Undaria pinnatifida and Saccharina japonica belong to Laminariales,
Ochrophyta both of which are macroalgae with important economic value in China.
Under current farming system, the spontaneous and cultivated populations of these two species are usually sympatric in the open sea. For the cultivated populations, if they were "contaminated" by the genes from the spontaneous populations, their agronomic characters may be changed and degraded. In addition, there are specific genes that evolved under the local environment in spontaneous populations, and they
are potential germplasm resources and ought to be prevented from excess influence from farmed populations. But until now, there has been little research about genetic connectivity between cultivated and spontaneous populations of the two algae.
Therefore, we collected U. Pinnatifida and S. japonica on a typical kelp farm in Dalian, China, and analyzed the genetic diversity, genetic structure and gene connectivity between the spontaneous and cultivated populations.
For U. Pinnatifida, we employed 10 pairs of microsatellite primers developed and selected in previous study to analyze two cultivated populations (F1&F2), two spontaneous populations (UWT&KWT) occurring on rafts and one spontaneous population (SW) inhabiting the subtidal zone from different years. In terms of Na (numbers of alleles) and He(expected heterozygosity), the population with the highest genetic diversity was UWT-18 (Na: 14.6; He: 0.874), and the lowest population was
F1-18 (Na: 8.1; He: 0.765). The maximum pairwise genetic distance was 1.470 ( between F1-18 and SW-18). Neighbor-joining (NJ) clustering analysis based on genetic distance divided the cultivated populations and subtidal population into two clusters with large genetic distance. The genetic structure analysis based on Bayesian model conducted by STRUCTURE 2.3.4 divided all the individuals into 3 clusters.
SW-18, KWT-18, and W17 were basically assigned to cluster 1, and the proportions of membership were 0.98, 0.96, and 0.93, respectively. Most of the individuals of F1-18, F1-17 and F1-15 were assigned to cluster 2, and their proportions of membership were 0.98, 0.97 and 0.97, respectively. Most individuals of F2-15 belonged to cluster 3, with a proportion of membership of 0.90.
The principal component discriminant analysis (DAPC) also assigned the farmed and the subtidal spontaneous population into two different clusters, which was consistent with the results of above-mentioned two clustering analysis.
These results reveal significant genetic diversity in both cultivated and subtidal spontaneous population. Clustering analysis revealed that the gene flow between cultivated and subtidal spontaneous population is very limited. Spontaneous sporophytes on farmed rafts contain pedigree from both farmed and subtidal spontaneous populations.
For Saccharina japonica, 10 pairs of microsatellite primers with higher
polymorphism were used to analyze three cultivation populations（ FB、 FJ、 XS） , one wild raft population（ RW） and two subtidal spontaneous populations（ SE&SW） collected in 2018. The value of Na and He was highest in FJ（ Na=3.8； He=0.496），lowest in SW（ Na=2.6； He=0.382. In terms of Na and He, the genetic diversity of cultivated populations was higher than that of subtidal spontaneous population. The
maximum pairwise genetic distance was 0.159， and it appeared between XS and SW.
All Fst values revealed significance except FJ and XS. Pairwise genetic distance-based NJ clustering analysis divided the cultivated and subtidal populations into two clusters with large genetic distance, and RW was between the two clusters. The Bayesian-model based genetic structure analysis divided all individuals into two clusters. The subtidal spontaneous population and the cultivated populations were principally assigned into cluster 1and cluster2, respectively. FB and FJ contained relatively high proportions of membership originating fromthe subtidal population(0.28 and 0.19). The results of DAPC were consistent with the above two
analysis, revealing that the cultivated populations and the subtidal spontaneous populations were clustered separately, and the raft spontaneous population was in between. Above results suggested that there was almost no pedigree of the cultivated populations flowing to subtidal spontaneous populations, indicating that influence from the former to the latter was very limited; however, some pedigree of subtidal
populations were detected in the cultivated populations, indicating that the gene flow from the subtidal populations to the cultivated populations was more prominent, and thus the gene flow between them was obviously asymmetry. The raft spontaneous population contained pedigree from both the cultivated and the subtidal populations.
These results provide a foundation for further understanding of interaction between cultivated and spontaneous populations of U.pinnatifida and S. japonica.