Abstract
BackgroundA large-scale systematical investigation of the influence of Pleistocene climate oscillation on mangrove population dynamics could enrich our knowledge about the evolutionary history during times of historical climate change, which in turn may provide important information for their conservation.ResultsIn this study, phylogeography of a mangrove tree Sonneratia alba was studied by sequencing three chloroplast fragments and seven nuclear genes. A low level of genetic diversity at the population level was detected across its range, especially at the range margins, which was mainly attributed to the steep sea-level drop and associated climate fluctuations during the Pleistocene glacial periods. Extremely small effective population size (Ne) was inferred in populations from both eastern and western Malay Peninsula (44 and 396, respectively), mirroring the fragility of mangrove plants and their paucity of robustness against future climate perturbations and human activity. Two major genetic lineages of high divergence were identified in the two mangrove biodiversity centres: the Indo-Malesia and Australasia regions. The estimated splitting time between these two lineages was 3.153 million year ago (MYA), suggesting a role for pre-Pleistocene events in shaping the major diversity patterns of mangrove species. Within the Indo-Malesia region, a subdivision was implicated between the South China Sea (SCS) and the remaining area with a divergence time of 1.874 MYA, corresponding to glacial vicariance when the emerged Sunda Shelf halted genetic exchange between the western and eastern coasts of the Malay Peninsula during Pleistocene sea-level drops. Notably, genetic admixture was observed in populations at the boundary regions, especially in the two populations near the Malacca Strait, indicating secondary contact between divergent lineages during interglacial periods. These interregional genetic exchanges provided ample opportunity for the re-use of standing genetic variation, which could facilitate mangrove establishment and adaptation in new habitats, especially in the context of global climate changes.ConclusionPhylogeogrpahic analysis in this study reveal that Pleistocene sea-level fluctuations had profound influence on population differentiation of the mangrove tree S. alba. Our study highlights the fragility of mangrove plants and offers a guide for the conservation of coastal mangrove communities experiencing ongoing changes in sea-level.
Highlights
A large-scale systematical investigation of the influence of Pleistocene climate oscillation on mangrove population dynamics could enrich our knowledge about the evolutionary history during times of historical climate change, which in turn may provide important information for their conservation
Low level of genetic diversity at the population level in S. alba Our results show that, S. alba hosts a relatively high level of genetic diversity at the species level, its genetic diversity at the population level was relatively low across its entire range, especially at the range margins (Fig. 1; Additional files 1 and 2)
In contrast to Wee et al [24], who proposed a blocking role of bifurcating ocean currents in preventing genetic exchange and maintaining genetic divergence between these two areas for R. mucronata, our results demonstrated that the Malacca Strait could provide opportunities for secondary contact during interglacial periods
Summary
A large-scale systematical investigation of the influence of Pleistocene climate oscillation on mangrove population dynamics could enrich our knowledge about the evolutionary history during times of historical climate change, which in turn may provide important information for their conservation. Mangrove plants constitute a highly productive ecosystem of both ecological and economic importance that lies at the interface of terrestrial and marine environments [1] They provide essential support to a variety of terrestrial and marine species through nutrient and organic matter sinking and transformation, protection from coastal erosion, and sediment control [2,3,4]. Mangrove communities have faced rapid decline; between 35 and 86% of the area once home to mangroves has been lost over the past 30 years, due to both climate change and human activity [8]. This loss poses a serious ecological problem and requires urgent conservation in the face of further increases in climate fluctuation. To better protect mangrove communities, a comprehensive understanding of the evolutionary demography of mangrove plants during historical climate oscillations is necessary
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