Thermal Differentiation Of The Two Morphotypes Of The Coral Galaxea Fascicularis

Global temperature rising is believed to be the main cause of coral reef bleaching.Different types of corals exhibit different phenotypes in the face of temperature rise.Therefore,the study of coral heat resistance is of great significance for coral reef protection.This thesis used the stony coral Galaxea fascicularis(a representative dominant species on inshore fringing reefs in the northern South China Sea)as the research object,revealing the difference of heat resistance in two types Galaxea fascicularis,the main results are as follows:1.Differences in heat resistance between the two types Galaxea fascicularis under slow-heatingand acute-heating conditionsAccording to the MpMs morphology and whether a mt DNA fragment(290 bp indel)is absent,Galaxea fascicularis is mainly divided into two types(sL and hS).In this study,acute-heating(32?,6-7?higher than average ambient temperature,108 h)and slow-heating experiments(increase 0.5?every 12 h,slowly heating to 31?from 25?,and maintaining at 31?for 9d,and then slowly decrease to 25?)were conducted on a Galaxea fascicularis population(n=14,7 sL and 7 hS,from West Island,Hainan).The results show that,the temperature increase will cause the bleaching of G.fascicularis.The statistics of bleaching rates were calculated by multiplying the samples of different bleaching situation:polyp wall whitening(mild bleaching,×0.25),coenosarc whitening and tissue shedding(moderate,×0.50),polyp wall and coenosarc shedding,tentacle retraction(severe,×1.00).The bleaching rates of sL and hS are significantly different(P<0.05),and the bleaching rate of sL type(22.6±30.0%/13.0±23.7%)was significantly higher than that of hS type(5.5±7.4.0%/0.0±0.5%).The bleaching of sL occured earlier than hS.and the rate of albino occurrence is significantly higher than that of hS clumped coral.In addition,the bleaching rate of sL type increased continuously during the recovery period,while that of hS type decreased.2.Transcriptome analysis of two types of G.fascicularisIn the slow-heating experiment of two types of G.fascicularis,transcriptome samples were taken from 24?to 29?at 44 h,31?at 60 h,and 90 h.By Illumina HiSeq TM2000sequencing,a total of 920,945,408 reads were obtained from all samples of the three individuals after quality control,and 62,575 genes were obtained by assembly.Of these,33,730 gene sequences were annotated to the GO database(53.9%)and 20,937unigenes to the KOG database(33.5%).The results show that the response modes of sL and hS type to simulated environmental warming(form 25±1?,1?/12 h)are significantly different.When the water temperature increased by 3?(to 28?)for 8 h,the number of genes significantly changed(P<0.05)in the hS type was 484,among which the number of up-regulated genes was 333,and the number of down-regulated genes was 151,while only8 genes were significantly up-regulated and 3 genes were down-regulated in sL type,the response degree of gene expression was significantly lower than that of hS type.When the temperature continued to rise by 3?(to 31?)for 54 h,although the bleaching rate of sL was significantly higher than that of hS type,there was no significant change in gene expression,while there were 159 significantly changed genes of hS type,among which 101were up-regulated and 58 were down-regulated.SL type lacks genetic adaptive physiological regulation for temperature rise,which may be one of the reasons for its poor heat resistance.3.Under the influence of calcium ions,the heat resistance of the two types of G.fascicularis is differentAnhydrous CaCl₂was used to dissolve CaCl₂in accordance with the water volume.In seawater filtered by 0.45um filter membrane,the mass to volume ratio of CaCl₂to seawater was 100 mg/L,and the pH value was stabilized at 8.12 by 0.5 mol/L NaOH solution.The effect of Ca²⁺on the heat resistance of two types of G.fascicularis was investigated by acute heating experiment.The experimental results showed that the two types of G.fascicularis produced different degrees of bleaching at high temperature(32?),but there was no significant difference in the bleaching rate of the two types of G.fascicularis in the Ca²⁺group and the control group,and the results showed that the bleaching rate of the two types of G.fascicularis was sL type>and hS type.Through the analysis of the results,the mass bleaching of Galaxea corals caused by the Ca²⁺in the water was not directly affected,and the heat resistance of sL corals was lower than that of hS corals,which was consistent with the results of the above acute heating experiment.4.Differences in heat resistance between the two morphological types of G.fascicularis under animal nutrition supplementationDuring acute heating experiments,brine shrimp larvae(Artemia)were added as animal nutrition sources.The experimental results show that the 32?high temperature will lead to coral bleaching,but the bleaching rate is significant different(p<0.05).At the same time,the bleaching time was late and the bleaching rate was low in the cluster coral individuals strengthened by animal nutrition,while the bleaching time was early and the bleaching rate was high in the same individuals in the 32?control group.In the whole experiment,the bleaching rate of sL type and hS type G.fascicularis was as follows:the bleaching rate of s type 32?control group>bleaching rate of s type 32?feeding group>bleaching rate of h type 32?control group>bleaching rate of h type 32?feeding group.Results show that the hS type is more heat-resistant than sL type,and cluster type helmet coral ingest exogenous helmet of animal nutrition and the cluster type heat resistant coral reef itself produce positive influence,after the body to strengthen,heat resistance of two types of G.fascicularis was increased,the heat resistance increase in sL type G.fascicularis was more obvious.Thus it can be seen that exogenous energy supplementation in the natural state is one of the means to deal with the high temperature stress in G.fascicularis.To sum up,the hS type of G.fascicularis has higher high temperature tolerance than the sL type.This difference is mainly determined by their own genetic differentiation.The transcriptome response of type hS to slow-heating is more than positive type sL.Exogenous animal nutrition has a positive effect on the improvement of heat resistance of sL type.G.fascicularis were differentiated into two morphological types with morphological and genetic markers,and significantly different in high temperature adaptability,so it became ideal materials to study environmental adaptability of reef-building corals.