Fitness-consequences of geitonogamous selfing in a clonal marine angiosperm (Zostera marina)

Plant mating systems have received considerable attention because the proportion of selfed vs. outcrossed progeny is an important evolutionary factor. In clonally reproducing plants, geitonogamous selfing between distant ramets belonging to the same genet is expected to be widespread, yet empirical data are sparse. Nothing is known about between-ramet selfing in aquatic flowering plants with subaqueous pollen transfer, most of which display pronounced clonal reproduction. From two locations in the western Baltic Sea, I present data on the effects of patch isolation and clonal diversity on the outcrossing rate of eelgrass, Zostera marina L., based on the genotypes of maternal plants and recently fertilized ovules scored at eight microsatellite loci. There were no differences in outcrossing rates between vegetation patches and continuous meadow although patches were nearly always composed of single genets. Quantitative effects of clonal diversity were present in the continuous vegetation where a significant positive correlation between genet diversity and the proportion of outcrossed offspring was detected (Kendall's tau = 0.82, P = 0.0017). On a population-scale as well, the genotypic diversity was positively correlated with outcrossing. The relative fitness of selfed offspring was low (omega +/- 95% confidence interval = 0.56 +/- 0.032 and 0.322 +/- 0.15) indicating that geitonogamy incurred substantial fitness costs. Selfing rates in Z. marina may not be in evolutionary equilibrium because of spatial and temporal heterogeneity of clonal size and diversity. The high prevalence of dioecy in seagrasses may have evolved to avoid the fitness costs associated with geitonogamy.