Trans-generational effects in the clonal invader Alternanthera philoxeroides

近年的研究已经揭示出表型可塑性可以通过营养世代遗传。从这个意义上讲,环境诱导的基因调控变化(例如DNA甲基化,即表观遗传变异)会导致可逆的塑性反应传递给后代。这种跨世代的可塑性在克隆植物中尤其重要,因为有性繁殖减少会降低通过遗传变异进行适应的可能性。许多最具侵略性的植物入侵者都具有克隆性,而且克隆性被认为是植物入侵性的关键。本研究中,我们的目标是确定在克隆入侵者喜旱莲子草(Alternanthera philoxeroides)中是否发生了跨代效应,以及这种效应在本地和非本地群落之间是否存在差异。在同质园实验中,我们将采自巴西(原产地)和伊比利亚半岛(非原产地)的喜旱莲子草种群分别种植在养分高和低的土壤中,后代植株被移植到高养分条件下作为对照。为了检验DNA甲基化对跨代可塑性的潜在作用,一半的母株用去甲基化剂5-氮杂胞苷处理。在来自原产地和非原产地的种群中均观察到了跨代效应。有趣的是,在原产地种群的生长变量(分株数、茎生物量、根生物量和总生物量)中发现了跨代效应,而在非原产地种群中发现了生物量分配具有跨代效应。在原产地种群中出现的跨代效应可以用“银勺”效应来解释,而非原产地种群的跨代效应可以归因于DNA甲基化引起的表观遗传传递。本研究强调了跨代效应对克隆植物生长的重要性,这有助于理解众多克隆植物能够成功繁殖扩张的机制。     

克隆植物的无性与有性繁殖对策

许多植物同时具有克隆生长与有性繁殖,两种繁殖方式间的平衡在不同物种间以及同一物种内不同种群间变化很大。旺盛的克隆生长可能会从多方面影响生活史进化。首先,许多克隆植物的有性繁殖与更新程度都很低,甚至有一些植物由于克隆生长而几乎完全放弃了有性过程,从而影响到克隆植物对局域环境的适应和地理范围进化。其次,克隆生长增大花展示进而增加了对传粉者的吸引,同时也增加了同株异花授粉的风险,而同株异花授粉往往会导致植物雄性和雌性适合度的下降。因此,克隆植物的空间结构与交配方式间可能存在着协同进化关系。最后,克隆生长与有性繁殖间可能存在着权衡关系:对克隆生长的资源投入将会减少对有性繁殖的资源投入。这种权衡关系可能是由环境条件、竞争力度、植物寿命和遗传等因素决定的。如果不同的繁殖方式是植物在不同环境下采取的适应性对策,那么我们可以预期:在波动和竞争力度大的生境中,植物应将大部分的繁殖资源分配给有性繁殖;而在相对稳定的环境中,克隆繁殖应该占据优势地位。但是自然选择对两种繁殖方式的选择结果是什么,以及控制这两种方式间平衡的生态和遗传因子究竟有哪些,到底是克隆生长单向地影响了植物的有性繁殖,还是与有性过程相伴随的选择压力同时塑造了植物的克隆习性?目前尚不清楚。同时从无性与有性繁殖两个方面综合考察克隆植物的繁殖对策是今后亟待加强的工作。     

植物边缘种群遗传多样性研究进展

边缘种群指地理分布边缘可检测到的一定数量的同种个体集合, 准确评价其遗传多样性对于理解第四纪冰期后气候变化对物种边缘扩展或收缩、遗传资源保护与利用以及物种形成等有重要意义。该文探讨了维持植物边缘种群遗传多样性的进化机制, 分析交配系统对物种边缘及其遗传多样性的影响, 比较了边缘与中心种群遗传多样性的差异及其形成的生态与进化过程, 并探讨了边缘种群遗传多样性与其所在的群落物种多样性的关系及理论基础。该文提出今后研究的重点是应用全基因组序列或转录组基因序列研究前缘-后缘种群之间或边缘-中心种群之间的适应性差异, 边缘种群与所在群落其他物种之间相互作用的分子机制, 深入解析边缘种群对环境的适应及边缘种群遗传多样性与群落物种多样性关系的生态与进化过程。     

