Climate‐induced range shifts shaped the present and threaten the future genetic variability of a marine brown alga in the Northwest Pacific

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 surviving 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.