genomes is often impractical for such organisms. The workflow the researchers present, a genome-skimming pipeline built around long-read sequencing, offers a practical template for other laboratories working with non-model organisms. Because skimming does not require a complete, high-coverage genome project, it can extract complete organellar genomes from modest amounts of sequencing data, and the long-read component resolves the very structural questions, circularity, fragmentation, and chromosome number, that short-read assemblies routinely obscure. The authors explicitly frame their approach as a strategy to expand mitogenomic resources in taxonomic groups that remain markedly underrepresented, and the D. renale result demonstrates the payoff: a discovery that would likely have been missed entirely with a short-read-only approach.

There are also implications for medicine and parasitology. Dioctophymiasis is rare in humans but serious when it occurs, and the parasite remains a persistent problem in domestic and wild carnivores across parts of the Americas and elsewhere. Mitochondrial genomes are the standard toolkit for species identification, surveillance, and population studies in parasitology, used to trace transmission routes, distinguish cryptic species, and monitor epidemiological patterns. A complete, reference-quality mitogenome for D. renale gives diagnostic and evolutionary work a foundation it previously lacked, and the revelation of its multipartite structure adds a caution for the field: mitochondrial genome organization cannot be assumed to follow the nematode textbook model, and surveillance markers should be chosen with the full architecture in mind.

The research was carried out by a consortium of Argentine institutions, with authors affiliated with the Universidad de Buenos Aires, CONICET, the Universidad Nacional de San Luis, and the Universidad Nacional de La Plata, and it was funded by Argentina’s National Agency for Science and Technology Promotion through projects PICT 2020 0513, PICT 2021 018, and PICT 2021-I-INVI-00795. The study was published open access in BMC Genomics, accepted on 2 September 2026 and released as a citable, peer-reviewed article with a permanent DOI on 5 September 2026, with a final Version of Record to follow.

For now, the giant kidney worm has delivered something rare: a mitochondrial genome that breaks the mold of its entire phylum’s vertebrate parasites, recovered through a method other researchers can replicate cheaply, and posing questions, about how eleven minichromosomes are replicated, segregated, and maintained inside a single cell, and what evolutionary pressures carved a genome apart, that will keep scientists busy for years. As sequencing technology continues to descend in cost and long reads become routine, the parasitic nematodes hiding further structural surprises may not stay hidden for long.