CAS Workshop on Ecosystem Succession Theory and Practice of Ecological Restoration

植被恢复生态学中的种群遗传学思考

王峥峰  彭少麟

（中国科学院华南植物研究所，广州 510650）

摘要：种群遗传学研究种群遗传组成和分布以及各种因子（生物、生境）如何影响这一组成和分布，当前主要研究对象是各物种种群的遗传多样性。遗传多样性是指地球上所有生物携带的遗传信息的总和，是生物多样性的重要组成部分。各物种种群的遗传多样性是这一遗传多样性的基本组成单元。物种种群的遗传多样性不但是维持其繁殖活力、抗病虫害能力和适应环境变化的基础，也是人类利用改良、创造新的栽培植物和家养动物品种的源泉。各种群遗传多样性及其差异构成了物种种群的遗传结构。

在植被恢复过程中，除了某些用于改良生境的先锋树种外（只需简单考虑其是否能成活，不需要考虑是否能交配生殖改良后代，但要注意特殊情况，见下文论述），各物种种群一般经历了由人工种植到成活定居，再由从较少的个体数量通过交配生殖向较多个体数量的发展过程（如演替后期种），在此遗传多样性和遗传结构起着非常大的作用，特别是引种初期。

1. 使物种种群拥有大的遗传多样性

维持物种最大遗传多样性是我们在植被恢复中所必须遵循的原则。假如我们只从较少的植株取种，不但将丢失大量的遗传变异，后代也可能会出现瓶颈效应，导致后代较小的遗传多样性，影响到个体对今后变化环境的适应性，这特别对自交不亲合的物种不利。因此在种源的选择上我们要从拥有最大遗传多样性的种群中采种，最好在“遗传同质区（genetically homogeneous regions，指具有相似的遗传多样性和遗传多样性组成的地区）”内广泛采集种源，力求遗传多样性最大。

如果注意到遗传多样性通常和种群某种性状相联系，我们就会看到对于有些无需繁殖后代的先锋树种，如中国科学院鹤山丘陵综合试验站植被恢复生态初期种植用来改良土壤等的大叶相思（Acacia. auriculaeformis）和马占相思（A. mangium）等，其大的遗传多样性可能代表了其有更大的生境改良能力，如固氮能力（与固氮菌作用）、对土壤元素的吸收转化能力等。

2. 防止近交衰退

近交衰退很早就得到人们的注意，因为人类本身就存在这一问题（指近亲结婚）。近交衰退产生原因是近交增加了有害等位基因纯合几率，导致个体适合度的下降。

在植被恢复初期，物种种群个体数量一般较少，此时如果不注意采种原则近交衰退现象不可避免，事实上，这一现象却未被广泛注意，例如在国内，多数绿化苗木的遗传多样性尚未研究，交配系统也不清楚，种源的采集多集中于同一棵植株或少数较大的植株上，没有广泛采种，这将在以后不可避免导致采用这些种源的人工林低的遗传多样性，并将使这些人工林的后代受到近交衰退的影响，影响人工林的自然更新。

如果考虑近交衰退的影响，在采集种子等过程中，我们先要了解采集的种子本身是否是近亲交配的结果，要避免在某一个体或遗传相似度大的个体中采集过多的种子，以减少近亲繁殖的潜在危害。在之后的播种等过程中，要有意识的把植株与植株之间的间距拉远一些，以得到更大的遗传多样性和遗传相似度低的后代以减少近亲繁殖。

3. 共适应（coadaptation）和远交衰退

共适应是指分布在不同生境（地区）中的种群个体，其不同位点的等位基因组合在一起产生的对环境的适应能力。当这些拥有不同等位基因组合的种群相互之间杂交（交配）将打破各自的等位基因组合，引起它们后代适应能力降低，即远交衰退。

在植被恢复过程中，如果我们不注意，就会把不同地区的种源（种群）混植在一起，这有可能导致在种群后代发生远交衰退现象，使后代存活率降低、生长不良或不育等，严重阻碍植被恢复的质量，延误恢复时间。

4. 防止遗传同化

所谓遗传同化（genetic assimilation），也有人称为遗传均一化（genetic homogenization）、遗传污染（genetic contamination）和遗传没化（genetic swamping），多指两个可相互杂交种群中的一方个体数量远大于另一方时，两者的相遇将使小种群个体更多与大种群的个体交配（杂交），减少了小种群个体之间交配产生属于自己的“纯”后代的比例，从而被大种群“稀释”掉。

