New markers - old questions: population genetics of seagrasses

Marine angiosperms, or seagrasses, continue to be a major focus of marine b iologists because of their important ecological role in many coastal ecosys tems. Seagrass population biology could benefit from a population genetic p erspective because genetic data enable the extraction of useful demographic information such as isolation and gene flow between demes. Moreover, popul ation genetic processes may contribute to the growing ecological risks of l ocal population extinction. Progress in seagrass genetics is partly driven by novel genetic markers which detect variation at the DNA level and overco me the limited polymorphism of allozymes. Key results of studies in the pas t decade, mostly using RAPD and microsatellites, were (1) considerable gene tic and genotypic (clonal) diversity is present in several species in contr ast to earlier notions of low polymorphism detected at allozyme loci, and ( 2) genetic differentiation among populations seems to be the rule despite e arlier reports of genetic uniformity. Pronounced genetic structure was dete cted between populations of 4 species examined thus far (Posidonia oceanica , P, australis, Zostera marina, Thalassia testudinum). The F-ST estimates v aried widely and ranged from 0.01 to 0.623 across studies and species. Gene tic differentiation at a systematic range of scales was only studied in eel grass Zostera marina, where it was positively correlated with geographic di stance. The high polymorphism of RAPD or microsatellite markers will allow the augmention of indirect estimates of gene flow by methods detecting indi vidual immigration events through paternity analysis or assignment tests. I mportant conservation related issues such as the level of inbreeding and th e effective population size have also been obtained from genetic marker dat a, but results are too scarce at the moment to allow generalizations. In Zo stera marina and Posidonia austrails, several population genetic attributes such as clonal diversity, mating system and effective population size vari ed among populations within species, highlighting that there is no 'typical ' population. An important gap in our knowledge is whether the effects of n atural population fragmentation and patchiness enhance the genetic isolatio n of populations due to anthropogenic disturbances. It is also unclear whet her genetic differentiation displayed at marker loci are correlated with fi tness-related plant traits, and whether genetic or genotypic diversity is i mportant for medium- to long-term meadow persistence. An assessment of the genetic and genotypic diversity at marker loci should be combined with expe riments on the ecological plasticity and reaction norms of genotypes compos ing the populations in question. This way, the role of genetic diversity fo r seagrass population maintenance and growth in the face of changing enviro nmental conditions can be evaluated.