入侵植物喜旱莲子草沿温度梯度的DNA甲基化变异：格局、稳定性及其适应意义

DNA methylation can produce heritable phenotypic variation without changing DNA sequences and thus play an important role in evolution. However, this role is heavily rely on its stability and adaptive significance. The breeding system of Alternanthera philoxeroides, an alien weed, is dominated by asexual reproduction in China. Despite the extremely low levels of DNA sequence variation, this weed can invade into a diverse climatic areas from Hainan (tropical) to Shandong (temperate). The adaptation to temperate change may be due to phenotypic plasticity and population differentiation of DNA methylation. In this project, we propose to collect samples following the strategy in landscape genetics. The following experiments will be performed: 1) to investigate the geographic pattern of DNA methylation using molecular markers and reduced represented bisulfite sequencing of epigenome, which will be further tested against the Isolation by Environment (IBE) model; 2) to analyze the environmental sensitivity and stability of DNA methylation in different environments through multiple methods including field monitoring in situ, common garden transplantation, and control experiment; 3) to examine how DNA methylation affect the gene expression, physiological tolerance, and relative growth rate, using manipulative experiment; and to perform genome scan and environmental association analyses to assess the adaptive significance of DNA methylation variation. Our aim is to reveal the adaptive and invasion mechanisms in terms of DNA methylation, which will deepen our understanding for adaptive evolution in asexual populations.

DNA甲基化可在不改变DNA序列的情况下产生可遗传的表型变异，在进化中发挥重要作用，但这种作用依赖于其稳定性和适应性。外来植物喜旱莲子草在中国以无性生殖为主，DNA序列变异极低，却能入侵从海南（热带）到山东（温带）的多样气候区，其对不同温度的适应可能与表型可塑性和甲基化变异的种群分化有关。本项目采用景观遗传学方法，沿自然温度梯度进行取样：1）利用分子标记和简化甲基化组技术分析自然种群甲基化变异的地理格局，检验其是否符合“环境隔离（IBE）”模型；2）利用原位监测、同质园交互移植、控制实验，分析甲基化变异对环境变化的敏感性及在不同环境中的突变率；3）利用操纵性实验，研究甲基化变异如何影响基因表达、生理耐受性和相对生长率；结合基因组扫描和环境关联分析，推断甲基化变异的适应意义。本项目将从DNA甲基化角度揭示喜旱莲子草的适应和入侵机制，促进对无性系种群适应性进化的认识。

表观遗传修饰可以在不改变DNA序列的情况下产生可遗传的表型变异，这对于无性繁殖的克隆植物的进化尤其重要，但这种作用依赖于表观遗传修饰的稳定性和适应性。本项目以喜旱莲子草为材料，研究表观遗传变异的空间格局、稳定性和功能意义。研究发现：1）喜旱莲子草在广阔的地理分布区内在DNA序列水平的变异程度很低，但在种群间蕴藏着丰富的DNA甲基化变异；其空间格局与环境因子有密切关联，但是表观遗传距离与环境距离并不是显著的线性相关；2）喜旱莲子草的DNA甲基化对环境变化表现出暂时的稳定性，既不像典型的可塑性那样完全随环境而变化，也不像典型的遗传变异那样完全独立于环境变化，而是随着时间延长而逐渐变化；环境胁迫与对照环境相比能够显著提高表观遗传修饰的突变率，导致群体水平的表观遗传多样性升高；3）采自分布区南北缘的不同种群的喜旱莲子草的抗寒性有显著差异，北方种群在低温下的存活率更高，关键抗性基因的表达水平更高，组蛋白修饰可能是不同种群抗寒能力适应性分化的重要原因。以上研究结果表明，表观遗传变异使得喜旱莲子草可以快速完成气候驯化，这为无性繁殖物种依靠表观遗传变异来适应环境异质性提供了实验证据，有助于丰富无性繁殖物种的进化理论，同时为制定更有效的入侵种防控策略提供科学依据。