Genetic differentiation between populations of the ascidian Aplidium falklandicum from South Georgia and South Orkney Islands

MILAGROS DEMARCHI; MARINA CHIAPPERO; RICARDO SAHADE

Ushuaia, Argentina

Workshop; IBMANT-ANDEEP (Interactions between the Magellan Region & the Antarctic-Antarctic Benthic Deep-sea Bioiversity) Intl. Symposium & Workshop; 2003

The genetic structure of populations of sessile marine animals depends largely on the dispersal abilities of the larval stages. It is expected that species with higher dispersal capabilities will present less genetic structure than those with larvae that disperse only relative short distances, which in turn would present small scale genetic differentiation. Among the factors that could affect the dispersion of the free-living stages, and therefore the gene flow between populations are the variable spawning and recruitment success, habitat availability, oceanographic conditions and physical barriers.   The Polar Front and the abissal depths that surround Antarctica can be considered as the main physical barrier in isolating the Antarctic system, particularly effective for shelf benthic species. It has been hypothesized that the Scotia Arc Islands could act as a bridge, or step stones, between the Magellan and Antarctic Regions. In this work we will test this hypothesis on Aplidium falklandicum, an ascidian species that due to its reproductive behaviour, short lived and lecithotrophic larvae, common to all the group, can be a good model for testing possible gene flow  throught the Scotia Arc in sessile benthic species. Genetic population structure of two populations of A. falklandicum at South Georgia and South Orkney Islands were determined using simple sequence repeats (ISSR). ISSR are semiarbitrary markers amplified by PCR. Three primers were used: MARA: [(AC)10 AA]; DO: [(AC)10 AG] and NA: [(GACA)5] and amplified a total of 93 bands. AMOVA and multivariate analysis clearly separated both populations (FST = 0.43 P < 0.01) and Nucleotide Diversity Indices indicated that South Orkney were more diverse than South Georgias population (SG: p = 0.0056; SO: p = 0.010). This pattern could be produced by serial bottle-neck processes suffered by SG population or even by local extinction and recolonization processes. Then alleles are eliminated from the population by genetic drift. These ideas are consistent with the environmental instability caused by the suggested variable position of the Polar Front in this area. While the separation of the populations suggest the absence of gene flow between them, is also true that even existing some actual conection this could be hindered by bottle-neck and genetic drift processes in SG population determining that alleles reaching the SO population would not be present any more in the source population and little more can be said with only two populations sampled. These results even fragmentary show the potential of molecular methods for a better understanding of ecological and evolutionary processes in the Antarctic system.