Genetic structure of sugar kelp in the St. Lawrence Estuary and Gulf (Québec, Canada)

The sugar kelp, Saccharina latissima, is cultivated at low scale in Quebec, Canada, and current practice involves seeding meiospores or gametophyte stocks onto spools carrying twine and transferring these to a seaweed farm site. As the stocks can originate from locations spanning several hundreds of kilometers from the farm sites, such practices could involve genetic contamination and disrupt local adaptations. Assessing genetic structure can inform us of the potential risks associated with these practices. Here, we characterized the genetic diversity and structure of S. latissima from locations in the St. Lawrence Estuary and Gulf at both microsatellite loci (308 sporophytes at 22 loci in 16 sites) and genomic markers (228 sporophytes at 6578 single nucleotide polymorphisms, or SNPs, in 13 sites). Several populations had low heterozygosity values and significant F_IS values at microsatellite loci. No genetic structure was observed among populations with microsatellite loci, but strong genetic structuring was observed with the genomic data. Population structure followed a geographic pattern and was congruent with major currents. Individuals from the wild population in the vicinity of the farm site were genetically distinct from the sporophytes on the growing lines that belong to a genetically distinct group. There was no significant genetic differentiation between wild individuals living in proximity to the farm site and another wild population of the same area. Hence, aquaculture practices have not resulted in changes in the genetic composition of the wild population on a large scale. Our results are important to guide future conservation efforts and the seaweed farming industry.