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Genetic structure of Rhizophora mucronata populations of Southeast Africa and its relationship with ocean currents and geographic distance
Lorent, S. (2015). Genetic structure of Rhizophora mucronata populations of Southeast Africa and its relationship with ocean currents and geographic distance. MSc Thesis. Vrije Universiteit Brussel: Brussel. 30 pp.

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Document type: Dissertation

    Rhizophora mucronata Poir. [WoRMS]

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    Long distance dispersal has a major importance in maintaining the evolutionary potential of species. Indeed, by enabling genetic exchanges between distant populations, it increases the genetic diversity within populations, augmenting their resilience towards future environmental changes. Long distance dispersal is particularly important in the present day context of increasing anthropogenic pressure and disturbances and climate change. Hence, in the past few years, there has been an increasing recognition of the importance of expanding the knowledge on the dispersal patterns of species as well as the factors shaping them. In this study, we used 5 microsatellite markers to investigate the genetic pattern of Rhizophora)mucronata Lam. in 13 populations of Southeast Africa ranging from Kenya to Southern Mozambique, including populations from Madagascar and the Seychelles. R.) mucronata is recognised to have high long distance dispersal potential as it possesses buoyant hydrochorous torpedoHshaped propagules capable of withstanding many days (or even months) at sea. The aim of this study was to gain knowledge on the patterns of connectivity, as well as on potential explanatory factors. For this reason, knowledge on ocean currents and a Mantel test was used to test the congruence between genetic structure and both geographic distance and ocean currents. Results indicated that populations of the studied region did not form a panmictic unit, and that both isolation by distance (IBD) and ocean currents were responsible for the present genetic structure of the 13 sampled populations. Ocean currents could either promote admixture between distant populations, or create a genetic break by preventing certain waters to mix. As an example, the South Equatorial Current brings admixture between eastward islands and westward mainland populations. On the contrary, as the same current arrives on the coastline, it bifurcates into a northward and southward component, preventing gene flow between the mainland populations situated North and South of the bifurcation. Moreover, we evidenced that populations sitting in central locations in terms of ocean currents and geographic distances with other populations, were more diverse than the more peripheral populations. This study presents insight on the connectivity of R.) mucronata populations in Southeast Africa, a region where no population genetic studies have yet been published for any mangrove species. Furthermore, it provides evidence that in addition to IBD, ocean currents are also a factor to consider when studying connectivity between populations and the repercussions that anthropogenic disturbances could have on it.

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