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.

Modelling mangrove propagule dispersal trajectories using high‐resolution estimates of ocean surface winds and currents

“…, Ngeve et al . ). Our results confirm that oceanic processes such as eddies could alter the dispersal trajectories of drifting organisms (Sponaugle et al .…”

“…, Ngeve et al . ). However, these models typically do not include interactions between wind and water currents as drivers for the transport of propagules with different traits.…”

“…Dispersal and connectivity models have previously been employed on local to global scales for describing the movement of marine organisms (Cowen et al 2006, Werner et al 2007, Treml et al 2008, Cowen & Sponaugle 2009, Mora et al 2012, Paris et al 2013, Simpson et al 2014, Wood et al 2014. For mangroves, propagule dispersal has been simulated on a local scale in the Pambala-Chilaw Lagoon Complex (Sri Lanka) (Di Nitto et al 2013), and recently also on regional scales for explaining population genetic structure (Kennedy et al 2016, Ngeve et al 2016. However, these models typically do not include interactions between wind and water currents as drivers for the transport of propagules with different traits.…”

Vicariance and Oceanic Barriers Drive Contemporary Genetic Structure of Widespread Mangrove Species Sonneratia alba J. Sm in the Indo-West Pacific

“…In the latter, the divergence was attributed to the fruiting season that coincided with northward-flowing ocean currents cutting through the South China Sea into the Pacific Ocean, thus preventing dispersal between populations from the east and west corners of the South China Sea [49]. Landscape level propagule release simulations, which account for ocean circulation patterns, will be able to cross-validate these inferences [50].…”

Landscape genetic structure of Scirpus mariqueter reveals a putatively adaptive differentiation under strong gene flow in estuaries

“…To date, only a few species in estuary have been studied on their genetic structure and researchers mainly focused on a small group of animals (Bible & Sanford, ; Bilton, Paula, & Bishop, ; Dennenmoser, Vamosi, Nolte, & Rogers, ; McCairns & Bernatchez, ; Sanford & Kelly, ) while few higher plant species have been studied (Bilton et al, ; but see, e.g., Ngeve, Stocken, Menemenlis, Koedam, & Triest, ; Ngeve, Stocken, Menemenlis, Koedam, & Triest, ). Additionally, these studies have shown that many are more structured than could be expected despite a lack of barriers of dispersal in estuarine system, suggesting some dispersal limitation through geographical distance, environmental variation, or other cryptic barriers promotes isolation (Kesäniemi, Hansen, Banta, & Knott, ).…”