Dispersal limitation of the mangrove Avicennia marina at its South African range limit in strong contrast to connectivity in its core East African region

Understanding the genetic composition and dynamics of mangrove species along a latitudinal range could provide insight as to how their biogeographical range evolved. In this study, we investigate the genetic composition of the widespread mangrove species Avicennia marina in its core region and southern range limit along the East African coast to test for the effect of geographical location and coastal geomorphology on the genetic diversity and differentiation at the southern range limit. A total of 388 A. marina individuals from 6 sites in Kenya and Tanzania (core region) and 6 in South Africa (southern range limit) were sampled and genotyped using 8 microsatellite markers. A high genetic differentiation was found between the core and range-limit populations, with strikingly high within-population inbreeding at the range limit with a consequent high fixation (allele fixation at locus level) and stronger between-population differentiation. Despite the fact that A. marina propagules may have the capacity to disperse between populations, the gene flow was found to be moderate to high within the core populations and extremely low within the range limit populations. This study highlights a genetically depauperate situation in peripheral populations, most likely as a consequence of historical sporadic arrival of founders with subsequent inbreeding and dispersal limitation due to the coastal geomorphology in combination with range-edge effects