Glacial refugia and recolonization pathways in the brown seaweed Fucus serratus

Abstract

The last glacial maximum (20,000-18,000 years ago) dramatically affected extant distributions of virtually all northern European biota. Locations of refugia and postglacial recolonization pathways were examined in Fucus serratus (Heterokontophyta; Fucaceae) using a highly variable intergenic spacer developed from the complete mitochondrial genome of Fucus vesiculosus. Over 1,500 samples from the entire range of F. serratus were analysed using fluorescent single strand conformation polymorphism. A total of 28 mtDNA haplotypes was identified and sequenced. Three refugia were recognized based on high haplotype diversities and the presence of endemic haplotypes: southwest Ireland, the northern Brittany-Hurd Deep area of the English Channel, and the northwest Iberian Peninsula. The Irish refugium was the source for a recolonization sweep involving a single haplotype via northern Scotland and throughout Scandinavia, whereas recolonization from the Brittany-Hurd Deep refugium was more limited, probably because of unsuitable soft-bottom habitat in the Bay of Biscay and along the Belgian and Dutch coasts. The Iberian populations reflect a remnant refugium at the present-day southern boundary of the species range. A generalized skyline plot suggested exponential population expansion beginning in the mid-Pleistocene with maximal growth during the Eems interglacial 128,000-67,000 years ago, implying that the last glacial maximum mainly shaped population distributions rather than demography.

Evolution and diversification within the intertidal brown macroalgae Fucus spiralis/F. vesiculosus species complex in the North Atlantic

“…The smoothing parameter (e) was estimated using the 'maximize optimization' option. A mitochondrial divergence rate of 2.0-3.4%/My was used based on an mtDNA-IGS molecular clock estimated for Fucus (Hoarau et al, 2007b).…”

“…Thus, post-LGM re-colonization of F. vesiculosus likely commenced from areas with high mtDNA haplotype diversity, possibly SW Ireland and the Hurd Deep, that were previously identified as glacial refugia for a number of marine organisms (reviewed in Maggs et al, 2008). As the Hurd Deep was not reconnected to the open North Atlantic until 15,000-13,000 yrs ago (see references in Hoarau et al, 2007b), initial re-colonization either stemmed from the SW Ireland refugium or from another putative refugium in the Brittany region. Both of the dominant haplotypes (c2, c3) must have been present in SW Ireland and initially expanded north, as evident by their co-occurrence on the islands of Shetland and The Faroes.…”

“…Additionally, frequent establishment of successful reproductive hybrids has been noted Coyer et al, 2007). Phylogeography of species comprising Lineage 1 is better understood (Hoarau et al, 2007b;Coyer et al, in press), largely because F. serratus and F. distichus are older taxa and reinforcement has minimized hybridization (Hoarau et al, unpub. data).…”

“…However, recent work on a variety of marine benthic invertebrate and algal species has demonstrated the existence of northern ice-free glacial refugia near Brittany (Hurd Deep) and SW Ireland along the eastern Atlantic, and along the Canadian Maritimes in the western Atlantic (reviewed in Maggs et al, 2008;Olsen et al, 2010). Within Lineage 1, F. serratus also displayed signatures of glacial refugia in Brittany and SW Ireland (Coyer et al, 2003;Hoarau et al, 2007b) and a glacial refugium for the sister species F. distichus may be present off the Canadian Maritimes (Coyer et al, in press). …”

“…The study addresses mechanisms underlying divergence and speciation in this species complex. Accordingly, our aims were to: (1) identify phylogeographic trajectories within the F. spiralis complex, including the High and Low shore entities, and F. vesiculosus, (2) track signatures of hybridization; (3) identify possible refugia for both species; and (4) estimate the effects of the LGM on F. spiralis and F. vesiculosus relative to other species of Fucus, which have been strongly influenced by the LGM and the immediately prior Eemian interglacial period (Coyer et al, 2003(Coyer et al, , 2006aHoarau et al, 2007b).…”