论克隆植物的遗传多样性

概述了克隆植物的类型与特点 ,对克隆植物的遗传多样性及其遗传结构的一些特点进行了综述 ,并讨论了克隆植物遗传变异的来源。总体而言 ,克隆植物拥有比早期推测大得多的遗传变异 ,虽然克隆种与其近缘有性繁殖种相比 ,遗传多样性较低 ,但广泛的遗传单态性却很罕见。克隆植物种群的遗传结构有所改变 ,广布基因型很少 ,大多数基因型仅分布于某一种群之内 ,种群间基因型多态性存在广泛的变异。不同克隆植物之间遗传多样性相差很大 ,遗传结构也有巨大差异。说明除生殖模式外 ,其他的一些因素 ,如地理分布范围、生境特点 ,散布方式和种群历史等都对克隆植物遗传多样性有重要影响。     

论克隆植物的遗传多样性

概述了克隆植物的类型与特点,对克隆植物的遗传多样性及其遗传结构的一些 特点进行了综述,并讨论了克隆植物遗传变异的来源。总体而言,克隆植物拥有比早期推测大得多的遗传变异,虽然克隆种与其近缘有性繁殖种相比,遗传多样性较低,但广泛的遗传单态性却很罕见。克隆植物种群的遗传结构有所改变,广布基因型很少,大多数基因型仅分布于某一种群之内,种群间基因型多态性存在广泛的变异。不同克隆植物之间遗传多样性相差很大,遗传结构也有巨大差异。说明除生殖模式外,其他的一些因素,如地理分布范围、生境特点,散布方式和种群历史等都对克隆植物遗传多样性有重要影响。     

植物边缘种群遗传多样性研究进展

边缘种群指地理分布边缘可检测到的一定数量的同种个体集合,准确评价其遗传多样性对于理解第四纪冰期后气候变化对物种边缘扩展或收缩、遗传资源保护与利用以及物种形成等有重要意义。该文探讨了维持植物边缘种群遗传多样性的进化机制,分析交配系统对物种边缘及其遗传多样性的影响,比较了边缘与中心种群遗传多样性的差异及其形成的生态与进化过程,并探讨了边缘种群遗传多样性与其所在的群落物种多样性的关系及理论基础。该文提出今后研究的重点是应用全基因组序列或转录组基因序列研究前缘-后缘种群之间或边缘-中心种群之间的适应性差异,边缘种群与所在群落其他物种之间相互作用的分子机制,深入解析边缘种群对环境的适应及边缘种群遗传多样性与群落物种多样性关系的生态与进化过程。     

被子植物性系统及其可塑性

繁殖是生物适合度的最终表现,有性繁殖相关性状的多态性极大地促进了物种分化和生物多样性的维持,并影响着植物对环境变化的响应。在种群水平上,被子植物的花有雌花、雄花和两性花三种性表型,三种性表型在种群中的分布和频率即定义了种群的性系统。被子植物的性系统包含植物影响性分配和交配的相关特性,决定着雌配子、雄配子在种群中的频率、交配机会及交配方式,是有性繁殖的关键性状,在被子植物中表现出丰富的多态性,在种群水平上分为性单态和性多态两大类。性单态为被子植物的古老性状,而性多态在100多个被子植物科中独立进化产生。被子植物性系统多态性及其变化机理一直是进化生物学与生态学的热点问题之一。该文以种群水平的性多态为对象,总结了被子植物性系统的类型、表达的遗传基础、分布频率,以及遗传因子、非生物环境和交配环境对性系统表达和性分配的影响。     

