The present study is the first report of genetic diversity, and genetic structure, among populations of the two dominant seagrass species Cymodocea rotundata and Enhalus acoroides spanning the diversity hotspot and range edge populations in Mindanao, (southern Philippines) using microsatellite (SSR) markers. Conforming novel genetic records for these two species of seagrass in the tropical areas and detection of high levels of polymorphism underlined impressively that genetic markers are powerful tools for assessing genetic diversity in seagrass.
Genetic diversity and connectivity
The species richness and structure of seagrass across regions is believed to be influenced by hydrodynamic regimes due to the intersection of favorable major and minor ocean currents meandering the extant seagrass communities (Jokiel and Martinelli 1992; Mukai 2010). Seagrasses have the potential to disperse over long distances via ocean currents during various life-history stages (McMahon et al 2014). Population genetic studies in combination with hydrodynamic models have increased understanding of the role/potential of connectivity in natural seagrass meadow recovery (e.g., Sinclair et al 2016, Smith et al 2018). A combined understanding of the dispersal mechanisms and reproductive biology of seagrasses will add to the overall understanding of spatial and genetic connectivity.
Genetic diversity is related to adaptive potential, and a loss in genetic diversity can increase the possibility of population extinction. Genetic diversity hotspots are located in a central area within the distribution range of a focal species (Diekmann and Serrão 2012). These are extremely important in seagrass conservation and serve as models for monitoring biodiversity in regions affected by anthropogenic disturbances and climate change (Olsen et al. 2004). The Philippines is likely to function as a genetic diversity hotspot influenced by populations of tropical seagrasses, owing to its location in a central habitat. Genetic diversity of C. rotundata and E. acoroides populations in Mindanao are ranging from low to high, and decreasing genetic diversity is usually found in the marginal ranges especially for C. rotundata. Upon observations, seagrass species confirmed in RIZ was dominated with E. acoroides and C. rotundata was intermittently found in shallow areas. The single genet of C. rotundata found in this meadow manifested that sexual reproduction is not apparent in this area and this is alarming for the conservation of this species in the area since anthropogenic activities are recurring with observations on the proliferation of the fast-growing algae that inhibited the penetrations of sunlight for photosynthesis. The environment in RIZ is not very suitable habitat for a seagrass species with short leaves therefore, environmental selection may have occurred, and the seagrass population in RIZ is likely to be endangered. In seagrass meadows often dominated by a single seagrass species, they are susceptible to pandemic disease outbreaks (Waycott et al 2009). It was further implicated by Reusch et al (2001) that decreasing genetic diversity of seagrasses may also correspond to the decrease of the resilience of meadows and the seagrass dwelling fish and invertebrates.
The overall allelic richness of E. acoroides (4.01) and C. rotundata (1.40) is higher compared with the previous studies of Nakajima et al (2017) for E. acoroides in the Guimaras Strait, Visaya, Philippines, and comparable with Arriesgado et al (2016) for C. rotundata (1.64 Norther Philippines, 1.78 Central Philippines and 1.94 Ryukyu Island), respectively. Furthermore, allelic richness of the two dominant seagrass species in Mindanao was comparable with some seagrass species; Halophila ovalis (1.56) in the Western Pacific Ocean (Nguyen et al 2014), Cymodocea nodosa (2.20) in the Atlantic regions (Alberto et al 2008) and Zostera marina (2.74) in San Juan Archipelago, Washington, USA (Wyllie-Echeverria et al 2010).
The decreased allelic richness in some populations is possibly the effect of drift because of small population size, as a result of reduced habitat and low gene flow and/or natural selection across life stages of some clonal species (e.g. C. rotundata) which favor clonal reproduction for environment fitness. Genetic diversity is related to adaptive potential, and a loss in genetic diversity can increase the possibility of population extinction. Genetic diversity hotspots are located in a central area within the distribution range of a focal species (Diekmann and Serrão 2012). These are extremely important in seagrass conservation and serve as models for monitoring biodiversity in regions affected by anthropogenic disturbances and climate change (Olsen et al 2004). The Philippines is likely to function as a genetic diversity hotspot influenced by populations of tropical seagrasses, owing to its location in a central habitat. However, in the present study even though Mindanao is located in the tropical region, the genetic diversity in some areas is low. The observation of low genetic diversity in some sites was also manifested in Japan and Hainan China in E. acoroides (Nakajima et al 2014)d rotundata (Arriesgado et al 2015). Decreasing genetic diversity of seagrasses may also correspond to the decrease of the resilience of meadows and the seagrass dwelling fish and invertebrates (Reusch et al 2001). In general, horizontal rhizome elongation is important for population maintenance in seagrasses (van Dijk and van Tussenbroek 2010), which was evidenced in RIZ population.
