First report of field occurrence of Myriophyllum rubricaule in Great Britain confirmed through molecular analyses

Abstract Background : The red-stemmed parrot’s feather, Myriophyllum rubricaule , was recently described as a new plant species following confusion over its identity and more broadly within the genus. In a molecular study using Myriophyllum samples collected as part of a classical biological control programme against M. aquaticum in 2021, two populations of a different species were found in Great Britain (GB). This species was thought likely to be M . rubricaule ; however, it had not been verified by molecular analysis, nor was this species reported to exist in the wild at that time. Methods : Sequence data from the rDNA-ITS, rbcL , trnH-psbA , trnK-matK ( trnK 5 intron, matK coding-region and trnK 3 intron) and trnL-trnF regions were compared between suspected M . rubricaule collected from GB and other Myriophyllum species, including M . rubricaule from the Netherlands (NL) and Chile. Phylogenetic analyses of the Myriophyllum samples using combined datasets of four regions (rDNA-ITS, rbcL , trnH-psbA and matK coding-region) and five regions (the rDNA-ITS, rbcL , trnH-psbA , trnK-matK and trnL-trnF ) were undertaken using a maximum likelihood method. Results : The sequencing revealed that the two field-collected samples and one from an aquatic plant nursery in GB were identical to M . rubricaule from NL. Phylogenetic analysis based on the four multilocus datasets showed the three GB samples grouped with M . rubricaule from NL and Chile and were clearly separated from M . aquaticum . Further phylogenetic analysis based on the five regions also showed that the three GB M . rubricaule were distinct from other Myriophyllum species, confirming their identification as M . rubricaule . Conclusion : This study has confirmed the first occurrence of M . rubricaule in the wild in GB through molecular identification. The potential invasive risk of this species given its apparent establishment at sites in GB and close relationship to M . aquaticum is also discussed.