Abstract:

Aims: Seaweeds are ecologically important foundation species in coastal marine waters. However, their ecosystem services are highly linked to levels of genetic diversity, distribution range, and ecological adaptation. This study aims to understand the complex genetic structure and biogeographic history of the brown macroalga Fucus distichus in the North Pacific, Northeast Atlantic, and Northwest Atlantic, including the identification of a marine glacial refugium in eastern Newfoundland, Canada, and to provide insights for conserving and managing seaweed resources under future global climate change.

Methods: We sampled five F. distichus populations from the Grand Banks of Newfoundland, and conducted PCR-based amplification and sequencing of two mitochondrial markers: 23S rRNA-tRNA-Val intergenic spacer (IGS) and COX1. By integrating these samples with molecular datasets published in 2011, we calculated the number of haplotypes, haplotype diversity (h) and nucleotide diversity (π) for each marker. We also constructed haplotype networks and evaluated phylogenetic affinities among haplotypes using maximum likelihood estimation and neighbour joining trees for each marker.

Results: IGS data showed that F. distichus populations from the Grand Banks each harbored 3-4 haplotypes of which most were private, whereas other populations from the North Pacific, Northeast Atlantic and Northwest Atlantic mostly had 1-2 haplotypes. In particular, the Grand Banks populations exhibited much higher haplotype (h = 0.6533) and nucleotide diversity (π = 0.0067) than other populations (h = 0.1487; π = 0.0022), with the highest genetic indices. Haplotype networks inferred from IGS and COX1 both showed that the ancestral haplotype was widely distributed in the Northeast Atlantic and Northwest Atlantic, including the Grand Banks. Phylogenetic trees further revealed a clear genetic divergence between the private haplotypes in the Grand Banks and others elsewhere. These phylogeographic results indicated that F. distichus populations on both sides of the North Atlantic experienced multiple large-scale extinction events due to sea-level fluctuations driven by glacial-interglacial cycles during the late Pleistocene. Afterwards, the surviving ancestor of F. distichus in the Arctic recolonized the Northeast Atlantic prior to the last glacial maximum, following with a trans-Atlantic migration from Europe to North America possibly during the Holocene. Our phylogeographic results also suggested that the Flemish Cap located to the east of the Grand Banks of Newfoundland, Canada was potentially a marine glacial refugium during the late Pleistocene ice ages.

Conclusion: Phylogeographic diversity patterns and processes can be influenced by various kinds of environmental factors. Adding geographically unique specimens such as isolated or ice-age survived populations during the paleoclimate change can largely expand our understanding of how species responded to historical environmental change, particularly the dynamic survival relics and dispersal routes associated with population diversification and speciation. These phylogeographic insights are also valuable for guiding natural resource conservation and management, and understanding of climate-driven ecological adaptation.

Key words: Fucus distichus, intergenic spacer, lineage diversity, last glacial maximum, sea-level fluctuation, climate change