Previous study suggested the importance of sexual reproduction in seagrass in the Philippines (Rollon et al 2003). This fact was also apparent from results of clonal diversity in Nakajima et al. (2014) for E. acroides (R = 0.47–1.00), Jahnke et al (2019) for T. hemprichii (R = 0.26–0.95), Arriesgado et al. (2016) for C. rotundata (0.21–1.0) and this study for E. acoroides (R = 0.9–1.00) and C. rotundata (0.7–1.0), respectively. The remarkable high genetic diversity of E. acoroides as compared to C. rotundata was explained by a hypothetical scenario of the possibility that because of the dispersal mechanisms exhibited by E. acoroides, of which the Mindanao Ocean currents is perhaps continuously transporting drifting flower in the surface water providing supply of new genotypes into the different seagrass meadows in the region (Fig. 3). This was also asserted in the previous study manifesting that the Kuroshio current greatly influenced the genetic diversity and structure of C. rotundata in the Ryukyu Island as propagules were carried and drifted by the water currents from the Philippines (Arriesgdo et al 2016). This was even confirmed by some studies on scleractinian corals (Veron and Minchin 1992; Chen and Shashank 2009), who found that the strong Kuroshio Current with high sea surface temperatures greatly influenced their distribution and genetic diversity across the Kuroshio triangle.
Genetic differentiation
The results of AMOVA and FST indicated pronounced genetic differentiation among C. rotundata and E. acoroides populations in the Mindanao region, suggesting limited gene flow. This is not surprising for C. rotundata considering the low dispersal capability of this clonal species which develop fruits at the base of shoots (den Hartog 1970) and seeds attach to the rhizome, which are frequently buried under the substrate (Larkum et al 2006). This characteristic of seed production inhibits seed dispersal, resulting in significant genetic differentiation among populations. On the other hand, the lower genetic differentiation of E. acoroides populations may be explained by the dispersal mechanism of this species influenced by major and minor currents in Mindanao. For instance, the hydrodynamics in Siargao Island is greatly influenced by the uninterrupted winds and currents coming from the Pacific Ocean, which was further intensified by the Mindanao Ocean currents running westward through the Siargao Strait. This has probably influenced the genetic connectivity among populations in Siargao and the southernmost seagrass meadows in Tawi Tawi. Furthermore, the significant isolation by distance (IBD; Fig. 2) manifested by C. rotundata is likely a consequence of habitat fragmentation as seen in other studies (Arriesgado et al 2016; Lima et al 2007; Diekmann & Serrão 2012; Assis et al 2013). The dominance of large genets indicates that these meadows are the result of ecological and evolutionary processes integrated over long time scales. Range edge populations are typically small and restricted to particular habitat islands within a matrix of unsuitable landscapes (Hampe & Petit 2005). These populations have persisted for longer time periods in relative isolation, which resulted in reduced genetic diversity (Petit et al 2003). Thus, their isolated life results in remarkably high population differentiations even at small geographical distances, which leads to extraordinary levels of regional genetic diversity (Hampe et al 2003; Martin & McKay 2004). A large genetic structure across spatial gradients was observed as a result of successive colonizations of the population having formed mosaics of genets to colonize space through vegetative elongation and/or produce seedling through reproduction among flowers of the same genet or its relatives, recruiting closed to the maternal plants. Selection for local adaptation to their environment is suggested to play an important role, which may result in the development of distinct ecotypes.
Implications for conservation
The finding of this study has important implications for conservation issues. The water currents play an important role in the recruitment and establishment of populations of this species in Mindanao, southern Philippines. The region was considered of extreme importance for conservation objectives and could be proposed as a model for monitoring biodiversity. It is important to conserve diversity in some range edge populations because such locally adapted genotypes uphold important evolutionary potential in face of future environmental change.
Genetic diversity and connectivity can inform decision-making and help to prioritize management actions. For example, connectivity estimates can be used to identify areas that are more likely to recover naturally following decline (e.g., areas that have steady supply of propagules from non-local sources) and areas that have limited recovery potential due to recruitment limitations (e.g., isolated areas expected to receive minimal or no propagule recruitment from non-local sources). Habitat enhancement and ecological engineering to encourage settlement would become priority management actions for areas showing limited signs of recovery despite expected propagule supply. In contrast, translocations (e.g., physical planting) in combination with habitat restoration investments would be needed in areas with limited propagule supply to ensure population establishment.
The level of genetic diversity of source and recipient seagrass meadows is also an important factor to consider when augmenting remnant seagrass meadows or establishing new meadows. Seagrass meadows at the edge of their range may have lower genetic diversity and higher levels of clonality or have reduced seed production as a result of pollen limitation. Small isolated populations often have similar issues (McMahon et al 2014). Overall genetic diversity is positively associated with population fitness (Connolly et al 2018), and standing genetic variation within populations is closely tied to adaptive capacity and resilience to environmental change (Leimu et al 2006). Consequently, selecting genetically diverse meadow as a donor source is important for maximizing restoration success. Local extinction, especially of organisms with low dispersal ability, in the coastal ecosystems is anticipated in the near future. This region should be monitored to conserve the coastal ecosystems and fisheries resources.
In general, seagrasses are in a vulnerable state and continuously declining. Effective management should equally espouse awareness factor through information, education and communication. This could be motivated by promoting and spreading public awareness about seagrasses and the importance of maintaining healthy seagrass habitats to the general public, environmentalists and policy makers which can help protect further loss and decline of these important habitats. If protected, healthy seagrass meadows will continue to support the many valuable and important creatures living within the meadows, and the biota of coral reefs and mangrove forests maintaining healthy interconnectivity of these three ecosystems as well.