Ocean Currents Drove Genetic Structure of Seven Dominant Mangrove Species Along the Coastlines of Southern China.

Chunlan Lian,Megumi K Kimura,Jianmin Tao,Zhongsheng Wang,Taizo Hogetsu,Hong Liu,Qifang Geng

doi:10.3389/fgene.2021.615911

Abstract

Mangrove forest ecosystems, which provide important ecological services for marine environments and human activities, are being destroyed worldwide at an alarming rate. The objective of our study was to use molecular data and analytical techniques to separate the effects of historical and contemporary processes on the distribution of mangroves and patterns of population genetic differentiation. Seven mangrove species (Acanthus ilicifolius, Aegiceras corniculatum, Avicennia marina, Bruguiera gymnorrhiza, Kandelia obovata, Lumnitzera racemosa, and Rhizophora stylosa), which are predominant along the coastlines of South China, were genotyped at nuclear (nSSR) and chloroplast (cpSSR) microsatellite markers. We estimated historical and contemporary gene flow, the genetic diversity and population structure of seven mangrove species in China. All of these seven species exhibited few haplotypes, low levels of genetic diversity (HE = 0.160–0.361, with the exception of K. obovata) and high levels of inbreeding (FIS = 0.104–0.637), which may be due to their marginal geographical distribution, human-driven and natural stressors on habitat loss and fragmentation. The distribution patterns of haplotypes and population genetic structures of seven mangrove species in China suggest historical connectivity between populations over a large geographic area. In contrast, significant genetic differentiation [FST = 0.165–0.629 (nSSR); GST = 0.173–0.923 (cpSSR)] indicates that populations of mangroves are isolated from one another with low levels of contemporary gene flow among populations. Our results suggest that populations of mangroves were historically more widely inter-connected and have recently been isolated, likely through a combination of ocean currents and human activities. In addition, genetic admixture in Beibu Gulf populations and populations surrounding Hainan Island and southern mainland China were attributed to asymmetric gene flow along prevailing oceanic currents in China in historical times. Even ocean currents promote genetic exchanges among mangrove populations, which are still unable to offset the effects of natural and anthropogenic fragmentation. The recent isolation and lack of gene flow among populations of mangroves may affect their long-term survival along the coastlines of South China. Our study enhances the understanding of oceanic currents contributing to population connectivity, and the effects of anthropogenic and natural habitat fragmentation on mangroves, thereby informing future conservation efforts and seascape genetics toward mangroves.

Highlights

Mangrove forests form conspicuous wetland ecosystems, fringing extensive areas of coastlines in tropical and subtropical regions (Mitsch et al, 2009; Barbier et al, 2011)
Low levels of genetic diversity in chloroplast SSR (cpSSR) were consistently observed in all species, as indicated by Nei’s unbiased haplotypic diversity
Accurate estimates of genetic diversity are very useful for optimizing sampling strategies, and for conserving and managing the genetic resources of trees (Hamrick et al, 1991; Schaal et al, 1991; Chalmers et al, 1992; Cardoso et al, 1998)

Summary

Introduction

Mangrove forests form conspicuous wetland ecosystems, fringing extensive areas of coastlines in tropical and subtropical regions (Mitsch et al, 2009; Barbier et al, 2011). Mangrove forests are extremely productive ecosystems that provide important ecological services, both to the marine environment (e.g., coastal protection) and to industries including fisheries, timber and plant products; and tourism (Alongi, 2002; Barbier et al, 2011; Manez et al, 2014) Despite their ecological and economic importance, mangroves are being fragmented and destroyed worldwide at an alarming rate due to both climate change and human activity. They include sea-level fluctuations affected by climate change, urbanization, industrial pollution, farmland conversion, embankments for aquaculture ponds, and so on (Farnsworth and Ellison, 1997; Li and Lee, 1997; Ge and Sun, 2001; Stellman et al, 2003; Ong and Gong, 2013; Nguyen, 2014). To better protect mangrove communities, a comprehensive understanding of the population dynamics and biographical processes associated with oceanic currents is necessary

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