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

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

Calm after the storm? Similar patterns of genetic variation in a riverine foundation species before and after severe disturbance

Abstract In summer 2011, Tropical storms Lee and Irene caused an estimated 90% decline of the submersed aquatic plant Vallisneria americana Michx. (Hydrocharitaceae) in the Hudson River of New York (USA). To understand the genetic impact of such large‐scale demographic losses, we compared diversity at 10 microsatellite loci in 135 samples collected from five sites just before the storms with 239 shoots collected from nine sites 4 years after. Although 80% of beds sampled in 2011 lacked V. americana in 2015, we found similar genotypic and genetic diversity and effective population sizes in pre‐storm versus post‐storm sites. These similarities suggest that despite local extirpations concentrated at the upstream end of the sampling area, V. americana was regionally resistant to genetic losses. Similar geographically based structure among sites in both sampling periods suggested that cryptic local refugia at previously occupied sites facilitated re‐expansion after the storms. However, this apparent resistance to disturbance may lead to a false sense of security. Low effective population sizes and high clonality in both time periods suggest that V. americana beds were already small and had high frequency of asexual reproduction before the storms. Dispersal was not sufficient to recolonize more isolated sites that had been extirpated. Chronic low diversity and reliance on asexual reproduction for persistence can be risky when more frequent and intense storms are paired with ongoing anthropogenic stressors. Monitoring genetic diversity along with extent and abundance of V. americana will give a more complete picture of long‐term potential for resilience

Principal Coordinate analysis (PCoA, above) and Neighbor-joining tree (NJ tree, below).

<p>The grouping of 11 populations of <i>Rhizophora racemosa</i> 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.</p

Analysis of Molecular Variance (AMOVA-Fst) based on a pooling of populations (Hierarchical) into estuaries and non-pooling (non-hierarchical).

<p>Analysis of Molecular Variance (AMOVA-Fst) based on a pooling of populations (Hierarchical) into estuaries and non-pooling (non-hierarchical).</p

Pairwise population genetic differentiation.

<p>Pairwise population genetic differentiation.</p

Gene flow barriers (red lines) and hypothetical barriers (Black dotted lines).

<p>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 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150950#pone.0150950.s006" target="_blank">S3 Table</a>), pairwise Fst, Pairwise D_est) used in the analysis.</p

Location of virtual propagules (dots) after 3 months of floating.

<p>Virtual propagules were released hourly during the months February, March, April, September and October 2012, since these months correspond with propagule release periods for <i>Rhizophora racemosa</i> 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.</p

Descriptive statistics of genetic diversity based on 11 nuclear microsatellite loci for 11 populations of <i>Rhizophora racemosa</i> from the Coast of Cameroon.

<p>Descriptive statistics of genetic diversity based on 11 nuclear microsatellite loci for 11 populations of <i>Rhizophora racemosa</i> from the Coast of Cameroon.</p

Box-whisker plot of mean population pairwise Fst values between “Within” ocean current population pairs group and ‘Between’ ocean current population pairs group.

<p>Box-whisker plot of mean population pairwise Fst values between “Within” ocean current population pairs group and ‘Between’ ocean current population pairs group.</p

Descriptive statistics of comparative genetic diversity of different estuaries.

<p>Descriptive statistics of comparative genetic diversity of different estuaries.</p