Defending LifeThe nature of host-parasite relations Elling Ulvestad 2007, XIX, 241 pp., Hardcover ISBN: 978-1-4020-5675-8 Springer (http://www.springer.com)

Reproduction of attached large brown algae is known to occur only by sexual zygotes.Using microsatellites we show evolution of asexual reproduction in the bladder wrack promoting population persistence in the brackish water Baltic Sea (< 6 psu). Here a dwarf morph of Fucus vesiculosus is dominated by a single clone but clonal reproduction is also present in the common form of the species. We describe a possible mechanism for vegetative reproduction of attached algae, and conclude that clonality plays an important role in persistence and dispersal of these marginal populations, in which sexual reproduction is impaired by low salinity.

Brown seaweeds of the genus Fucus occupy a wide variety of temperate coastal habitats. The genus is evolutionary dynamic with recent radiations to form morphologically distinct taxa. In the brackish Baltic Sea, fucoids are the only perennial canopy-forming macroalgae. The most northern populations of Fucus occur permanently submerged in extremely low salinity (3-5 psu). These are currently referred to as Fucus vesiculosus L. but are morphologically distinct with a narrow frond without bladders. We report here that a population of this unique morphotype is reproductively isolated from a truly sympatric population of common F. vesiculosus and conclude that the northern morphotype represents a previously undescribed species. We describe Fucus radicans sp. nov., which is attached and dioecious with broadly elliptic receptacles, characterized by a richly branched narrow flat frond (2-5 mm), short thallus (o26 cm), and a high capacity for vegetative recruitment of attached plants. Analysis of five highly polymorphic microsatellite DNA loci showed genetic differentiation between sympatric populations of F. radicans and F. vesiculosus, whereas allopatric populations of the same species revealed a coherent pattern of genetic variation. Sequences of the RUBISCO region in F. radicans were identical to or differing at only one to two dinucleotide positions from those of F. vesiculosus, indicating a recent common origin of the two species.

Recent studies showing consequences of species' genetic diversity on ecosystem performance raise the concern of how key ecosystem species are genetically structured. The bladder wrack Fucus vesiculosus L. is a dominant species of macroalga in the northern Atlantic, and it is particularly important as a habitat-forming species in the Baltic Sea. We examined the genetic structure of populations of F. vesiculosus with a hierarchical approach from a within-shore scale (10 m) to a between-seas scale (Baltic Sea-Skagerrak, 800 km). Analysis of five microsatellite loci showed that population differentiation was generally strong (average F ST = 12%), being significant at all spatial scales investigated (10 1 , 10 3 , 10 4-5 , 10 6 m). Genetic differentiation between seas (Baltic Sea and Skagerrak) was substantial. Nevertheless, the effects of isolation by distance were stronger within seas than between seas. Notably, Baltic summer-reproducing populations showed a strong within-sea, between-area (70 km) genetic structure, while Baltic autumnreproducing populations and Skagerrak summerreproducing populations revealed most genetic diversity between samples within areas (<1 km). Despite such differences in overall structure, Baltic populations of summer-and autumnreproducing morphs did not separate in a cluster analysis, indicating minor, if any, barriers to gene flow between them. Our results have important implications for management and conservation of F. vesiculosus, and we raise a number of concerns about how genetic variability should be preserved within this species.