Historical isolation and contemporary gene flow drive population diversity of the brown alga Sargassum thunbergii along the coast of China.

BACKGROUND: Long-term survival in isolated marginal seas of the China coast during the late Pleistocene ice ages is widely believed to be an important historical factor contributing to population genetic structure in coastal marine species. Whether or not contemporary factors (e.g. long-distance dispersal via coastal currents) continue to shape diversity gradients in marine organisms with high dispersal capability remains poorly understood. Our aim was to explore how historical and contemporary factors influenced the genetic diversity and distribution of the brown alga Sargassum thunbergii, which can drift on surface water, leading to long-distance dispersal. RESULTS: We used 11 microsatellites and the plastid RuBisCo spacer to evaluate the genetic diversity of 22 Sargassum thunbergii populations sampled along the China coast. Population structure and differentiation was inferred based on genotype clustering and pairwise F ST and allele-frequency analyses. Integrated genetic analyses revealed two genetic clusters in S. thunbergii that dominated in the Yellow-Bohai Sea (YBS) and East China Sea (ECS) respectively. Higher levels of genetic diversity and variation were detected among populations in the YBS than in the ECS. Bayesian coalescent theory was used to estimate contemporary and historical gene flow. High levels of contemporary gene flow were detected from the YBS (north) to the ECS (south), whereas low levels of historical gene flow occurred between the two regions. CONCLUSIONS: Our results suggest that the deep genetic divergence in S. thunbergii along the China coast may result from long-term geographic isolation during glacial periods. The dispersal of S. thunbergii driven by coastal currents may facilitate the admixture between southern and northern regimes. Our findings exemplify how both historical and contemporary forces are needed to understand phylogeographical patterns in coastal marine species with long-distance dispersal.

Phylogeographic data revealed shallow genetic structure in the kelp Saccharina japonica (Laminariales, Phaeophyta).

BACKGROUND: Population structure and genetic diversity of marine organisms in the Northwestern Pacific Ocean exhibited complex patterns. Saccharina japonica is a commercially and ecologically important kelp species widely distributed along the coast of Japan Sea. However, it is still poorly known about population genetics and phylogeographic patterns of wild S. japonica populations on a large geographic scale, which is an important contribution to breeding and conservation of this marine crop. RESULTS: We collected 612 mitochondrial COI and trnW-trnL sequences. Diversity indices suggested that S. japonica populations along the coast of Hokkaido exhibited the highest genetic diversity. Bayesian Analysis of Population Structure (BAPS) revealed four clusters in the kelp species (cluster 1: Hokkaido and South Korea; cluster 2: northwestern Hokkaido; cluster 3: Far Eastern Russia; cluster 4: China). The network inferred from concatenated data exhibited two shallow genealogies corresponding to two BAPS groups (cluster 2 and cluster 3). We did not detect gene flow between the two shallow genealogies, but populations within genealogy have asymmetric gene exchange. Bayesian skyline plots and neutrality tests suggested that S. japonica experienced postglacial expansion around 10.45 ka. CONCLUSIONS: The coast of Hokkaido might be the origin and diversification center of S. japonica. Gene exchange among S. japonica populations could be caused by anthropogenic interference and oceanographic regimes. Postglacial expansions and gene exchange apparently led to more shared haplotypes and less differentiation that in turn led to the present shallow phylogeographical patterns in S. japonica.

Hidden diversity and phylogeographic history provide conservation insights for the edible seaweed Sargassum fusiforme in the Northwest Pacific.

Understanding the evolutionary processes that have created diversity and the genetic potential of species to adapt to environmental change is an important premise for biodiversity conservation. Herein, we used mitochondrial trnW-L and cox3 and plastid rbcL-S data sets to analyze population genetic variation and phylogeographic history of the brown alga Sargassum fusiforme, whose natural resource has been largely exterminated in the Asia-Northwest Pacific in the past decades. Phylogenetic trees and network analysis consistently revealed three major haplotype groups (A, B, and C) in S. fusiforme, with A and B distributed in the Japan-Pacific coast. Group C consisted of three subgroups (C1, C2, and C3) which were distributed in the Sea of Japan, the Yellow-Bohai Sea, and East China Sea, respectively. Isolation-with-migration (IM a) analysis revealed that the three groups diverged approximately during the mid-Pleistocene (c. 756-1,224 ka). Extended Bayesian skyline plots (EBSP) showed that groups A and B underwent relatively long-term stable population size despite a subsequent rapid demographic expansion, while subgroups C2 and C3 underwent a sudden expansion at c. 260 ka. FST and AMOVA detected low population-level genetic variation and high degrees of divergence between groups. The cryptic diversity and phylogeographic patterns found in S. fusiforme not only are essential to understand how environmental shifts and evolutionary processes shaped diversity and distribution of coastal seaweeds but also provide additional insights for conserving and managing seaweed resources and facilitate predictions of their responses to future climate change and habitat loss.