• Energy Research

The North Atlantic red alga Mastocarpus stellatus is characterized by two life histories (sexual-type and direct-type), which correspond to two geographically isolated breeding groups. These features enable M. stellatus to be an interesting model to investigate how environmental shift and apomictic propagation have influenced its population genetic structure, historical demography and distribution dynamic. To test these ideas, we obtained 456 specimens from 15 locations on both sides of the North Atlantic and sequenced portion of the nuclear internal transcribed spacer (ITS), mitochondrial cox2-3 region (COX) and plastid RuBisCo spacer (RLS). Median-joining networks and ML trees inferred from COX and RLS consistently revealed two gene lineages (mtDNA: CN, CS; cpDNA: RN, RS). The concatenated COX and RLS markers yielded three cytotypes: a northern CN-RN, a southern CS-RS and a mixed cytotype CS-RN, which enabled us to roughly separate samples into D (direct-type life-cycle) and S (sexual-type life-cycle) groups (northern CN-RN and mixed cytotype CS-RN=D; southern CS-RS=S). Pairwise FST analysis of the D group revealed a high level of genetic differentiation both along European coasts and across the Atlantic basin. Bayesian skyline plots (BSPs) and IMa analyses indicated that M. stellatus underwent slight demographic expansion at the late-Pleistocene, with the beginning of divergence between lineages dating to c. 0.189Ma (95%HPD: 0.083-0.385Ma). IMa analyses also revealed asymmetric genetic exchange among European populations and a predominant postglacial trans-Atlantic migration from Norway and Galway Bay to North America. Our study highlights the importance of phylogeographic approaches to discover the imprints of climate change, life histories and gene flow in driving population genetic connectivity and biogeographic distribution of intertidal seaweeds in the North Atlantic.

AbstractGlaciation‐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.

AbstractSeagrasses play a vital role in structuring coastal marine ecosystems, but their distributional range and genetic diversity have declined rapidly in recent decades. To improve conservation of seagrass species, it is important to predict how climate change may impact their ranges. Such predictions are typically made with correlative species distribution models (SDMs), which can estimate a species’ potential distribution under present and future climatic scenarios given species’ presence data and climatic predictor variables. However, these models are typically constructed with species‐level data, and thus ignore intraspecific genetic variability, which can give rise to populations with adaptations to heterogeneous climatic conditions. Here, we explore the link between intraspecific adaptation and niche differentiation inThalassia hemprichii, a seagrass broadly distributed in the tropical Indo‐Pacific Ocean and a crucial provider of habitat for numerous marine species. By retrieving and re‐analysing microsatellite data from previous studies, we delimited two distinct phylogeographical lineages within the nominal species and found an intermediate level of differentiation in their multidimensional environmental niches, suggesting the possibility for local adaptation. We then compared projections of the species’ habitat suitability under climate change scenarios using species‐level and lineage‐level SDMs. In the Central Tropical Indo‐Pacific region, models for both levels predicted considerable range contraction in the future, but the lineage‐level models predicted more severe habitat loss. Importantly, the two modelling approaches predicted opposite patterns of habitat change in the Western Tropical Indo‐Pacific region. Our results highlight the necessity of conserving distinct populations and genetic pools to avoid regional extinction due to climate change and have important implications for guiding future management of seagrasses.

Natural hybridization can play a significant role in evolutionary processes and influence the adaptive diversification and speciation of brown seaweeds. However, this phenomenon is as yet unknown in Saccharina kelps. Saccharina angustata and two varieties of Saccharina japonica (S. japonica var. japonica and S. japonica var. diabolica) partly overlap in distribution along the Pacific coast of Hokkaido, which makes them a good model system to study hybridization and introgression among species of the genus Saccharina. Based on 13 highly variable nuclear microsatellites and a mitochondrial marker, we assessed the genetic diversity levels of S. angustata for the first time and populations from Muroran to Shiranuka (western part of the Pacific coast in Hokkaido) exhibited highest genetic diversity. Genetic diversity of S. japonica was higher in S. japonica var. japonica as compared with S. japonica var. diabolica. There was significant genetic differentiation (FST > 0.25, p < 0.05) between S. japonica and S. angustata based on both markers. Moreover, there was poor genetic connectivity and limited interspecific hybridization among these closely related Saccharina species. Ecological niche models projected a northward expansion of both S. japonica and S. angustata under future climate scenarios and a range overlap between two species along the coast of Okhotsk Sea in Kamchatka Peninsula. The interspecific hybridization and genetic diversity among these kelps provide insights for kelp selection and cultivation as well as future conservation strategies of wild stocks.

Eucheuma is one of the most important commercial red seaweeds in Southeast Asia, and plays an important role in the global seaweed aquaculture. It is expected to exhibit great responses to ocean warming. Here, we used maximum entropy species distribution models (SDMs) to estimate the suitable habitat of Eucheuma denticulatum under present conditions, and to predict the future range dynamics under the four representative concentration pathway (RCP) scenarios. The best marine environmental factors for E. denticulatum distribution modeling were distance to shore, sea surface temperature and currents velocity. Our results showed that E. denticulatum' distributions would contract in the Central Indo-Pacific Ocean, especially the regions of the Sunda Shelf, while expanding poleward along the south coast of Australia in 2100. Our study provided important knowledge for the prediction of the tropical seaweed distribution, conservation and sustainable developments of E. denticulatum in the future.