Marine genomics meets ecology : diversity and divergence in South African sea stars of the genus Parvulastra

• Main Author: Dunbar, Katherine
• Published: Cardiff University 2006
• Subjects: 576.58
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583987

The coast of South Africa is situated between the warm Indian and the cold Atlantic Oceans, resulting in an extreme intertidal temperature gradient and potentially strong opposing selection pressures between the east and west coasts. Several intertidal biogeographic divides have been identified, including one at Cape Point between the cold west coast and the temperate south coast provinces. However, few studies have investigated the effects of these opposing environments on phylogeography or gene flow in intertidal organisms. A small intertidal sea star, Parvulastra exigua, was chosen as a model organism to investigate these issues. Taxonomic confusion in this species and its systematic relationship with a related South African Parvulastra species, Parvulastra dyscrita, was resolved using nuclear (Actin intron sequences and AFLP) and mtDNA molecular markers. At least one cryptic species was identified within Parvulastra in South Africa, which occupied an extremely restricted geographic distribution and therefore may be a candidate for conservation. Molecular and morphological evidence confirmed that P. exigua and P. dyscrita are separate species. An ecological survey was conducted on P. exigua at 19 locations in South Africa covering a distance of 2500 km. P. exigua samples from each location were sequenced for mtDNA and screened for 421 AFLP loci. AFLP was also used to identify outlier loci that were potentially under selection. An 'unmottled' colour morph was distributed from the Namibian border to Cape Point and a 'mottled' morph was distributed from Cape Point to the Mozambique border, with an area of sympatry around Cape Point. The unmottled morphs were positively influenced by under boulder and bare rock habitats, but negatively affected by canopy, coralline algae and sand. Mottled morphs were positively influenced by under boulder, protected habitats, encrusting algae and bare rock, and negatively affected by algal tufts and sand. MtDNA revealed two divergent, reciprocally monophyletic clades, one comprising the east coast samples and the other encompassing the west coast samples. Both clades showed evidence for a recent, rapid population expansion. The genetic break-point was located on the south coast, but did not coincide with the divergence in colour morphs, being approximately 500 km to the east. AFLP indicated a strong isolation by distance pattern of genetic structure among sampling locations and did not recapitulate the mtDNA genetic divide. Such incongruence among data sets might be caused by a vicarance event if sea level changes separated the east and west coast populations, which expanded in isolation, followed by secondary contact, restoring present day gene flow between the coasts. Population genomic analysis revealed approximately 7% of the genome to potentially be under divergent selection, and the phenotype frequencies of the 'diverging outlier loci' revealed high directionality (spatial correlation). This suggests strong selection pressures between the east and west coasts may be acting on these loci, which could have arisen when the populations were in allopatry. The habitat and colour morph differences of P. exigua between the two coasts are potentially also influenced by selection. However, the isolation by distance pattern indicates that divergent selection pressures are not strong enough to cause reproductive isolation, or disrupt gene flow between the east and west coast populations.