人为活动是造成当前遗传同化的主要因素，如道格拉斯冷杉（Douglas fir，Pseudotsuga menziesii）的原生种群被保护在美国加利福利亚地区的几个保护区内，但作为优良林木，道格拉斯冷杉已被广泛种植在保护区的周围，这些人工种群（来自不同地区）的花粉不可避免地影响到了原生种群的遗传结构，使原生种群失去了应有的遗传特异性，减少了保护的价值。相似的例子还有北加利福利亚黑胡桃树（north California black walnut，Juglans hindsii），大面积人工种植的黑胡桃树花粉大面积传播实际上摧毁了原生种群的遗传特异性，因此在植被恢复的过程中，我们要注意大面积人工种植的物种种群不要威胁到恢复地周围的近缘种（种群）的遗传独特性。

5. 采取的措施

由上，我们注意到，在植被恢复的过程中，我们要在引种初期就要从种群遗传学角度开展相应的调查研究工作，增大引种种群的遗传多样性，减少近交衰退和远交衰退的可能，保持用于生态恢复种群后代的最大生存力，同时不要造成人工植被对当地植被的遗传同化。

关键词：植被恢复生态学；种群遗传学；遗传多样性；近交衰退；远交衰退；遗传同化 

Plant restoration ecology and population genetics

Wang Zhengfeng  Peng Shaolin

(South China Institute of Botany, Chinese Academy of Sciences, Guangzhou 510650, China)

Population genetics seeks to understand how the genetic composition of populations changes over times, and what factors influence the change. The main aspects of population genetics are population genetic diversities, which are the core issues of current biological diversities. Population genetic diversities are the fundament for population survival such as pathogens resistance and environmental adaptation, and the resources for improvement of captive plants and animals by human being. Genetic structure is the genetic diversities of populations and their distribution pattern among populations.

During the ecological restoration, species populations, such as later successional species, will undergo establishment (introduced by human beings), growth and reproducing. Population size will increase from few individuals to more individuals. Then, population genetic diversities play a very important role during these processes, especially at the time of introduction (planting).

1. Keeping high genetic diversity in population

Keeping high genetic diversity is the rule of thumb for ecological restoration. If we only collect seeds from a few mother plants, we will lose a lot of genetic diversity when using these seeds. Low genetic diversity may also result in genetic bottleneck which can decrease the adaptation of progeny. Therefore, to make high genetic diversity we need to collect seeds for restoration from variety of individuals and large area in genetically homogeneous regions.

For some pioneer species, the high genetic diversity also means high ability to adapt and meliorate environment (e.g. soils).

2. Avoiding inbreeding depression

Inbreeding depressing has long been observed by human being because it exists in ourselves due to intermarriage. Inbreeding may reduce population fitness through the expression of recessive deleterious alleles in homozygous form.

During the early stage of ecological restoration, introduced population usually owns a few individuals which may cause inbreeding. However, it is given less recognition. For examples, in China, most of plant species are lacking investigation of genetic diversity, not to mention their breeding system. People are accustomed to collect seeds from a few or only one big mother tree, which will definitely decrease the overall genetic diversity and may result in inbreeding depressing if we use them for restoration.

On the other side, if we take inbreeding into consideration, we need to distinguish the seeds from the inbreeding origin before collecting them, and avoid collecting more seeds from only a few or near mother trees. When planting, we need to distance these seeds to get high genetic diversity.

3. Coadaptation and outbreeding depression

For individuals from different habits, its fitness may come from the combination of alleles, which is known as coadaptation. The phenomenon of loss of fitness upon breakup of coadaptated gene complexes is termed outbreeding depression. If we are unconscious to mix genetic divergent individuals from different populations, outbreeding depression will result in mal-adaptation, lowering the fitness of progeny, which actually delay the time of restoration.

4. Avoiding genetic assimilation

Genetic assimilation, or genetic homogenization, genetic contamination, genetic swamping will dilute the small population when it hybrid with the large other speices population. That is, no “pure” progeny will be produced by small population due to their parents have the high probability to produce hybrids.

Human activities are the main reason to cause the genetic assimilation, such as indigenous Douglas fir (Pseudotsuga menziesii) and north California black walnut (Juglans hindsii) population. They are both destroyed by largely planted artificial (domestic) populations around them.

5. What should we do?

During ecological restoration, all in all, we need pay more attention to keep high genetic diversity of introduced (planted) populations, avoiding inbreeding and outbreeding depression and genetic assimilation.

Key word: plant restoration ecology; population genetics; genetic diversity; inbreeding depression; outbreeding depression; genetic assimilation 

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