The contribution of intraspecific variation to future climate responses of brown algae

Intraspecific variation, which may arise through the interplay between environmental heterogeneity and adaptive variation, has consequences for the resilience of species to climate change. Here, we revealed the levels of intraspecific genomic variation and its causes in a widespread brown alga (Sargassum horneri). Our datasets included both benthic and floating samples from 20 populations sampled along a latitudinal gradient in the Chinese marginal seas. We prepared two datasets: 1) 4628 filtered single nucleotide polymorphisms (SNPs) for all samples, and 2) 20,735 filtered SNPs for only benthic samples. We conducted genotype-environment association analyses and outlier tests to identify adaptive loci for each dataset. Both neutral and adaptive SNPs revealed a deep genetic split between benthic populations in the North Yellow Sea (NYS) and East China Sea. Redundancy analysis demonstrated that temperature is a major factor driving adaptive divergence of benthic populations. Predictions of the lineage-level species distribution model revealed severe habitat loss of S. horneri in the NYS and eastern coasts of Korea under RCP8.5 in the 2100 s; however, the low-latitude boundary remained stable. Based on these results, we speculate that low-latitude populations may have high resilience to global warming. The dispersal routes of floating samples were determined by coastal currents; however, restricted gene flow between regions was detected. Thus, we speculate that opportunities for the recovery of depleted populations through natural gene flow may be limited. This study highlights the importance of applying conservation strategies (e.g., assisted gene flow) to protect coastal species from local extinction in the future.