Population genetic structure of the threatened tropical seagrass Enhalus acoroides in Hainan Island, China
Introduction
Seagrass habitats are one of the most important and dominant ecosystems along coastal regions. Their ecosystem services include providing food and habitat for animals, as well as nutrient cycling and sediment stabilization (Orth et al., 2006). In China, there are approximately 22 seagrass species distributed in the intertidal and subtidal zones, spanning nearly 9000 ha across the temperate and tropical coastline (Zheng et al., 2013). Unfortunately, many seagrass ecosystems along China coastline are under threat caused by pollution (e.g. domestic sewage), physical disturbance (e.g. fishing and dredging, coastal reclamations and harbor constructions) and aquaculture, resulting in large areas of seagrass habitat loss, habitat fragmentation and even local or regional extinctions (Zheng et al., 2013; Jiang et al., 2014). Conservation is necessary and urgent for the seagrass ecosystems in China. Genetic diversity is an important part of conservation biology which provides information for making conservation, restoration and ecological management decisions (Waycott et al., 2006; Williams, 2001; Nakajima et al., 2017), because genetic make-up determines the long-term evolutionary processes and informs ecosystem functions like species diversity (Procaccini et al., 2007; Hughes et al., 2008; Evans et al., 2016).
Population genetic diversity and structure are influenced by mutation, gene flow, genetic drift and natural selection, which in turn are shaped by geography, environment, biogeographical history and their synergistic interactions (Olsen et al., 2004; Arnaud-Haond et al., 2007; Hernawan et al., 2017; Kendrick et al., 2017). Previous studies have revealed that genetic variance patterns for seagrasses were chaotic in some regions as a consequence of complex ecological and environmental processes (Procaccini et al., 2007; Sinclair et al., 2014). Firstly, ocean currents are expected to play an important role in promoting gene flow among populations in marine ecosystems, and thus the frequency of long distance dispersal (LDD) of propagules maybe more common in the marine environment than their terrestrial counterparts (McMahon et al., 2014; Jahnke et al., 2016). In particular, the LDD of rafting reproductive shoots of Zostera marina with seeds derived by currents may increase the gene flow among populations (Reusch, 2002). Nevertheless, limited gene flow still exists due to geographical barriers, such as peninsulas (Muñiz-Salazar et al., 2005), isolation created by current circulation and direction (White et al., 2010), or historical events, such as the last glacial maximum (Olsen et al., 2004; Arnaud-Haond et al., 2007). Additionally, heterogeneous environmental stressors (e.g. water depth, temperature, salinity and sediment type) could have impact on life histories, mating systems, dispersal and recruitment, resulting in genetic divergence within or among populations (Procaccini et al., 2001; Reusch and Wood, 2007; Van Dijk and van Tussenbroek, 2010).
Enhalus acoroides is a morphologically large, long-lived, dioecious seagrass with the capacity for both sexual and clonal reproduction (den Hartog, 1970). In China, E. acoroides occurs along the east coastline of Hainan Island, which is the northern margin of its Indo-Pacific distribution (Short et al., 2007). E. acoroides is a dominate species in this tropical island, forming continuous monospecific beds or mixed meadows with Halophila ovalis, Thalassia hemprichii, Cymodocea rotundata and Halodule uninervis. The distribution area and aboveground cover of E. acoroides have been declining dramatically in the past 10–15 years (Chen et al., 2015), while its population genetic information is still poorly understood in China. In the present study, eight E. acoroides populations were collected covering the entire species geographic range in China, including 3 lagoonal populations and 5 offshore open-water populations. We aimed to 1) understand the genetic diversity of E. acoroides on a regional spatial scale; 2) assess the genetic structure of E. acoroides and its potential causes; and 3) identify populations for priority conservation based on their genetic contribution.
Section snippets
Sample collection
To assess genetic diversity and structure of E. acoroides, we collected seven resident populations and one “rafting population” along the east coast of Hainan Island, China (Fig.1). In each resident population, 26–39 shoots were collected. The distance between shoots was at least 10 m to limit the effect of clonal architecture on the estimates of genetic diversity. After scraping the attached epiphytes, leaves were washed with fresh water and then dried and preserved in silica gel. Among the
Clonal and genetic diversity
Among the 259 shoots, 252 distinct MLGs were identified as genets that were derived from different sexual reproduction events. One MLG was shared between the two adjacent populations GL and GD. Clonal richness of each population ranged between 0.862 and 1.00 (Table 1).
After removing repeated shoots, Hardy-Weinberg equilibrium (HWE) was estimated using the genotype data of genets. Deviation from HWE was found in the BS and XC populations. No linkage disequilibrium was found between any pair of
Clonal and genetic diversity
The clonal richness of E. acoroides was high using a 10 m sampling interval in this study, and ranged from 0.862 to 1.000 in each population, which was consistent with previous study for E. acoroides with similar spatial intervals (Nakajima et al., 2014). In contrast, fine-scale genetic study of E. acoroides in the LA population indicated a slightly lower clonal richness using a shorter sampling interval of 1 m (0.645 – 0.789; Yu et al., unpublished data). Clonal richness also differs across
Conclusions
In present study, clear genetic structure was found among the eight populations of E. acoroides. Lagoonal populations were highly isolated, while connectivity existed among the open-water populations. This pattern seemed to be as a consequence of geographic isolation and ocean currents. Based on the genetic contribution analysis that incorporated both genetic variation with in a population and distinctiveness among populations, we suggest that populations TL and GD need to be a protection
Acknowledgments
We thank Min Liu for helpful discussions and comments on the early edition of the manuscript and Changhao Zhou and Guanghui Li for helping field sampling. This research was supported by the National Specialized Project of Science and Technology (2015FY110600), the National Natural Science Foundation of China (No. 41606182; No. 41730529), the National Basic Research Program of China (2015CB452905) and the Strategic Priority Research Program of the Chinese Academy of Science (No. XDA13020204).
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