This species has had the greatest number of publications in the genetic analysis literature for seagrasses (Moore et al., Chapter 16). Nonetheless, only re cently a first study has been produced that documents the genetic diversity and genetic structure of the species throughout almost its entire geographical range (Olsen et al., 2004). This shortcoming may be due in part to the particularly widespread distribution of this species which occurs across the majority of available habitats in the northern hemisphere. Studies along the Pacific coasts of North America date back to 1992-1994 and have been carried out using RFLP (Fain et al., 1992), allozymes (Laushman, 1993) and DNA fingerprinting analyses (Alberte et al., 1994). Of these studies, DNA fingerprinting showed higher polymorphism within meadows, as expected for this marker type. More recently, other studies have utilized allozymes to compare distinct Z. marina meadows in California and Chesapeake Bay, showing Fst values ranging from 0.06 to 0.335, depending on the geographic distance among meadows (Williams and Davis, 1996; Williams and Orth, 1998). In the last few years, genetic diversity in Z. marina has been studied extensively along the northern coasts of Europe using polymorphic microsatellite loci (Reusch, 1999c among the others). The use of the same markers shows high al-lelic richness in the Pacific populations in respect to the Atlantic ones and clear genetic distinction between southern and northern east-Pacific populations (Olsen et al., 2004). Unfortunately, the values of polymorphism observed using microsatellites could not be compared directly with results obtained from markers utilized previously.
The aforementioned study on the distribution of microsatellite genetic diversity along the whole geographic range of the species (Olsen et al., 2004)
provides a much comprehensive, comparable picture of the overall phylogeographic pattern of this species. Among the previous broader scale studies on Zostera marina to date, pronounced genetic sub-structuring was observed among eight European populations, with a strong linear relationship of genetic differentiation along geographic distances of 12-4,500 km (Reusch et al., 2000). However, on a larger geographic scale the observed pattern was counterintuitive, with two North American populations clustering with those from the Baltic Sea and North Sea. New data support recent genetic exchange in Z. marina, between the east Pacific, west and east Atlantic, suggesting a still-active trans-Arctic connections (Olsen et al., 2004). Instead, a weak correlation of genetic and geographic distance was found in populations sampled in the northern Wadden Sea and south-western Baltic Sea, indicating enhanced metapopulation dynamics in the area (Reusch, 2002). Recolonisation in the two areas may have occurred recently, masking any signal resulting from recent gene flow among populations. In general, populations showed high connectivity (d = 0.018), despite the fact that populations are annual in the Wadden Sea and perennial in the Baltic. Theoretically annual Wadden Sea populations may show higher connectivity due to greater propagule production and stronger tidal currents. The vectors of such high gene flow are still unclear. Assignment tests conducted on rafting reproductive shoots shows that they can have an important role in dispersing genotypes up to 50 km (Reusch, 2002).
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