Abstract
BackgroundLong-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.ResultsWe 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 FST 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.ConclusionsOur 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.
Highlights
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
Two biogeographic scenarios have been proposed for species isolated in the Yellow-Bohai Sea (YBS)/East China Sea (ECS) basin (Fig. 1): I) species possessed a homogeneous population structure as reported in the barnacle Chthamalus challengeri [3] and the alga Sargassum horneri [4] (Fig. 1a); and II) species comprised two genetic clusters (YBS vs. ECS) which were separated by the line of Changjiang estuary, as illustrated in the gastropod Cellana toreuma [15] and the brown alga Sargassum fusiforme [16] (Fig. 1b)
Haplotype R1 was mainly detected in the Yellow-Bohai Sea (YBS, 97.9%), whereas haplotype R5 was mainly detected in the East-China Sea (ECS, 90.4%) (Fig. 2 and Additional file 1: Table S1)
Summary
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. Two biogeographic scenarios have been proposed for species isolated in the YBS/ECS basin (Fig. 1): I) species possessed a homogeneous population structure as reported in the barnacle Chthamalus challengeri [3] and the alga Sargassum horneri [4] (Fig. 1a); and II) species comprised two genetic clusters (YBS vs ECS) which were separated by the line of Changjiang estuary, as illustrated in the gastropod Cellana toreuma [15] and the brown alga Sargassum fusiforme [16] (Fig. 1b) These contrasting biogeographic patterns indicate that coastal species co-distributed along the coast of China might exhibit different ecological responses to climatic shifts
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