Molecular Phylogenetics and Taxonomy of Planarian Flatworms

Planarian flatworms, a diverse assemblage of free-living Platyhelminthes, have long challenged taxonomists owing to their limited morphological features and prevalence of cryptic diversity. Over the past decade, molecular phylogenetics has transformed our understanding of their evolutionary relationships and taxonomy. By integrating mitochondrial genomes, nuclear ribosomal RNA genes and multi-marker datasets, researchers have uncovered deep divergences among major lineages, revealed repeated invasions of novel habitats and identified previously unrecognised species complexes. Advances in sequencing technologies now permit assembly of complete mitogenomes and paralogous rRNA clusters, while multilocus analyses employing SSU, LSU and protein-coding genes have refined clade boundaries from family to genus level. This molecular framework has driven taxonomic revisions across Tricladida, Geoplanidae and related groups, elucidated routes of global dispersal for invasive taxa and provided robust markers for ongoing biodiversity assessment and conservation planning.

Research from Nature Portfolio

Recent studies have combined short- and long-read sequencing to generate the first complete mitochondrial genome of a notorious invasive land flatworm. This work uncovered extensive tandem repeats, an unusually long cox2 gene and two paralogous nuclear rRNA gene clusters differing in length and copy number. Phylogenetic reconstruction using maximum likelihood methods realigned several species between tribes within the subfamily, revealing that some taxa traditionally placed in Caenoplanini instead group with Rhynchodemini. Consistent structural features across Rhynchodeminae mitogenomes were characterised, providing new synapomorphies for subfamily-level definitions and prompting reassessment of generic boundaries.

Molecular Phylogenetics and Taxonomy of Planarian Flatworms publication trend

The graph below shows the total number of articles in molecular phylogenetics and taxonomy of planarian flatworms across all publications each year (not limited to Nature Index journals).

Technical terms

Mitogenome: The complete mitochondrial DNA sequence of an organism, used for phylogenetic reconstruction.

Next-generation sequencing (NGS): High-throughput methods for rapid DNA sequencing, enabling whole-genome and mitogenome assembly.

Cytochrome c oxidase I (COI): A mitochondrial gene commonly used as a DNA barcode for species identification and phylogenetic analysis.

Small and large subunit ribosomal RNA (SSU, LSU): Conserved nuclear rRNA genes used as phylogenetic markers across broad taxonomic scales.

Maximum likelihood phylogeny: A statistical method for inferring evolutionary relationships by finding the tree that maximises the likelihood of observed molecular data.

Clade: A group of organisms that includes an ancestor and all its descendants, representing a single branch on the tree of life.

Cryptic species: Genetically distinct species that are morphologically very similar or identical, requiring molecular data for recognition.

References

1. The invasive land flatworm Arthurdendyus triangulatus has repeated sequences in the mitogenome, extra-long cox2 gene and paralogous nuclear rRNA clusters. Scientific Reports (2024).
2. Evolutionary Analysis of Mitogenomes from Parasitic and Free-Living Flatworms. PLOS ONE (2015).
3. Planarian (Platyhelminthes, Tricladida) Diversity and Molecular Markers: A New View of an Old Group. Diversity (2014).
4. Hammerhead flatworms (Platyhelminthes, Geoplanidae, Bipaliinae): mitochondrial genomes and description of two new species from France, Italy, and Mayotte. PeerJ (2022).
5. A new species of Dugesia (Platyhelminthes, Tricladida, Dugesiidae) from China, with an account on the histochemical structure of its major nervous system. Zoosystematics and Evolution (2020).