The common brown alga Scytosiphon lomentaria (Scytosiphonaceae) has generally been regarded as a species complex. In the Pacific, molecular data have shown that it includes more than one species; the diversity in other oceans, however, has not been examined. In the present study, the genetic diversity of S. lomentaria was investigated in 57 samples from the north-east Atlantic (mainly from Ireland) and two from the Mediterranean, which were compared with seven samples from Japan. Phylogenetic analyses based on the mitochondrial cytochrome oxidase subunit 3 gene (cox3) and nuclear ribosomal internal transcribed spacers (ITS1 and 2) revealed four separate monophyletic groups. Clades A1 and A2 consisted of the samples from the north-east Atlantic; the Mediterranean samples also belonged to A1. The affiliation of four samples was incongruent between cox3 and ITS trees, suggesting directional mitochondrial introgression from A2 to A1. Clades P1 and P2 contained nine samples from the north-east Atlantic, i.e. three Irish, all Norwegian and Icelandic samples, as well as all Pacific samples; in these clades, the cox3 and ITS data were congruent. This is the first documented record of Pacific lineages of Scytosiphon in the north-east Atlantic. Our results indicate that at least four species are passing under the name S. lomentaria in the north-east Atlantic and the Mediterranean.
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