Seed enzyme polymorphism and autogamy of the seagrass Posidonia oceanica from the western Mediterranean

An exceptionally productive flowering season was observed in the Mediterran ean meadows of Posidonia oceanica (L.) Delile in the autumn of 1994. At tha t time some hermaphroditic inflorescences were bagged, and set fruits, indi cating that this species can be self-fertile. By spring 1995, the fruits of Tt oceanica had matured; the seeds from nine Mediterranean meadows [French continental (3 populations), Corsica (5 popul ations) and Tunisian coasts (1 population)] were harvested, weighed and ana lysed for enzyme diversity. Genetic variability was estimated based on the enzymatic polymorphism of 9 loci. Three of the loci were found to be monomo rphic, two were frequently polymorphic within only one population and the r emaining four were polymorphic in most of the populations. This demonstrated there was very little genetic variability, with an averag e observed heterozygosity of 0.0205 for all loci within all populations. Ac cording to the Hardy-Weinberg equilibrium, expected heterozygosity was 0.02 26. The observed lack of heterozygotes was not statistically significant. T he enzyme polymorphism of seeds did not show a geographic diversity pattern although the bio-mass of seeds increased along a north-south gradient sugg esting an increased reproductive fitness towards the southern Mediterranean populations. These results support the hypothesis that P. oceanica meadows are made up of a few clones extending over large areas. The clonal growth and self-fertilisation in these populations suggest a predominance of autog amy resulting in a high degree of inbreeding.