The Integrative Taxonomy and Mitochondrial Genome Evolution of Freshwater Planarians (Platyhelminthes: Tricladida): The Discovery of a New Clade in Southern China.

BACKGROUND

The genus (Platyhelminthes: Tricladida) includes a large diversity of free-living freshwater flatworms and is important for studies on regeneration and evolution. This study aims to describe a newly discovered asexual planarian species from southern China and explore its genetic characteristics and regenerative abilities.

METHODS

An integrative taxonomic analysis was conducted using morphology, karyology, histology, molecular phylogeny (18S, 28S, COI, mitogenome), and genome size estimation via flow cytometry. Regeneration was assessed by standardized amputations, and long-term asexual propagation was observed under laboratory conditions for three years.

RESULTS

Phylogenetic analyses using nuclear (18S, 28S rDNA) and mitochondrial (COI, mitogenome) markers confirmed that Guang Yang & Zhinan Yin, sp. nov. forms a distinct clade within . Its 18,125 bp mitogenome contains 36 genes but lacks atp8. displays a distinctive morphology, notably a pharynx located near the head. All body fragments regenerated into complete individuals within nine days. Remarkably, one individual produced ~10⁵ clonal descendants over three years via repeated amputation, maintaining stable regenerative ability and growth across generations. Karyological analysis revealed a diploid karyotype (2 = 16) consisting of eight chromosome pairs. The nuclear genome size was estimated at approximately 2.5 Gb using as an internal standard. Histological examination showed no detectable reproductive organs, confirming the species as an exclusively asexual lineage.

CONCLUSIONS

represents a new planarian strain with stable propagation and regeneration. These features make it a valuable resource for regenerative biology and comparative genomic studies.