| Evolutionary theory generally assumes that genetic variation needed for evolution mainly results from DNA sequence variation,so the genetic diversity in population is the raw material of adaptive evolution.However,epigenetic variation can produce heritable phenotypic variation without affecting DNA sequence variation.In principle,it can form an evolutionary path independent of DNA sequence,so the epigenetic diversity in the population may also affect the evolutionary process.Epigenetic diversity can be derived from random spontaneous epigenetic variation,or it can be induced by environment.In this paper,Thlaspi arvense,a widespread annual herbaceous plant in Cruciferae,was used to examine the changes of epigenetic diversity under salt stress,and to confirm whether the offsprings(G1 and G2)of the stressed plants(G0)still have epigenetic stress memory effect after stress relief.After germination,two groups of seeds from the same plant were planted under salt stress and control conditions for three consecutive generations.The stress group was transferred to control conditions after one or two generation under stress.The leaf tissues of 80 plants were collected and analyzed using the molecular markers of Methyl-Sensitive Amplification Polymorphism(MSAP).A total of 306 bands were amplified using two pairs of endonucleases and 10 pairs of primers.The changes of epigenetic diversity of Thlaspi arvense under salinity stress and after transplanting were studied.In addition,the data of plant height,leaf length,seed quantity and quality of the same plants were collected for phenotypic analysis.The results are as follows:1.Salinity stress can significantly increase the population epigenetic diversity of Thlaspi arvense.The stress group had significantly higher epigenetic diversity than those of control group in several diversity index,including the Shannon’s diversity index(0.013 VS 0.037),the expected heterozygosity(0.009 VS 0.024)and the percentage of polymorphic sites(2.94% VS 6.86%)in the stress group increased significantly.Epiloci analysis showed that the epigenetic variation was not consistent among different individuals under the same treatment.Even in the control condition without salinity stress,different epigenetic variation will be accumulated among individuals,and the accumulation was faster under salinity stress.The above results suggest that the epigenetic variation under salinity stress may mainly come from random epigenetic mutation.2.The effect of salinity stress was more significant with the extension of stress time.In the first generation cultivation,the Shannon’s diversity index of the stress group increased by 23.53%(P=0.6014)compared with that of the control group,while that of the stress group increased by 217.65%(P=0.0349)compared with that of the control group after two generations of continuous cultivation under stress.Compared with the control group,the unmethylation was more than 82% in the stress group,and the change was demethylation.With the increase of stress generation,the demethylation rate was also increasing(7.25% VS 11.96%),indicating that the main change caused by salinity stress was demethylation3.The effect of salinity stress on the epigenetic diversity of population can form stress memory,which can last at least two generations.After the stress was relieved,the epigenetic diversity of the offspring of the treatment group that completed growth under the control conditions was still significantly higher than that of the control group offspring.The Shannon’s diversity index of the second generation was 0.029 and 0.017(P = 0.1513),and the Shannon’s diversity index of the third generation was 0.025 and 0.013(P = 0.1553).Site analysis showed that about 60% of the variation could be passed on to the offspring stably.4.Salinity stress had a similar effect on phenotypic variation.Compared with the control group,the plant height(18.39 cm VS 24.81cm)(P<0.0001),number of seeds(228 VS 345)(P<0.0001)and hundred-grain weight(0.088 g VS 0.108g)(P=0.0015)in the G0 generation of the salinity stress group decreased significantly.Similarly,compared with the control group,the G1 generation of two successive generation of the salinity stress group also decreased significantly,the plant height(16.32 cm VS 24.75cm)(P=0.0002),number of seeds(192 VS 323)(P<0.0001)and hundred-grain weight(0.031 g VS 0.075g)(P<0.0001).However,at the population level,the phenotypic coefficients of variation of plant height(0.191 VS 0.066),number of seeds(0.170 VS 0.085)and hundred-grain weight(0.090 VS 0.042)in G0 generation of the salinity stress group were higher than those in the control group,and the phenotypic coefficients of variation of plant height(0.314 VS 0.114),number of seeds(0.330 VS 0.169)and hundred-grain weight(0.309 VS 0.042)in the two generation salinity stress group of G1 generation were higher than those in the control group.In other words,salinity stress can significantly improve the phenotypic variation range of plants.Based on the above results,salinity stress can significantly improve the epigenetic diversity of the population,which can be maintained for at least two generations in the offspring through epigenetic stress memory.At the same time,salinity stress can significantly increase the range of phenotypic variation of important functional traits.The internal mechanism of parallel variation of epigenetic variation and phenotypic variation needs further study.These results deepen the understanding of the interaction between epigenetic variation and environmental stress,at the same time,considering the application value of various aspects,these results provide basic data for genetic breeding and development and utilization of Thlaspi arvense. |
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