Clonality And Its Population Genetic And Ecological Consequences In Halophila Ovalis (Hydrocharitaceae)

Seagrasses beds are one of the most important ecosystems owing to important ecological and economic functions in the worldwide. Global seagrasses are declining at an increasing rate as a consequence of increasing human activities and natural stress, and it is urgent to protect and recovery seagrasses beds. Seagrasses are typical clonal plants and strong clonality is the main mechanism for seagrasses beds maintaining.To understand the effect of clonality on population spatial genetic and the correlation between clonal diversity and ecological impacts under different environmental stress in seagrasses, we selected the dominate species Halophila ovalis (R. Br.) Hartog along coastal regions of Southern China to conduct a survey of the clonal characteristics and biomass allocation in different water depth gradient and to develop its specific microsatellite primers, then based on microsatellite loci and different clonal characteristics we examined the genetic and ecological effects of clonality. At last, we analyzed the genetic structure of populations from coastal regions of Southern China and identified the population for priority conservation.The main results were as following:1. In December 2008, we investigated the cloanl characteristics and biomass of H. ovalis in four seagrass beds at different water depth along cosat regions of Hepu Guangxi. It was demonstrated that water depth can affect the clonal characteristics and biomass allocation of H.ovalis.We found no significant differences in converage and leaf pair density between four monospecific H. ovalis beds, Shabei, Xialongwei, Beimu and Yingluo, which have different water depths at maximum tidal level (MTL) but otherwise similar environmental conditions. However, horizontal internodal length, leaf length, width, rhizome diameter and shoot height all increased significantly with the increases in water depth from 2 to 9m MTL and decreased when the water depths were greater than 9m MTL. No significant difference in above-or belowground biomass between the seagrass beds was found. However, the ratio of above-to belowground biomass was significantly higher in the shallowest site (Yingluo) compared to the other three seagrass beds, indicating that more biomass was stored belowground in deeper water.2. Using the biotin-streptavidin capture method, we developed ten polymorphic microsatellite primers for the seagrass H. ovalis and all primers were successfully cross-amplified in H. minor.The number of alleles per locus ranged from 2 to 12 across 80 H. ovalis individuals, and the average number of alleles per locus was 7.1. Four loci, HO3, HO20, HO31and HO51, were monomorphic across 30 H. minor individuals. The observed number of alleles in H. minor ranged 2 to 6.3. Based on 8 microsatellite loci we analyzed the spatial genetic structure (SGS) within two H. ovalis population, Xialognwei and Yingluo, behaving signiciantly different clonal characteristics. It showed that clonality can enhance SGS and there were significant SGS between two populaitons. The comparison of SGS between two populations showed the extent of SGS were stronger in Yingluo where H. ovalis grew with phalanx-like strategy than that in Xialong where H. ovalis grew with guerilla-like strategy.For two populations, significant SGS were found at ramet-, central genet-and random genet-level, indicating limited gene flow on fine-scale within each population. The extents of SGS at ramet-level were stronger than those at genet-level. And genetic relationship coefficient (rij) was larger significantly at ramet-level than that at genet-level within 0-2m for Xialongwei and within 2-4m for Yingluo respectively. The tendency of greater genetic correlation at ramet-level declined with increasing spatial distance ranged from 0 to 45m in two populations.The extent of SGS was stronger with higher Sp statistics in Yingluo than that in Xialongwei based on all dataset levels. The clonal richness were high in two H. ovalis populations Xialongwei(0.799) and Yingluo(0.697). In Yingluo the number of large clone with more than 2 sampling units and clonal subrange was more than twice and more than four times of thoes in Xialongwei respectively. For Yingluo significant genetic correlation were found within 0-8m and 0-6m at ramet-and genet-level respectively. For Xialongwei, the distances where significant genetic correlation observed at two levels were shorter,0-6m and 0-4m respectively. There was no significant difference of total SGS between two populations at two levels, but there was significantly different genetic correlation within 0-2m between two populations based on ramet-level.4. In June 2009,27 and 32 quadrats of 1×1m were established in different sites and tidal zones in Xialongwei and Yingluo respectively. Then we analyzed the multilocus genotypic diversity for each quadrat based on 7 microsatellite loci. For each quadrat the leaf pairs density, coverage and epiphyte biomass of H. ovalis were investigated in summer and in winter. It was showed that clonal diversity had positive ecological consequences for H. ovalis.The clonal diversity of H. ovalis was not related to biomass of epiphytic algae both in summer and in winter. And there were not significant linear relationship between clonal diversity and growth such as leaf pair density and coverage in summer. In contrast, in winter leaf pair density and coverage both increased significantly with increasing clonal richness and the biomass of epiphyte increased significantly with increasing cloanl heterozygousity (the mean proportion of heterozygous loci of clone in a plot). Furthermore, there were significant interactions between clonal diversity (clonal richness or heterozygousity) and environmental factors for growth of H. ovalis, and the effect of clonal richness on growth was obvious under different environment.5. Based on ten microsatellite loci, it was revealed that the genetic diversity was high in 10 H. ovalis populations from coastal regions of Southern China, the unbiased expected heterozygousity(HE) and clonal richniss(R) were 0.90±0.05 and 0.459±0.050 respectively. And the genetic differentiation among populations was also high (GsT=0.324).The consistent results of cluster analyses using UPMGA methods and STRUCTURE were detected, it was showed that 10 populations fell into two groups. One group contained Beigang population in Hainan, Liusha population in Guangdon and Yingluo populations in Guangxi, and the other group contained 7 populations distributed the coastal regions of Guangxi and from east to west coastal of Hainan island. Ocean currents were the main approach for gene flow. There was no significant isolation-by-distance observed among 10 populations, but was among 7 populations of the large group. Moreover, the estimation of population pairwise migrate using MIGRATE revealed that the population pairwise migrate within group were larger than these between two groups. The significant isolation-by-distance and barriers for ocean current resulted in limited dispersal and genetic divergence.Among the 10 H. ovalis populations from coastal regions of Southern China, Yingluo population in Guangxi was identified for priority conservation owing to the highest genetic contribution, Dongjiao popualtin in Hainan and Shabei population in Guangxi in the next space.