Abstract
Glaciation‐induced environmental changes during the last glacial maximum (LGM) have strongly influenced species' distributions and genetic diversity patterns in the northern high latitudes. However, these effects have seldom been assessed on sessile species in the Northwest Pacific. Herein, we chose the brown alga Sargassum thunbergii to test this hypothesis, by comparing present population genetic variability with inferred geographical range shifts from the LGM to the present, estimated with species distribution modelling (SDM). Projections for contrasting scenarios of future climate change were also developed to anticipate genetic diversity losses at regional scales. Results showed that S. thunbergii harbours strikingly rich genetic diversity and multiple divergent lineages in the centre‐northern range of its distribution, in contrast with a poorer genetically distinct lineage in the southern range. SDM hindcasted refugial persistence in the southern range during the LGM as well as post‐LGM expansion of 18 degrees of latitude northward. Approximate Bayesian computation (ABC) analysis further suggested that the multiple divergent lineages in the centre‐northern range limit stem from post‐LGM colonization from the southern survived lineage. This suggests divergence due to demographic bottlenecks during range expansion and massive genetic diversity loss during post‐LGM contraction in the south. The projected future range of S. thunbergii highlights the threat to unique gene pools that might be lost under global changes.
Highlights
In coastal marine environments, climate oscillations during the last glacial maximum (LGM, c. 21 thousand years ago [ka]) drove coastline configurations and the disjunction of marine glacial refugia (Assis et al, 2017; Hu et al, 2011; Maggs et al, 2008)
The present study addresses two alternative process-based evolutionary hypotheses for S. thunbergii, based on both the previously reported patterns of genetic structure, and on the sharp geographical isolation of the Northwest Pacific marginal seas during the LGM (Wang, 1999): (i) S. thunbergii persisted in demographically isolated northern and southern refugia in the Northwest Pacific during Quaternary ice ages; (ii) S. thunbergii survived in southern glacial refugia and expanded northwards following the glaciers retractions, causing homogeneous low diversity regions across the wide northern range and leaving behind most endemic diversity at rear edges
The southern source scenario during the LGM was supported with the maximum posterior probability (0.9696, 95% CI: 0.9642–0.9749) relative to almost zero support for the other five scenarios, including colonization from the northern source and for each deriving from an extinct common ancestor separately (Figure S1)
Summary
Climate oscillations during the last glacial maximum (LGM, c. 21 thousand years ago [ka]) drove coastline configurations and the disjunction of marine glacial refugia (Assis et al, 2017; Hu et al, 2011; Maggs et al, 2008). The present study addresses two alternative process-based evolutionary hypotheses for S. thunbergii, based on both the previously reported patterns of genetic structure, and on the sharp geographical isolation of the Northwest Pacific marginal seas during the LGM (Wang, 1999): (i) S. thunbergii persisted in demographically isolated northern and southern refugia in the Northwest Pacific during Quaternary ice ages; (ii) S. thunbergii survived in southern glacial refugia and expanded northwards following the glaciers retractions, causing homogeneous low diversity regions across the wide northern range and leaving behind most endemic diversity at rear edges. This information allowed testing whether past climate-driven edge-of-range shifts have influenced present genetic diversity hotspots and if these key areas are at risk of disappearing under projected global warming (Morelli et al, 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
