Population Genetics, Phylogeographic Structure And Evolutionary Histories Of Four Typical Marine Macroalgae

We concluded phylogeographic studies of four marine algae, Palmaria palmata, Mastocarpus stellatus from North Atlantic and Sargassum thunbergii, Sargassum fusiforme from Northwest Pacific, using multi- locus markers. The phylogeographic patterns and evolutionary histories was discussed to assess the relative effects of historical(e.g. paleoclimatic oscillation) and contemporary(e.g. ocean currents) factors in shaping popualtion structure and distribution shifts of genetic diversity.(1) Mitochondrial cox2-3 and plastid rpl12-rps31-rpl9 sequences were obtained from 15 Palmaria palmata populations from North Atlantic. The mt DNA cox2-3 haplotype network showed that North Atlantic P. palmata divided into two genetic lineages in association with mid-Pleistocene(0.453 Ma) climate change. Multiple glacial refugia may have existed for European populations and long-term isolation may have contributed greatly to deep genetic differentiation. The North American P. palmata populations are characterized by comparatively low genetic diversity and shallow phylogeographic structure, likely resulting from population bottleneck effect. IMA 2 analysis did not reveal gene flow between the European and North A merican coasts, indicating long-term isolation.(2) We sampled 15 Mastocarpus stellatus populations from both sides of the North Atlantic and sequenced portion of the nr DNA ITS, mt DNA cox2-3 and cp DNA rbc L-S. Median-joining networks and ML trees inferred from cox2-3 and rbc L-S consistently revealed two gene lineages(mt DNA: CN, CS; cp DNA: RN, RS), dating to c. 0.189 Ma(95%HPD: 0.083–0.385 Ma). The concatenated cox2-3 and rbc L-S yielded three cytotypes: a northern CN-RN, a southern C S-RS and a mixed cytotype CS-RN, which enabled us to roughly separate samples into D(direct-type life-cycle) and S(sexual-type life-cycle) groups(northern CN-RN and mixed cytotype CS-RN = D; southern CS-RS = S). S group was found only in southern areas(English C hannel and southern Galway Bay), and D group was mainly found in northern Europe and northeastern Canada. Pairwise FST analyses revealed a high level of genetic differentiation within D group. IMA analyses also revealed asymmetric genetic exchange among European populations and a predominant postglacial trans-Atlantic migration from Norway and Galway Bay to North America.(3) N uclear internal transcribed spacer-2 and mitochondrial cox3 sequences were obtained from 35 Sargassum thunbergii populations. Several lines of evidence indicate that S. thunbergii is characterized by shallow population structure. Pairwise FST estimates and analyses of molecular variance(AMOVA) at various hierarchical levels(latitude criteria, longitude criteria, marine provinces, biogeographical basins and zoogeographical zones) were conducted to elucidate population genetic differentiation. The consistent result was that around 70 percent of genetic variance occurred within sampling localities. Geographic distances do not correlate with population pairwise genetic differentiations. MIGRATE analyses revealed high levels of asymmetric gene flow among S. thunbergii populations, with the numbers of migrants largely corresponding to the directions of oceanic current systems in the Northwest Pacific. O ur integrative evidence suggests that the population genetic structure of S. thunbergii was mainly shaped by oceanic currents in the Northwest pacific.(4) We sampled 26 populations of the brown alga Sargassum fusiforme over the entire distribution range and investigated phylogeographic diversity and demographic history using mitochondrial trn W-L, cox3 and plastid rbc L-S sequences. The concatenated mitochondrial markers revealed three major genetic clades(A, B, C), with A and B located in the Japan-Pacific ocean and C located in the Sea of Japan, Korean and Chinese coasts. IMA estimates revealed deep inter-clade divergence in association with the mid-Pleistocene climate changes(c. 0.858–1.224 Ma). The divergence time for the three subclades of clade C was c. 0.106–0.128 Ma: subclade C1 was located in the Sea of Japan; subclade C2 was located in Korea and North China; subclade C3 was located in South C hina. C lades A and B had relatively long-term stable population size, whereas sub-clades C2 and C3 underwent sudden expansion at c. 0.26 Ma. We synthesize that the population relic of S. fusiforme in isolated marginal seas during the Pleistocene glacial periods contributed most to the deep genetic split, and the present-day geographic and hydrological barriers maintain such kinds of genetic differentiation.