Phylogeographic patterns in attached and free-living marine macroalga Fucus vesiculosus (Fucaceae, Phaeophyceae) in the Baltic Sea

Abstract Sequencing of a mitochondrial intergenic spacer and 23S subunit was used to investigate the phylogeographic patterns in Fucus vesiculosus. Samples originated from 21 sites spanning six subbasins of the Baltic Sea. We identify a putative ancestral mitochondrial haplotype that entered the Baltic Sea from the Atlantic, colonising extensively throughout the species’ distribution. The dominance of this haplotype is seen in the low overall haplotype diversity (Hd = 0.29). Moreover, there is indication of few spatially aggregated patterns in the deeper demographic time scales (Fct = 0.040; Fst = 0.049). Tajima’s D (−0.685, p-value 0.297) and Fu’s FS (0.267, p-value 0.591) showed no significant signals of extreme demographic changes. The Baltic Sea free-living Fucus is confirmed as F. vesiculosus or a closely related species. Haplotype diversities are comparable between forms (attached Hd = 0.306; free-living Hd = 0.268). The relatively short temporal scale for colonisation alongside low variance in the Fucus mitochondrial genome results in a rather panmictic structure across the Baltic Sea. Our data suggest that the mitochondrial intergenic spacer and 23S poorly describe the evolutionary dynamics of Fucus spp. in such a young, postglacial environment, yet this concatenated-barcode advances our understanding of the colonisation dynamics of F. vesiculosus over deeper demographic timescales.