Genetic variability does not support species delimitation within the Zannichellia palustris complex in temperate Eurasia

Biodiversity researches rely heavily on trustworthy species identification. Morphological variability within the Zannichellia palustris complex is quite low, which results in significant differences in the published diagnostic characteristics. Genetic variability of the complex in Eurasian scale remains unclear. Endemic species in the section Zannichellia also were described but their taxonomic status has not been confirmed genetically. Thus, we aimed to study variation of the proposed diagnostic traits and genetic variability (nuclear ribosomal DNA (ITS) and plastid DNA (trnH-psbA) markers) on mass material of the Z. palustris complex on the area-wide scale. The sampling primarily covered the former USSR; some samples from North America, Israel, Turkey, China, Mongolia, and West Europe were also studied. We did not manage to reveal any distinct morphotypes within the complex. There was also no morphological differentiation of haplotypes and ribotypes. Thus, we were not able to distinguish any species within Z. palustris complex. Endemic taxa from Kamchatka (Z. komarovii) and from the Lower Volga (Z. clausii) also were not differentiated genetically. Additional studies based on analysis of low-copy nuclear genes along with an assessment of the ploidy of the specimens is necessary to further clarify the taxonomy of the complex.