Genetic Structure And Connectivity Of Scleractinian Coral Porites Lutea Around Hainan Island

Coral reef ecosystem is a special ecosystem on the Earth.It maintains high biodiversity and primary productivity.It is often referred to "tropical rain forest in the sea" and considered as the upper limit of the evolution of ecosystem.It is of great significance to the optimization of the marine environment and marine ecosystems.China's reef-building corals are mainly distributed in the seawaters around Hainan Island,Xisha and Nansha Islands.In recent years,due to the rising global temperatures,ocean acidification,seawater pollution,and human-induced disturbances,coral reef ecosystems are suffering extensive degradation.China's coral reefs are also facing an unprecedented crisis and have been reduced by at least 80%in the past 30 years.Therefore,actions are urgently needed to protect the coral reefs.At present,the main strategy of coral reefs protection and restoration is establishing a network of coral reef reserves,and combining with human-assisted evolution of reef-building corals.Rational planning of the coral reef reserve network requires the analysis of the genetic structure of reef-forming coral populations,as well as the genetic connectivity of their sexual reproduction larvae through the spread of surface currents;and the artificial assisted evolution of reef-building corals needs to explorethe population genetic differentiation of reef corals and their differences in adaptability.Porites lutea is widely distributed in Hainan Island and Xisha Islands.It might exist for a long time and can adapt to a relatively harsh living environment.In this study,the analysis of distribution,connectivity of populations and the community structure of the symbiotic zooxanthellae was used to study the genetic diversity and distribution of corals Porites lutea around Hainan Island,which would be of great significance for the protection and restoration of coral reefs in the South China Sea.The microsatellite markers of coral and zooxanthellae symbiont in Porites lutea were developed through the excavation and re-sequencing of GenBank sequence resources.The microsatellite markers of coral and zooxanthella were identified by pure zooxanthellae DNA assisted validation.Using this polymorphic and co-dominant marker,the genetic structure and genetic connectivity of the population of the 10 populations of Porites lutea around Xisha and Hainan Island were analyzed.The main findings are as follows:1.We created a system to culture symbiotic zooxanthellae.The zooxanthellae were cultured in vitro for 122 days to obtain pure zooxanthella DNA for identification of the microsatellite loci of Porites lutea.At the same time,it can also provide necessary materials for the exploration of zooxanthella living environment,the symbiotic mechanism of coral and zooxanthellae,and the exploration of coral bleaching mechanism.2.Through the excavation of GenBank sequence resources,83 and 100 microsatellite sequences with sufficient flanking sequences were screened out from 142 genomic microsatellite sequences of Porites lobata and 11,516 EST sequences of P.astreoides.Primers were designed to test and amplify the DNA of Porites lutea using six different geographical populations and the sequencing products were resequenced.The microsatellite markers collected from the re-sequencing analysis sequence were amplified simultaneously using DNA pools of coral and zooxanthellae to identify the microsatellite markers belonging to the Porites lobata.Finally,20 Porites lutea microsatellite markers were identified.3.Those 20 microsatellite markers were used to scan 9 Porites lutea populations around Hainan Island and 1 Porites lutea population from Xisha Yongxing Island.It was found that there were certain genetic homogeneity(18/20 marker genotypes)individuals in a population.The homogeneity rate was the highest in the Tongguling(pt)population with 6 individuals,accounting for 35.29%of the population;the Leigongdao(pg)and the Linchangjiao(p1)populations each had 1 genetically homogenous individual;other groups did not find any individual with genetic homogeneity and genetic homogeneity accounted for 5.16%of the total.After removing these genetic homogeneity individuals in each group,the genetic diversity was low.The average number of alleles in the 10 populations ranged from 3.2(Basuo and Tongguling)to 4.6(Mulantou);the average allele abundance ranged from 2.4(Basuo)to 3.2(Linchangjiao);the heterozygosity ranged from 0.29(Tongguling)to 0.48(Dachanjiao);the corresponding average expected heterozygosity ranged from 0.52(Basuo)to 0.66(Haiwei),respectively.4.Among the 10 populations,most of the sites(>6/10)were in line with Hardy Weinberg equilibrium,e.g.Dazhou Island,Leigong Island,Mulantou,Haiwei,Dazhangjiao,Lichangchangjiao,and Basuo.Xisha population mainly exhibited free mating characteristics;while the Tongguling only had six sites that were consistent with Hardy Weinberg equilibrium,and mainly exhibited non-free mating characteristics.The inbreeding coefficient Fis was between-0.04(Basuo)and 0.4(Tongguling)with an average of 0.177,indicating that the Basuo had a lower inbreeding rate,while the Tongguling had a higher inbreeding rate.5.Xisha and Hainan Island have clearly differentiated in the group of Porites lutea.From the perspective of Nei's genetic distance,they have the highest genetic distance with Dachanjiao population and the lowest genetic distance with the Hawei population.The cluster analysis showed that the Mulantou and Leigongdao in the northern part of Hainan Island were clustered first,while the Luhuitou in the south was clustered with the west Basuo and the Lichangjiao as the first branch.After the two aggregated and the eastern Tongguling and Dazhoudao formed.The clade of Dachanjiao and the Haiwei population in the west,showed a large genetic distance with the surrounding populations,and was closer to the genetic distance of the Xisha population.The genetic differentiation factor Fst of the Xisha and the Basuo populations was the highest,being 0.347,and the lowest among the populations of the Xiasha and the Haiwei population was 0.276,indicating that the populations around Xisha and Hainan Island have obvious genetic differentiation.The genetic differentiation coefficient between the eastern and northern populations is larger,and the genetic differentiation coefficient between the southern population and the northern and western regions is smaller.Combined with the genetic flow between different groups,the Qiongzhou Strait in the northern part of Hainan Island and the Porites lutea group in the southwest have the highest genetic transmission connectivity is the most active area for reef-building coral larvae drifting and colonization.It is speculated that the Porites lutea in Hainan Island originate from the Xisha Islands,enter the Beibu Gulf from the south with the currents from the west,and arrive through the Qiongzhou Strait.From Hainan to the northeast part,on the other hand,from the south of Hainan Island to the north with the currents on the east side.The genetic differentiation between the populations of the Porites lutea in the west of Hainan Island is not proportional to the geographical distance,which may be related to the differences in the current direction and the structure of the reef disk.