Identification of Hybrids in Potamogeton: Incongruence between Plastid and ITS Regions Solved by a Novel Barcoding Marker PHYB

Potamogeton is one of the most difficult groups to clarify in aquatic plants, which has an extensive range of interspecific morphological and ecological diversity. Internal transcribed spacer (ITS) is prevalent for phylogenetic analysis in plants. However, most researches demonstrate that ITS has a high percentage of homoplasy in phylogenetic datasets. In this study, eighteen materials were collected in Potamogeton from China and incongruence was shown between the rbcL and ITS phylogenies. To solve the discrepancy, we employed a novel barcode PHYB to improve resolution and accuracy of the phylogenetic relationships. The PHYB phylogeny successfully resolved the incongruence between the rbcL and ITS phylogenies. In addition, six hybrids were confirmed using PHYB, including P. compressus × P. pusillus, P. octandrus × P. oxyphyllus, P. gramineus × P. lucens, P. distinctus × P. natans, P. distinctus × P. wrightii, and S. pectinata × S. amblyophylla. Whereas, only one hybrid was identified (P. compressus × P. pusillus) by ITS, indicating that ITS homoplasy was present in Potamogeton and ITS was completely homogenized to one parental lineage. Thus, ITS might have limited utility for phylogenetic relationships in Potamogeton. It is recommended that a three-locus combination of chloroplast DNA gene, ITS and PHYB is potential to effectively reveal more robust phylogenetic relationships and species identification.