Comparison of mitochondrial DNA sequences points to the European populations as the direct origin of Undaria pinnatifida that has spread to the northern Wadden Sea

The increasing spread of Undaria pinnatifida has caused concern in many parts of the world in recent decades, and this alga has been listed as invasive in the introduced countries. Its most recent spread in Europe was reported on Sylt island in the northern Wadden sea (German Bight, Eastern North Sea) between 2016 and 2017. Its direct origin remained unknown. In the present study, we obtained the mitochondrial DNA sequences including the partial coding region of cox3 and intergenic noncoding loci tatC-tLeu, atp8-trnS and trnW-trnI from one drifting population and one attached population of U. pinnatifida on Sylt and compared them with the available sequences published in previous studies. For the concatenated sequences of atp8-trnS and trnW-trnI, two haplotypes (Up01 and Up03) were detected in Sylt populations with Up01 being the dominant haplotype, which was most similar to the haplotype composition identified previously in European populations. For the concatenated sequences of cox3 and tatC-tLeu, two haplotypes (H1 and H9) were found in Sylt populations and they were the same as those identified previously in Brittany, France. These results suggest that European populations were most likely the direct origin of the newly established U. pinnatifida population on Sylt. The combined use of these sequences will be a robust tool to infer the origins of newly established populations of this seaweed in the future.