Influence of the genetic variability and plasticity on the fitness of a common intertidal seaweed

The ability of species to adapt to environmental change forms an important part of their stability and resilience. Many organisms show large phenotypic variation across environmental gradients and both terrestrial plant and sessile marine species often exhibit very different ecotypes in different environments. This phenotypic variation can be driven by a combination of genetic and non-genetic processes but their relative importance may differ between sites. In seaweeds, the potential to express phenotypic plasticity appears common and is of ecological importance as it increases fitness by allowing a response within the organism's life-time. In contrast, much less is known about the factors that might cause local adaptation in seaweeds. The habitat-forming seaweed Hormosira banksii plays an important role in structuring intertidal communities in southern Australia and several ecotypes are recognised across its distributional range. These different ecotypes are thought to reflect phenotypic responses to variable environmental conditions. In this thesis, I examined patterns of morphological variation in Hormosira and assessed the extent to which those patterns reflect environmental or genetic effects via reciprocal transplant experiments and their correlation with molecular variation. There was substantial morphological variation in Hormosira across its geographic distribution in Tasmania, Australia. In particular north coast individuals had a distinct 'bushy' morphology, smaller vesicles and shorter fronds, compared to other regions and tidal regime was identified as the best predictor of morphological differences between these regions. Semi-diurnal tides at Tasmania's north coast create more frequent and greater temperature fluctuations for Hormosira which likely results in higher levels of thermal and desiccation stress. To test whether the local environmental factors determine phenotypic variation, juvenile Hormosira recruits were reciprocally transplanted between sites on the north and east coasts and their phenotype assessed after 12 months. These transplantations showed that the site of origin strongly affected morphology in Hormosira individuals after 12 months with none of the transplanted Hormosira developing a similar overall morphology to local Hormosira at the recipient site. Hormosira from the eastern populations did change in some traits when transplanted to the north coast indicating that certain morphological traits are more plastic than others which potentially allows them to adjust their morphology to conditions on the more stressful north coast. In contrast, individuals originating from the north coast had no plastic response when transplanted to the east coast suggesting stronger genetic control on morphology in those populations or, that the northern phenotype is established early in development. Moreover, transplantation mostly resulted in reduced growth rates and local individuals usually showed higher growth rates compared to foreign individuals, indicative of local adaptation. A second reciprocal transplant experiment of Hormosira embryos showed that embryos originating from the north coast had greater survival and growth in their local north coast environment compared to the east coast environment, reinforcing the idea that the northern origin phenotype may be locally adapted to greater emersion stress at the north coast. Survival of embryos from eastern populations did not vary between the local and foreign habitat but growth was enhanced in the foreign environment, indicating a capacity to benefit under the novel conditions. The phenotypic variation among Hormosira populations in Tasmania appears decoupled from cytochrome oxidase 1 (CO1) sequence variation with most Tasmanian populations having a single haplotype. However, across all southeastern Australia, there was clear partitioning of populations into three major haplotype groups (western, central and eastern) which likely reflects events and changes prior to and during the Last Glacial Maximum (approximately 25,000 years ago), and subsequent patterns of recolonisation of Hormosira into Tasmania. Current boundaries and break-points between haplotypes correspond with biogeographical provinces in this region and are possibly maintained by the short distance dispersal of Hormosira and a combination of habitat discontinuity and complex oceanographic features conditions that decrease the probability of long distance dispersal. Overall, this study indicates that morphological variation in Hormosira is influenced by both genetic and environmental factors but their relative importance appears populationspecific. In particular, limited plasticity shown by both juveniles and embryos originating from the north coast highlight the significance of fixed genetic effects in possibly allowing the north coast morph to adapt to the greater environmental fluctuations on that coast.