High levels of intra- and inter-individual polymorphism in the rDNA ITS1 of Caulerpa racemosa (Chlorophyta)

The coenocytic green alga Caulerpa racemosa colonized the Mediterranean som e time after the opening of the Suez Canal in 1869. For most of the past ce ntury it has remained confined to the southeastern Mediterranean, but over the past several years the species has been reported in abundance along the western coasts of Italy, Sardinia and the Marseille area of southern Franc e. Whether this constitutes a natural range expansion or a possible new int roduction is under discussion. The purpose of this study was to establish t he ITS signatures of populations inside and outside the Mediterranean. ITS1 sequences were compared among 78 clones from 21 individuals representing 1 1 populations of C. racemosa, collected from the Western Mediterranean, the Canary Islands, Panama and Western Australia. Intraindividual polymorphism of ITS1 was found to be as high or higher than inter-individual polymorphi sm across all samples tested. Polymorphism parsimony analysis, which takes into account intra-individual variation, revealed strong separation between individuals from Panama, Fuerteventura (Canary Is.) and the Mediterranean. In contrast, none of the six Mediterranean populations could be distinguis hed from one another. In addition, individuals and their clones collected f rom Perth (Australia) and Gran Canaria were spread throughout the Mediterra nean clade. Possible reasons for the lack of homogenization of ITS arrays i n C. racemosa under the mechanisms of concerted evolution and implications for phylogeographic interpretation are discussed and compared with Caulerpa taxifolia.