克隆植物的表型可塑性与等级选择

表型可塑性是指生物个体生长发育过程中遭受不同环境条件作用时产生不同表型的能力。进化的发生有赖于自然选择对种群遗传可变性产生的效力以及各基因型的表型可塑性。有足够的证据说明表型可塑性的可遗传性,它实际上是进化改变的一个成分。一般通过优化模型、数量遗传模型和配子模型来研究表型可塑性的进化。植物的构型是相对固定的,并未完全抑制表型可塑性。克隆植物因其双构件性而具有更广泛的、具有重要生态适应意义的表型可塑性。构件性使克隆植物具有以分株为基本单位的等级结构,从而使克隆植物的表型选择也具有等级性。构件等级一般包含基株、克隆片段或分株系统以及分株3个典型水平。目前认为克隆植物的自然选择有两种模式,分别以等级选择模型和基因型选择模型表征。等级选择模型认为:不同的等级水平同时也是表型选择水平,环境对各水平具有作用,各水平之间也有相互作用,多重表型选择水平的净效应最终通过繁殖水平——分株传递到随后的世代中。基因型选择模型指出:克隆生长引起分株的遗传变异,并通过基株内分株间以及基株间的非随机交配引起种子库等位基因频率的改变,产生微进化。这两种选择模式均突出强调了分株水平在自然选择过程中的变异性以及在进化中的重要性,强调了克隆生长和种子繁殖对基株适合度的贡献。基因型选择模型包含等级选择模型的观点,是对等级选择模型的重要补充。克隆植物的表型可塑性表现在3个典型等级层次上,由于各层次对自然选择压力具有不同的反应,其表型变异程度一般表现出“分株层次>分株片段层次>基株层次”的等级性反应模式。很多证据表明,在构件有机体中构件具有最大的表型可塑性,植物的表型可塑性实际上是构件而非整个遗传个体的反应。这说明克隆植物的等级反应模式可能具有普适性。如果该反应模式同时还是构件等级中不同“个体”适应性可塑性反应的模式,那么可以预测:1)在克隆植物中,分株层次受到的自然选择强度也最大,并首先发生适应性可塑性变化,最终引起克隆植物微进化;2)由于较弱的有性繁殖能力,克隆植物在进化过程中的保守性可能大于非克隆植物。克隆植物等级反应模式的普适性亟待验证。     

啮齿动物的母体效应

母体可通过基因遗传之外的其他方式影响后代的表型,包括激素分泌、胎盘渗透性、母乳成分和母体抚育行为等。这种对后代形态和行为的影响很大且持续终生。母体效应是子代对环境异质性的一种表型反应,在进化生态学研究中是一个非常重要的变异来源,对种群生态学而言,也可能是种群调节机制的一个重要的因子。着重介绍了母体状况对啮齿动物后代生长发育、性比、性特征和对性选择的影响,对啮齿动物母体效应的进一步研究进行了展望。     

通过遗传多样性探讨极小种群野生植物的致濒机理及保护策略: 以裸子植物为例

遗传多样性是生物多样性的重要组成部分, 然而由于资源的过度开发利用和生境的破碎化影响了物种的遗传多样性, 甚至威胁到物种的生存适应性和生物多样性。极小种群野生植物是亟待保护的国家重点保护濒危植物,遗传多样性研究对揭示极小种群致濒机理及保护策略具有重要意义。生境破碎化会造成物种遗传多样性降低、种群间分化增加、基因流减少等, 使种群濒危。但在某些物种中, 繁殖特征、进化历史等生物和生态因素的不同也可能造成近期生境破碎化后遗传效应的延迟。裸子植物进化历史悠久, 包含许多孑遗物种, 由于生活史周期长, 遭受生境破碎化后可能短期内显示不出遗传效应的改变, 但长期很难恢复。本文以裸子植物为例综述了濒危植物的遗传多样性研究的案例, 探讨了濒危裸子植物应对环境恶化的维持机制、致濒因素和保护方案, 旨在说明通过遗传多样性研究充分认识极小种群致濒机理对高效保护极小种群野生植物的重要性。     

The role of transgenerational effects in adaptation of clonal offspring of white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis>) to drought and herbivory

Environmentally induced transgenerational effects can increase success of offspring and thereby be adaptive if offspring experience conditions similar to the parental environment. The ecological and evolutionary significance of these effects in plants have been considered overwhelmingly in the context of sexual generations. We investigated whether drought stress and jasmonic acid, a key hormone involved in induction of plant defenses against herbivores, applied in the parental generation, trigger transgenerational effects in clonal offspring of Trifolium repens and whether these effects are adaptive. We found that drought stress experienced by parents significantly affected phenotypes of offspring ramets. Offspring ramets were bigger if they were produced in the parental water regime (control/drought). Repeated application of jasmonic acid to parents increased the subsequent growth of offspring ramets produced by stolons after they were disconnected from the parental clone. However, these offspring ramets experienced similar herbivory by the generalist Spodoptera littoralis caterpillar as did control offspring ramets, indicating that this jasmonic acid application in the parental generation did not result in a transgenerational effect comprising increased herbivory resistance. We conclude that, overall, environmental interaction in the parental generation can trigger transgenerational effects in clonal plants and some of these effects can be adaptive. Moreover, transgenerational effects in clonal plants that significantly influence their growth and behavior can ultimately affect the evolutionary trajectories of clonal populations.     

