THE MATING SYSTEM OF AN HYDROPHILOUS ANGIOSPERM POSIDONIA-AUSTRALIS (POSIDONIACEAE)

The hydrophilous seagrass Posidonia australis has a wide range of mult ilocus outcrossing rates (t), which vary from 0 to 0.89, with ''appare nt'' outcrossing rates varying from 0 to 0.42 among the seven populati ons sampled. This pattern of outcrossing rate indicates that water pol lination (hydrophily) is less uniform than wind pollination and more s imilar to animal pollination in its variability. Variation in levels o f outcrossing between populations may be due to differences in water m ovement; for example, open bays have greater pollen dispersal and high er outcrossing rates. Considerable pollen movement within meadows was inferred from a high frequency of nonmaternal alleles in the pollen po ol. The distribution of genetic diversity among populations (G(ST) = 0 .229) suggests moderate gene flow on the local scale. These results de monstrate that successful submarine cross-pollination occurs in the hy drophile P. australis, which has a diverse mating system with populati ons that range from predominantly inbred to predominantly outcrossed.