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, 11 (3), e0150950

Contrasting Effects of Historical Sea Level Rise and Contemporary Ocean Currents on Regional Gene Flow of Rhizophora Racemosa in Eastern Atlantic Mangroves


Contrasting Effects of Historical Sea Level Rise and Contemporary Ocean Currents on Regional Gene Flow of Rhizophora Racemosa in Eastern Atlantic Mangroves

Magdalene N Ngeve et al. PLoS One.


Mangroves are seafaring taxa through their hydrochorous propagules that have the potential to disperse over long distances. Therefore, investigating their patterns of gene flow provides insights on the processes involved in the spatial genetic structuring of populations. The coastline of Cameroon has a particular geomorphological history and coastal hydrology with complex contemporary patterns of ocean currents, which we hypothesize to have effects on the spatial configuration and composition of present-day mangroves within its spans. A total of 982 trees were sampled from 33 transects (11 sites) in 4 estuaries. Using 11 polymorphic SSR markers, we investigated genetic diversity and structure of Rhizophora racemosa, a widespread species in the region. Genetic diversity was low to moderate and genetic differentiation between nearly all population pairs was significant. Bayesian clustering analysis, PCoA, estimates of contemporary migration rates and identification of barriers to gene flow were used and complemented with estimated dispersal trajectories of hourly released virtual propagules, using high-resolution surface current from a mesoscale and tide-resolving ocean simulation. These indicate that the Cameroon Volcanic Line (CVL) is not a present-day barrier to gene flow. Rather, the Inter-Bioko-Cameroon (IBC) corridor, formed due to sea level rise, allows for connectivity between two mangrove areas that were isolated during glacial times by the CVL. Genetic data and numerical ocean simulations indicated that an oceanic convergence zone near the Cameroon Estuary complex (CEC) presents a strong barrier to gene flow, resulting in genetic discontinuities between the mangrove areas on either side. This convergence did not result in higher genetic diversity at the CEC as we had hypothesized. In conclusion, the genetic structure of Rhizophora racemosa is maintained by the contrasting effects of the contemporary oceanic convergence and historical climate change-induced sea level rise.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Fig 1
Fig 1. The coastal area of Cameroon indicating the studied estuaries and major dispersal corridor and barrier to gene flow.
Map generated from Hydrography shape files obtained from the World Resource Institute (WRI) Congo Basin Forest Atlases webpage:
Fig 2
Fig 2. The coastal area of Cameroon with the 11 sites as in Table 1 and the proportions of two inferred clusters of individuals for each population.
Proportions of inferred clusters (Q-values) per population are represented as bar charts at their respective geographical locations on the map.
Fig 3
Fig 3. Principal Coordinate analysis (PCoA, above) and Neighbor-joining tree (NJ tree, below).
The grouping of 11 populations of Rhizophora racemosa along the entire coast of Cameroon into 3 groups with high admixtures of populations from the Rio Del Rey Estuary and the Cameroon Estuary complex and isolation of first group of populations of the Loukondje Estuary (Kribi, site 10) and the Ntem Estuary (Campo, site 11). The second group is made-up of 2 landward populations from the Rio Del Rey Estuary (MBO, site 2 and EKO, site 1) (>75% bootstrapping). A third large group consisting of one seaward population from the Rio Del Rey Estuary (BEK, site3) and 5 other populations from the Cameroon Estuary complex (sites 5–9). Bootstrap values ≥ 75 are indicated on each node of the NJ tree and site numbers (1–11) are indicated beside the pop ID’s.
Fig 4
Fig 4. Pairwise contemporary migration rates between populations based on Bayesian estimates using individual multilocus genotypes.
Fig 5
Fig 5. Strong isolation by distance (IBD) among Rhizophora racemosa populations along the coast of Cameroon.
The pairwise actual genetic differentiation (Dest) increases from 0 to 7% over a direct flight geographic distance of up to 300 km), p < 0.001.
Fig 6
Fig 6. Box-whisker plot of mean population pairwise Fst values between “Within” ocean current population pairs group and ‘Between’ ocean current population pairs group.
Fig 7
Fig 7. Location of virtual propagules (dots) after 3 months of floating.
Virtual propagules were released hourly during the months February, March, April, September and October 2012, since these months correspond with propagule release periods for Rhizophora racemosa in the study area (personal observation). Hence, in total, 3626 virtual propagules were released in each of the locations. Author-defined release locations correspond with the localities where samples for our genetic analyses were collected, which were subsequently shifted to the closest ocean point (rectangles) to ensure the possibility of particle movement. Site numbers (1–11) are indicated beside the corresponding point.
Fig 8
Fig 8. Gene flow barriers (red lines) and hypothetical barriers (Black dotted lines).
The thickness of the red line indicates the importance of the barriers based on the 3 different distance matrices (pairwise Nei’s genetic distances between populations (S3 Table), pairwise Fst, Pairwise Dest) used in the analysis.

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Grant support

This study was financed by the Vrije Universiteit Brussel—International Relations and Mobility Office (VUB—IRMO) Doctoral Scholarship awarded to M.N. Ngeve. The Doctoral School NSE of the VUB also awarded a travel grant (NSE-TG-2013-82) to M.N. Ngeve. The BAS 42 funding of the VUB also supported the laboratory analyses of this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.