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Summary Evolutionary theory predicts that clonal organisms are more susceptible to extinction than sexually reproducing organisms, due to low genetic variation and slow rates of genetic evolution. However, plants that reproduce clonally are among the oldest living organisms on our planet. Here, we test the hypothesis that clonal seagrass meadows display epigenetic diversity that compensates for the lack of .genetic diversity. In a clonal meadow of the seagrass Zostera marina we characterized DNA methylation among 42 shoots. We correlated methylation patterns with photosynthetic performance under exposure to, and recovery from 27°C. Here we show for the first time that methylation variation in clonal seagrass promotes variation in fitness-related traits of ecological relevance: photosynthetic performance and heat stress resilience. The recovered shoots memorized heat responsive methylation changes for >five weeks. While genotypic diversity has been shown to enhance stress resilience and invertebrate diversity in seagrass meadows composed of several genotypes, we suggest that epigenetic variation plays a similar role in clonal meadows with the potential to secure function and resilience not only of Z. marina plants, but of the entire associated ecosystem. Consequently, conservation management of clonal plants must include epigenetic variation as indicator of resilience and stability.

The coenocytic green alga Caulerpa racemosa colonized the Mediterranean some time after the opening of the Suez Canal in 1869. For most of the past century 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 France. Whether this constitutes a natural range expansion or a possible new introduction is under discussion. The purpose of this study was to establish the ITS signatures of populations inside and outside the Mediterranean. ITS1 sequences were compared among 78 clones from 21 individuals representing 11 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 polymorphism 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 distinguished from one another. In addition, individuals and their clones collected from Perth (Australia) and Gran Canaria were spread throughout the Mediterranean clade. Possible reasons for the lack of homogenization of ITS arrays in C. racemosa under the mechanisms of concerted evolution and implications for phylogeographic interpretation are discussed and compared with Caulerpa taxifolia.

Phylogenetic relationships in the Desmarestiales (Phaeophyceae) were inferred among the monotypic Arthrocladia (Arthrocladiaceae) and 27 isolates from Desmarestiaceae, representing 17 taxa of Desmarestia and the monotypic Antarctic genera Himantothallus and Phaeurus. Phaeurus and Arthrocladia were used as outgroups. Parsimony analyses of nuclear ribosomal DNA internal transcribed spacer (ITS1 and ITS2) sequences, in which gaps were both included and excluded yielded well-resolved trees with a consistent general branching pattern. A parallel analysis of nine morphological and life-history characters and three ecological characters yielded a similar tree but provided little resolution in the terminal clades. The position of the monotypic Arthrocladia villosa within the Desmarestiales is consistent with monophyly for the order, but its position as the most primitive desmarestialean is not resolvable from the molecular data set. The basal position of Phaeurus, the Antarctic Desmarestia species, and Himantothallus is consistent with the hypothesis of a Southern Hemisphere origin for the family Desmarestiaceae. The more recent Northern Hemisphere ''aculeata'' clade evolved from an Antarctic ancestor. A ''D. aculeata-like'' species was ancestral to a lineage characterized by annual sporophytes with high sulfuric acid content, which radiated into many species, widely distributed in both hemispheres. Mapping of morphological and ecological characters onto the molecular tree confirm the informativeness of sulfuric acid-containing vacuoles and unilocular sporangial types. There is good congruence between phylogenetic tree topology and temperature imprints in relation to biogeographic distribution, supporting the theory that temperature tolerance is a conservative trait.

Phylogenetic relationships in the Arctic-Antarctic disjunct seaweed species Acrosiphonia arcta (Dillwyn) J. G. Agardh (Acrosiphoniales, Chlorophyta) were examined using restriction fragment-length polymorphism analysis of the fast-evolving nuclear ribosomal intergenic spacer (IGS) region and random amplified polymorphic DNA (RAPD) markers. Twenty-two isolates collected from 10 different locations in both hemispheres were compared. Five IGS length classes were identified among the 10 locations. Throughout the North Atlantic, IGS regions were found to be extremely homogeneous whereas RAPD patterns revealed subdivided populations that suggest founder effects. Acrosiphonia arcta populations found in the Arctic and North Atlantic oceans are hypothesized to be of Pacific origin. Extensive differences found between Arctic Greenland populations and those in the North Atlantic suggest that colonization of Arctic Greenland occurred as an independent event. Recolonization of the Antarctic peninsula from Southern Chile is favored, whereas the directionality of transequatorial passage along the western coast of the Americas could be in either direction.