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Published March 10, 2016 | Published + Supplemental Material
Journal Article Open

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

Abstract

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.

Additional Information

© 2016 Ngeve et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Received: November 12, 2015; Accepted: February 22, 2016; Published: March 10, 2016. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: 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. Acknowledgments: We extend sincere gratitude to Ma Christiana Nalova Ngenye, Mr. and Mrs. Ndoko Joseph (Ekondo-Titi), Pr. and Mrs. Oben Samuel (Bekumu), Mr. Peter Elive Ikome (Douala), Pr. Eban Daniel, Pr. Ntakim Clarence, Rev. Okala Dieudonne Bienvenue, Pr. and Mrs. Atiko Pierre (Kribi), Elder Atkin Egbe Obie (Kribi), Eugene N. Ngeve, and Ms Natalie Kana for logistical and field assistance.We are also grateful to Tim Sierens for technical and laboratory assistance. The numerical ocean modeling component of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA). Author Contributions: Conceived and designed the experiments: MNN. Performed the experiments: MNN TVdS. Analyzed the data: MNN. Contributed reagents/materials/analysis tools: MNN TVdS DM LT. Wrote the paper: MNN TVdS. Originally formulated the idea: MNN NK LT. Collected the samples: MNN. Reviewed the manuscript: MNN TVdS DM NK LT.

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