Mangrove ecosystems of the South China Sea, along the coastal area of the northernmost submerged edge of the Sunda Shelf became established during the Holocene. The colonization potential by mangrove tree species is supposed to be high due to the dispersal properties of their propagules and the adaptation to estuarine conditions and coastal currents. Rhizophora is expected to disperse better than Avicennia and hence is expected to show less genetic structure over greater distances. We tested this hypothesis for the common and wide-ranging mangrove tree species A. marina and R. stylosa, with a sampling design along a same coastline in Northeastern Vietnam. Genetic structure and connectivity at both large and fine scale were analyzed at individual and population level using microsatellite markers in 229 A. marina individuals and 167 R. stylosa individuals. These revealed an overall low level of allele and gene diversity for both species, with significant levels of inbreeding of both species throughout the area. Over distances up to 160 km, A. marina populations revealed a genetic substructure of several gene pools whereas a single admixed gene pool for R. stylosa was apparent. A fine-scaled structure was observed in northernmost A. marina populations, but seldom and much weaker in R. stylosa. A strong relatedness and structure within 1 km distance in A. marina was found while R. stylosa revealed a much lower relatedness. Overall, the differences in connectivity levels, as detected from relatedness for R. stylosa and A. marina along a same coastline and at site level, thus could be related to their respective propagule dispersal properties. |
