Genetic structure of the subtidal red alga Delisea pulchra

We used random amplified polymorphic DNA (RAPDs) to examine small-scale spa tial genetic structure in the red alga Delisea pulchra (Greville) Montagne at two locations near Sydney, Australia. We examined genetic structure amon g plants at four spatial scales ranging from 2 km apart down to <50 cm apar t between locations, among sites within locations, among quadrats within si tes, and among plants within quadrats. Haploid stages of D. pulchra were ab sent from the populations studied, suggesting that they are maintained thro ugh asexual reproduction of diploid plants. Consistent with this, we found that 19 RAPD phenotypes scored in this study had multiple individuals, indi cating the presence of clones in these populations. However, there were no RAPD phenotypes common to two locations separated by only 2 km. Analysis of molecular variance revealed that strong genetic differences occurred betwe en plants from these two locations, with 46.3% of the total genetic variati on occurring at this scale, most probably reflecting limited gene flow. Wit hin each location, <25% of the genetic variation was attributable to differ ences among sites or quadrats, indicating gene flow at those smaller scales . Most of the variation within each location occurred at the smallest spati al scale, among plants within 0.25 m(2) quadrats. Nonetheless, some pairwis e genetic distances (phi(ST)) between sites or quadrats within locations we re large, indicating some genetic divergence on smaller scales. Genetic dis tance was independent of spatial distance within both locations, suggesting that fine-scale differences within locations were most probably caused by variation in fine-scale patterns of water movement or fine-scale natural se lection. We assessed the impact of one potential selective agent, grazing s ea urchins, on the fine-scale genetic structure of D. pulchra. There was no evidence that grazing by sea urchins affected the genetic structure of D. pulchra. In combination with demographic data, our results indicated that l ocal populations of D. pulchra within locations were relatively open and th at fine-scale genetic structure was probably constrained by gene flow. At t he larger scale however, strong genetic differentiation indicated little ge ne flow between locations and restricted dispersal of spores.