Transgenerational effects of stress exposure on offspring phenotypes in apomictic dandelion

Heritable epigenetic modulation of gene expression is a candidate mechanism to explain parental environmental effects on offspring phenotypes, but current evidence for environment-induced epigenetic changes that persist in offspring generations is scarce. In apomictic dandelions, exposure to various stresses was previously shown to heritably alter DNA methylation patterns. In this study we explore whether these induced changes are accompanied by heritable effects on offspring phenotypes. We observed effects of parental jasmonic acid treatment on offspring specific leaf area and on offspring interaction with a generalist herbivore; and of parental nutrient stress on offspring root-shoot biomass ratio, tissue P-content and leaf morphology. Some of the effects appeared to enhance offspring ability to cope with the same stresses that their parents experienced. Effects differed between apomictic genotypes and were not always consistently observed between different experiments, especially in the case of parental nutrient stress. While this context-dependency of the effects remains to be further clarified, the total set of results provides evidence for the existence of transgenerational effects in apomictic dandelions. Zebularine treatment affected the within-generation response to nutrient stress, pointing at a role of DNA methylation in phenotypic plasticity to nutrient environments. This study shows that stress exposure in apomictic dandelions can cause transgenerational phenotypic effects, in addition to previously demonstrated transgenerational DNA methylation effects.     

Joint impact of competition,summer precipitation,and maternal effects on survival and reproduction in the perennial <Emphasis Type="Italic">Hieracium umbellatum</Emphasis>

Most studies on consequences of environmental change focus on evolutionary and phenotypic plastic responses, but parental effects represent an additional mechanism by which organisms respond to their local environment. Parental effects can be adaptive if they enhance offsprings ability to cope with environments experienced by their parents, but can also be non-adaptive for instance when offspring from benign environments are just better provisioned and hence perform better than offspring from less benign environments. Parental effects originate from both the abiotic and biotic environmental variation. However, the effects of the parental abiotic and biotic environment are rarely studied together. We make use of an experimental set-up containing plots in a natural heath land, where summer precipitation was manipulated to reflect either ambient or drought conditions. In both plot types, competition from grasses was prevalent. We assessed survival and reproduction of Hieracium umbellatum offspring originating from ambient and drought plots grown in a factorial design with two levels of moisture (control and drought) and two levels of competition (grown with and without a local perennial grass). The maternal environment strongly affected offspring performance. Biomass and reproduction was higher in offspring from ambient plots in agreement with the hypothesis of a better maternal provisioning in the most benign environment. However, adding competition revealed potentially adaptive responses to survival, and altered allocation to reproduction in offspring from maternal drought plots. Under combined competition and drought (mimicking maternal drought plots), survival was only reduced in offspring from ambient plots, and offspring from drought plots survived best. When grown in competition under control watering conditions mimicking maternal ambient plots, offspring from drought plots (growing in an environment different from their maternal one) showed a 25% reduction in reproduction. Potential adaptive responses to the home maternal environment were only revealed when jointly manipulating levels of competition and water availability.     

Parental and developmental temperature effects on the thermal dependence of fitness in Drosophila melanogaster

Cross-generational effects refer to nongenetic influences of the parental phenotype or environment on offspring phenotypes. Such effects are commonly observed, but their adaptive significance is largely unresolved. We examined cross-generational effects of parental temperature on offspring fitness (estimated via a serial-transfer assay) at different temperatures in a laboratory population of Drosophila melanogaster. Parents were reared at 18 degrees C, 25 degrees C, or 29 degrees C (Tpar) and then their offspring were reared at 18 degrees C, 25 degrees C, or 29 degrees C (Toff) to evaluate several competing hypotheses (including an adaptive one) involving interaction effects of parental and offspring temperature on offspring fitness. The results clearly show that hotter parents are better; in other words, the higher the temperature of the parents, the higher the fitness of their offspring, independent of offspring thermal environment. These data contradict the adaptive cross-generational hypothesis, which proposes that offspring fitness is maximal when the offspring thermal regime matches the parental one. Flies with hot parents have high fitness seemingly because their own offspring develop relatively quickly, not because they have higher fecundity early in life.     

