Biogeography of the Genus Sargassum (Heterokontophyta: Phaeophyceae) and the Phylogeographic Patterns of Sargassum spp. in Northwest Pacific.

摘要

The biogeographical pattern of the brown macroalgal genus Sargassum as well as the phylogeography of selected Sargassum spp. along NW Pacific coast were elucidated using analytical biogeographical and comparative phylogeographical tools.;Two allopatrically distributed varieties of S. hemiphyllum, v. chinense and v. hemiphyllum, are genetically distinct in terms of their internal transcribed spacer 2 (ITS2) and Rubisco spacer. The genetic break between these two varieties, with v. chinense distributed in southern Chinese coast and v. hemiphyllum in Japan and Korea, is situated in a region that includes Bohai, Yellow Sea and East China Sea, all of which were heavily influenced by the Yangtze and Yellow Rivers in China. An introgression of the mitochondrial (Mt) genome from v. chinense to v. hemiphyllum, possibly mediated by the Kuroshio Current, is evident based on the Mt marker TrnW_I spacer. Hybridization between the two varieties may still be ongoing since the concerted evolution of ITS2 is not yet saturated in the Korean population located geographically in-between the distribution of the two varieties. In contrast, no variation in ITS2 and Rubisco spacer is revealed in S. muticum, including the native Asian populations and introduced populations in Europe and North America. There is a fixed one-nucleotide difference in the TrnW_I spacer, between the population in eastern Japan and all the other populations examined. This finding supports the earlier suggestion that the source of the introduced S. muticum populations is western and central Japan (Seto Inland Sea), where the germlings of S. muticum have been associated with the Pacific oysters previously introduced for farming in Canada, UK and France in earlier years.;To investigate the effect of freshwater outflow from Yangtze and Yellow Rivers in eastern China in shaping the genetic population structure of Sargassum spp., a comparative phylogeographic study was conducted on four closely related Sargassum species showing either continuous (Sargassum thunbergii and S. muticum ) or discontinuous (S. hemihyllum and S. fusiforme) distribution patterns along the Chinese coast. The results showed discontinuously distributed species to exhibit more haplotypes (e.g. four in TrnW_I spacer) among their populations than those with continuous distribution (two in TrnW_I spacer) pattern. Little or no population differentiation is revealed in species with a continuous distribution. Their occurrences in the brackish Bohai region may be attributed to the presence of inherited physiochemical traits that allow them to tolerate lower salinity waters in estuaries. The discontinuously distributed species, however, exhibited a deep genetic divergence among populations, as revealed by various genetic markers. There are two main lineages of S. fusiforme based on ITS2 and TrnW_I sequences, but the geographical region associated with this genetic break between the two lineages in eastern and southwestern Japan is different from that of S. hemiphyllum. Analysis of molecular variance (AMOVA) results indicate that the maintenance of the population structure of S. fusiforme appears not to be correlated with the outflow of the two rivers. For S. hemphyllum, reduced salinity as the suspected genetic barrier was investigated directly in the laboratory to elucidate its effect on the growth and survival of S. hemiphyllum var. chinense . Statistically significant difference was observed in the relative growth rate (calculated based on wet weight) of branches cultured under different salinities, with the optimal growth under salinity level of 33 ppt. The lethal limit of vegetative growth was between 0 and 10 ppt. Germlings cultured in 15 ppt attained the highest survivorship. The optimal growth of the germlings occurred at 25 ppt, while the lowest lethal limit was within the range of 0 ppt and 5 ppt. Germlings reared under low salinity were deficient in rhizoid development, making them highly unlikely to grow into large thallus in the natural environment with strong waves. Compared with the optimal and lethal salinity level of S. mutium, the lethal limits of both vegetative branches and germlings of the two species are comparable. The optimal growth of branches of S. muticum occurred under salinity level of 27 ppt, in contrast to the optimal salinity level of S. hemiphyllum at 33 ppt. This could have explained the absence of S. hemiphyllum in brackish water and support the suggestion that river discharge serves as a barrier for the exchange of genetic materials among its populations. (Abstract shortened by UMI.)