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.
['Li|Jing-Jing|JJ|0000-0003-3241-2248', 'Hu|Zi-Min|ZM|', 'Sun|Zhong-Min|ZM|', 'Yao|Jian-Ting|JT|', 'Liu|Fu-Li|FL|', 'Fresia|Pablo|P|', 'Duan|De-Lin|DL|']
D001499:Bayes Theorem D002681:China D016000:Cluster Analysis D018742:DNA, Chloroplast D017753:Ecosystem D051456:Gene Flow D014644:Genetic Variation D005828:Genetics, Population D006239:Haplotypes D018895:Microsatellite Repeats D009792:Oceans and Seas D058974:Phylogeography D025341:Principal Component Analysis D044647:Sargassum
Historical isolation Gene flow Sargassum thunbergii Long-distance dispersal Microsatellite Plastid RuBisCo spacer Population genetic diversity