Molecular ecology and biogeography of mangrove trees towards conceptual insights on gene flow and barriers: A review

In this review the most recent contributions to the field of molecular ecology and biogeography of mangrove trees are considered. Emphasis is on the obtained information of the different molecular marker methods used in mangrove genetics and on the potential to infer biogeographical patterns. Isozymes on average showed low or even no polymorphism in mangrove trees similar as known in seagrasses. The outcrossing Avicennia seems to be the most variable mangrove tree for isozymes. Both low amounts of interpretable allozymes and difficulties in maintaining the enzyme activity have reduced the number of successful studies during the isozyme era. Dominant marker methods (RAPD, AFLP and ISSR) were successful to demonstrate differences in amplified DNA products at large-scale geographical distances within Avicennia species and to estimate species relationships. Hybrid testing seldom revealed hybridization among tree species. The most promising markers (microsatellites or SSR) were only recently developed and will continue to provide evidence in future studies. SSR loci in Avicennia seem to show relatively low levels of polymorphism, though clearly demonstrating that populations located at the edge of the species range can be even more depauperated. Populations located more central in their native range and situated along the same coastline such as reported in Rhizophora, are expected to be only weakly differentiated due to increased levels of gene flow. Haplotypic chloroplast variants (PCR-RFLP) or sequences revealed strong genetic structuring between populations of Avicennia, Kandelia and Ceriops from different biogeographical oceanic regions. Recent views on long-distance dispersal and on gene flow across oceans as well as along the same coastline are discussed. A comparative analysis on genetic variables across species and regions indicated general trends in the partitioning of genetic variation. A conceptual map with a worldwide overview of those regions where high levels of gene flow were reported and of other regions that were considered as effective barriers, is presented. As an aim to increase the number of reliable comparisons of genetic variables across species or regions and to increase the relevance of mangrove genetics for local conservation issues, recommendations on the molecular markers and on the sampling design of individuals and populations are given within a conceptual context of evolutionary significant units.