Expansion of the mangrove species Rhizophora mucronata in the Western Indian Ocean launched contrasting genetic patterns

Abstract

Estimates of population structure and gene flow allow exploring the historical and contemporary processes that determine a species’ biogeographic pattern. In mangroves, large-scale genetic studies to estimate gene flow have been conducted predominantly in the Indo-Pacific and Atlantic region. Here we examine the genetic diversity and connectivity of Rhizophora mucronata across a > 3,000 km coastal stretch in the Western Indian Ocean (WIO) including WIO islands. Based on 359 trees from 13 populations and using 17 polymorphic microsatellite loci we detected genetic breaks between populations of the (1) East African coastline, (2) Mozambique Channel Area (3) granitic Seychelles, and (4) Aldabra and northern Madagascar. Genetic structure, diversity levels, and patterns of inferred connectivity, aligned with the directionality of major ocean currents, driven by bifurcation of the South Equatorial Current, northward into the East African Coastal Current and southward into the Mozambique Channel Area. A secondary genetic break between nearby populations in the Delagoa Bight coincided with high inbreeding levels and fixed loci. Results illustrate how oceanographic processes can connect and separate mangrove populations regardless of geographic distance.

Avicennia Genetic Diversity and Fine-Scaled Structure Influenced by Coastal Proximity of Mangrove Fragments

“…This adds to the importance of coastal connectivity through gene flow (i.e., propagule flow) that is the only natural cohesive force between estuaries on longer term for a species to maintain its evolutionary units. Barriers to genetic connectivity however may come from land masses and different migration histories (Triest, 2008;Hodel et al, 2018;Wee et al, 2020;Triest et al, 2021a), opposite ocean currents (Mori et al, 2015;Ngeve et al, 2017) or very large rivers (Triest et al, 2018). Overall, the expectation that migration routes followed major oceanic and coastal currents is largely confirmed from genetic diversity and genetic structure approaches at population level.…”

Inheritance of DNA methylation differences in the mangrove Rhizophora mangle

“…While most mangrove species are not considered to be threatened, habitat loss continues to be a serious threat, with current average annual rates of loss of 1%–2% (Alongi, 2008; Polidoro et al, 2010; Triest et al, 2021). The resultant loss of diversity could pose risks for these coastal foundation species in the future, particularly as sea levels are projected to rise between 0.2 and 2 m over the next century due to anthropogenic climate change (Melillo et al, 2014).…”

Complete Chloroplast Genome Variants Reveal Discrete Long-Distance Dispersal Routes of Rhizophora in the Western Indian Ocean

“…populations along the Malay Peninsula could be explained by prevailing ocean currents and dispersal potential of the species, and not necessarily by a geographical distance (Wee et al, 2014(Wee et al, , 2020. Overall, ocean currents explain much of the patterns, both in the case of barriers caused by land masses (Triest, 2008) or opposing currents (Wee et al, 2014(Wee et al, , 2015(Wee et al, , 2020, as in the case of the bifurcation of equatorial currents (Mori et al, 2015;Ngeve et al, 2017;Triest et al, 2021b).…”

“…Rhizophora mucronata exhibits low levels of nuclear microsatellite diversity throughout its range despite the separation of WIO populations from other regions (Wee et al, 2015). The population genetic structure and estimates of gene flow and migration directionality of R. mucronata in the WIO using nuclear microsatellites revealed several genetic breaks and patterns of connectivity aligned with major ocean currents (Triest et al, 2021b). Along the African mainland coast, the bifurcation of the South Equatorial Current (SEC) separated an allelically more diverse region (East Africa) from the Mozambique Channel Area (MCA) with a genetic break located south of Delagoa Bight and characterized by monomorphism and strong inbreeding (Triest et al, 2021b).…”

“…The population genetic structure and estimates of gene flow and migration directionality of R. mucronata in the WIO using nuclear microsatellites revealed several genetic breaks and patterns of connectivity aligned with major ocean currents (Triest et al, 2021b). Along the African mainland coast, the bifurcation of the South Equatorial Current (SEC) separated an allelically more diverse region (East Africa) from the Mozambique Channel Area (MCA) with a genetic break located south of Delagoa Bight and characterized by monomorphism and strong inbreeding (Triest et al, 2021b). The Aldabra Atoll populations strongly differs genetically from the population of the African mainland with a less genetic differentiation from North Madagascar populations.…”