Evolution Research On Saccharina By Uncovering Their Mitochondrial Genomes

Saccharina,a kind of large marine algae(kelp),plays significant roles in multiple aspects, including optimizing the structure of the marine ecological system, increasing marine carbon cycle, alleviating eutrophication, adjusting ocean acidification and cleaning the marine environment. The Saccharina of multicellular large body is farmed widely in China, creating tremendous output value every year. Thus, it becomes more and more meaningful to study the germplasm resources and population genetic diversity of Saccharina.Saccharina are widely utilized as raw materials in chemical industry, biological,pharmaceutical, food processing, military, and so on. As one of the primary producer in the marine ecosystems, Saccharina absorb carbon dioxide and release oxygen by photosynthesis and maintain the balance and stability of the marine ecosystem. It also provides essential living and breeding environment for marine animals.Studies focused on the origin and the evolution of this algal has great potential to explore many genetic traits. China has the top production of Saccharina in the world.However, in recent years, since the long-term single-lineage cultivation, Saccharina strains showed genetic recession tendencies and the quality of the germplasm is declining, reminding an urgent task to screen or introduce superior high-yielding strain for Saccharina breeding. Developing molecular markers are also necessary to assist the molecular breeding.As a cellular organelle, the mitochondrion has relatively simple structure and small molecular weight. Due to its presumed lack of recombination, maternal inheritance, and a relatively rapid mutation rate, mitochondrial DNA sequences are extensively used for comparative and evolutionary genomics, phylogenetics, molecular evolution, species identification, and population genetics. Mitochondrial DNA sequencing of Saccharinastrains would promote the study in genetic relationship and genetic diversity, and provide valuable information for selective breeding.In this research, we resolved the mitochondrial genomes of 10 Saccharina strains,including 3 cultivable strains from the coast of Bohai and Yellow sea and 7 wild species strains from the northern Japan, eastern of Russia and coastal areas of northern Germany respectively. They were Saccharina sp. ye-C5(NHD1939);Saccharina sp.ye-C6(NHD1940);Saccharina latissima ye-C14(NHD1941);Saccharina sp. ye-G(NHD1942);Saccharina sp. ye-C2(NHD1943);Saccharina sp. ye-C12(NHD1945);Saccharina sp. ye-F(NHD1946);Saccharina sp. ye-C(NHD1947);Saccharina sp.ye-B(NHD1948); Saccharina sp. ye-W(NHD1949). Firstly we extracted and sequenced the mitochondrial DNA of these 10 Saccharina strains. Using bioinformatics software processing, we obtained the mitochondrial genome data of 10 Saccharina strains by mapping all reads to a reference mitochondrial genome of Saccharina japonica. The mitochondrial genomes of 10 Saccharina strains all have 38protein-coding genes totally. Their length of completed of mitochondrion genome range from 37,609 bp to 37,673 bp. We obtained the genetic map of mitochondrial genes for each of the 10 strains Saccharina based on their genetic structure respectively. A phylogenetic analysis was performed combined with the other sequenced mitochondrial genome of 12 brown algae. It indicated that Saccharina sp. ye-C12、Saccharina sp.ye-C2、Saccharina sp. ye-W and Saccharina sp. ye-C6 were closely related as sister taxa; Saccharina sp. ye-B、Saccharina sp. ye-G and Saccharina sp. ye-F were closely related; Saccharina sp. ye-C5 was closely related to Saccharina longissima; Saccharina sp. ye-C was closely related to Saccharina coriacea; Saccharina latissima ye-C14 was closely related to Saccharina latissima.We drew the schematic diagram using bioinformatics tools according to the reference sequence of Saccharina japonica that represented gene site mutation and gene arrangement of 10 strains Saccharina of mitochondrial DNA with the phylogenetic tree.It indicated that the cultivated Saccharina in our country originated from Saccharina sp.ye-C6, one of wild species from the northern Japan. We concluded that single-species of Saccharina led to its low level of genetic diversity. We analyzed the gene site mutation of 10 strains Saccharina and found that the cultivated populations exhibited relatively low levels of genetic diversity compared with the wild populations. We demonstratedthat the quality of the germplasm has declined. The gene arrangement result showed that the massive gene arrangement of mitochondrial DNA didn’t happened to the 10 strains Saccharina, indicating that they diverged recently and no large mutations occurred.