Hidden founders or high connectivity or both? Investigating the expansion pathway of mangroves along the Cameroonian coastline

The probability of mangrove propagules released from their mother trees to leave the immediate environment of their mother trees is dependent on several factors, such as availability of dispersal vector (water) at the time of release, density of trees and root network in the vicinity of the mother trees, as well as other barriers. And although mangrove propagules have potentials of long distance dispersal, short dispersal distances are mostly observed, giving rise to leptokurtic dispersal kernels. Rhizophora spp. form dense root networks which strongly increases retention of large propagules, such as those of Rhizophora spp. Genetically, the expected end result of high retention rates over time is spatial aggregation of genetically related individuals. Although Rhizohora species have high potential for such a fine scale genetic structure, several site specific features can alter expected outcomes. We investigated comparative fine scale genetic structure of Rhizophora racemosa populations from the entire coast of Cameroon, using 11 polymorphic microsatellite markers. Populations of the most anthropogenically disturbed mangrove area in Cameroon (the Cameroon Estuary complex) were those with significant fine scale genetic structure, and these populations also had evidence of significant reduction in effective population sizes (Bottlenecks). This led to us hypothesizing that due to the high anthropogenic pressures on these sites, these were the most “recent” mangroves in Cameroon, due to recent expansions replacing the lost mangrove sites. We therefore tested four expansion models of mangroves along this coastline, which supported this claim. Although we observed high levels of gene flow in mangroves of this estuary, based on our evidences, we conclude that both high connectivity and hidden founder effects characterize the spatial patterning of genetic diversity in the Cameroon Estuary complex.

Publication year:2017