Phylogeographical structure, distribution and genetic variation of the green algaeUlva intestinalisandU. compressa(Chlorophyta) in the Baltic Sea area

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Abstract
The marine algaeUlva intestinalisandU. compressaare morphologically plastic with many overlapping characters and are therefore difficult to distinguish from each other. The present distribution ofU. intestinalisandU. compressais investigated along the salinity gradient in the Baltic Sea area through analyses of internal transcribed spacer (ITS) sequence data. Also, the amount and distribution of intraspecific genetic polymorphism in the ITS region is studied allowing inferences on the phylogeographical pattern and postglacial recolonization of the Baltic Sea area. The data show that of the two species onlyU. intestinalisoccurs in the Baltic Sea. The distribution ofU. compressais more restricted than previously reported, and it was not found in salinities lower than 15 ppt. All of Scandinavia and the Baltic Sea were covered with ice during the last ice age and the organisms in the Baltic Sea must have colonized the area after the ice had started to melt. The genetic diversity ofU. intestinalisandU. compressain the Baltic Sea and the neighbouring area was found to be reduced compared to that in the British Isles. This reduction may be the result of either a historical reduction of diversity or an adaptation of specific clones to the northern environmental conditions.