Phenotypic influences on the reproductive strategy of the facultative sexual rotifer <Emphasis Type="Italic">Brachionus rubens</Emphasis> (Monogononta)

Facultative sexual species employ a dual reproductive strategy (heterogony) comprising primarily asexual reproduction with intermittent sexual reproduction. Given the higher relative costs of sexual reproduction, elucidating the triggers underlying these transitions might help our understanding of the evolution of (obligate) sex in general. Existing hypotheses into how and when facultative sexuals invest into sex focus largely either on environmental (habitat-deterioration and resource-demanding hypotheses) or genetic factors (condition-dependent hypothesis), but tend to lack experimental evidence, especially with respect to within-population variation. To address this deficit, we examined the influence of several variables that potentially affect fitness (food quality, water temperature, physiological acclimation, and all combinations thereof) on both the lifetime reproduction (total number of offspring) and investment into sexual offspring per female in a clonal population of the monogonont rotifer Brachionus rubens. Investment into sex, both absolutely and relative to lifetime reproduction, was tied most closely to and positively correlated with individual fitness (i.e., lifetime reproduction): individuals with higher fitness invested more into sexual reproduction. These results run contra to the condition-dependent hypothesis and indicate an energy-budget analogue of the resource-demanding hypothesis. Furthermore, investment into sex increased after a period of physiological acclimation to the new conditions, probably because of the amelioration of short-term stress effects or clonal selection. Our results underscore that life history and general phenotypic considerations—here, energetic provisioning of offspring, the presence of a sexual resting stage, and the relative timing of sexual versus asexual reproduction—can modify existing hypotheses based either on environmental or genetic factors alone.     

Effects of parental light environment on growth and morphological responses of clonal offspring

• Environments experienced by parent ramets of clonal plants can potentially influence fitness of clonal offspring ramets. Such clonal parental effects may result from heritable epigenetic changes, such as DNA methylation, which can be removed by application of DNA de‐methylation agents such as 5‐azacytidine.
• To test whether parental shading effects occur via clonal generation and whether DNA methylation plays a role in such effects, parent plants of the clonal herb Alternanthera philoxeroides were first subjected to two levels of light intensity (high versus low) crossed with two levels of DNA de‐methylation (no or with de‐methylation by application of 5‐azacytidine), and then clonal offspring taken from each of these four types of parent plant were subjected to the same two light levels.
• Parental shading effects transmitted via clonal generation decreased growth and modified morphology of clonal offspring. Offspring responses were also influenced by DNA methylation level of parent plants. For clonal offspring growing under low light, parental shading effects on growth and morphology were always negative, irrespective of the parental de‐methylation treatment. For clonal offspring growing under high light, parental shading effects on offspring growth and morphology were negative when the parents were not treated with 5‐azacytidine, but neutral when they were treated with 5‐azacytidine.
• Overall, parental shading effects on clonal offspring performance of A. philoxeroides were found, and DNA methylation is likely to be involved in such effects. However, parental shading effects contributed little to the tolerance of clonal offspring to shading.

     

A unified approach to the evolutionary consequences of genetic and nongenetic inheritance

Inheritance-the influence of ancestors on the phenotypes of their descendants-translates natural selection into evolutionary change. For the past century, inheritance has been conceptualized almost exclusively as the transmission of DNA sequence variation from parents to offspring in accordance with Mendelian rules, but advances in cell and developmental biology have now revealed a rich array of inheritance mechanisms. This empirical evidence calls for a unified conception of inheritance that combines genetic and nongenetic mechanisms and encompasses the known range of transgenerational effects, including the transmission of genetic and epigenetic variation, the transmission of plastic phenotypes (acquired traits), and the effects of parental environment and genotype on offspring phenotype. We propose a unified theoretical framework based on the Price equation that can be used to model evolution under an expanded inheritance concept that combines the effects of genetic and nongenetic inheritance. To illustrate the utility and generality of this framework, we show how it can be applied to a variety of scenarios, including nontransmissible environmental noise, maternal effects, indirect genetic effects, transgenerational epigenetic inheritance, RNA-mediated inheritance, and cultural inheritance.