羊草种群克隆多样性的初步研究

 采用水平淀粉凝胶电泳技术检测了松嫩草原上两种叶色类型(灰绿型和黄绿型)羊草(Leymus chinensis)种群的克隆多样性。羊草种群中克隆多样性较高,Simpson指数平均为0.983,97.7% 的基因型为局部分布;在克隆特性上,灰绿型和黄绿型羊草也明显不同,黄绿型种群的克隆多样性低于灰绿型。由基株数据计算的种群遗传多样性与由分株计算的结果一致,进一步确认了黄绿型和灰绿型羊草间存在着明显的遗传分化。种群中不时有实生苗补充及环境异质性引起的分化选择作用可能是维持种群内基因型多样性的重要因素。     

内蒙古典型草原建群种羊草基因型多样性抑制群落物种多样性的生态功能

植物群落的物种多样性以及群落建群种的基因型多样性对群落生态功能是否存在交互影响已成为群落生态学研究的热点内容。以内蒙古典型草原群落内常见物种为研究对象,研究了群落物种多样性与建群种羊草(Leymus chinensis)基因型多样性及其交互作用对群落生物量生态功能特性的影响。结果表明:(1)羊草基因型多样性、物种多样性及其交互作用对群落地上、地下和总生物量无显著影响(P0.05);(2)羊草基因型多样性、物种多样性及其交互作用对多样性效应(净多样性效应、互补效应和选择效应)有显著影响(P0.05)。羊草基因型多样性抑制多样性净效应的发挥,且主要抑制互补效应;而物种多样性则促进多样性净效应的发挥,主要表现为选择效应对地上生物量的正效应;(3)互补效应对群落生物量多样性净效应起主要贡献。实验所得结果不仅为探讨多样性效应在物种水平以及群落水平上对群落生物量的影响因素提供了重要启示,而且为内蒙古草原种质资源的保护及合理利用,乃至生态系统的恢复和重建提供理论指导。     

贺兰山丁香自然居群克隆生长格局及遗传多样性的ISSR分析

运用ISSR分子标记技术,通过制定挖掘采样、"+"形采样及"垂直"采样3种采样方案,对贺兰山丁香(Syringa pinnatifolia var.alanshanica)不同居群的克隆多样性、克隆生长格局及其遗传多样性进行了分析.克隆多样性分析表明:挖掘采样方式采到的3个克隆系内,各自所包含的单株间具有完全相同的基因型;"垂直"采样及"+"形采样的7个居群、 239个样品表现出136个不同的基因型或克隆,显示贺兰山丁香具有较高的克隆多样性(D=0.994)及基因型分布均匀性(E=0.985).克隆生长空间格局分析表明,贺兰山丁香为密集型克隆植物.每个居群都由多克隆组成,克隆生长只发生在同一丛内,多数基株只含有1个分株,最多可达8个.物种水平上的平均克隆大小(N_C)和平均基因型比例(PD)分别为1.757和0.569.对贺兰山丁香遗传多样性分析的结果显示,在居群水平和物种水平上都保持着较高的遗传多样性,其遗传变异主要存在于居群内;但居群间分化程度较低(G_ST=0.320),表明自然居群间基因交流有限.     

海草克隆性及其种群遗传效应

海草是适应在海洋环境中生存和繁殖的单子叶植物,由于所处环境常存在潮汐、风暴等的干扰,海草形成了一系列适应特征,克隆性是其中突出的一个.所有的海草都具有水平根状茎,许多海草也具有垂直根状茎,在一些海草中,也观察到无性生殖(无融合生殖).与克隆生长有关的参数(如节间长度、间隔子长度、分枝角度以及延伸速率和分枝率等)对于海草的克隆生长有着决定性影响,但繁育系统对克隆斑块大小也有较大影响.强烈的克隆性影响着海草的遗传变异.总体来看,海草种群内遗传多样性比陆生植物低,也低于另一类海洋高等植物-红树植物,利用DNA标记观察到的多样性高于等位酶标记.在一些海草植物种群中观察到较高的克隆多样性,但也有一些种群由单一基因型或少量基因型组成,其原因主要是由于奠基者效应和克隆生长.通常克隆植物中基因流有限,但是海草的克隆片段可能远距离扩散,从而提高种群间的基因流.就克隆生长对种群空间结构和交配系统的影响进行了综述.     

艾纳香野生种群克隆多样性及克隆结构研究

艾纳香是具有克隆生长习性的多年生宿根性草本植物,其广布于中国南部,为了更有效地保护和合理利用艾纳香资源,本文利用RAPD分子标记技术,对4个野生艾纳香种群进行了克隆结构和克隆多样性(单克隆种群或多克隆种群)进行了初步研究。结果表明:(1)10对10bp随机引物共检测到70条谱带,其中多态带为60条,占85.71%,检测到64个基因型,且全部为局限基因型;(2)与Ellstrand Roose(1987)总结的克隆多样性平均值(PD=0.17,D=0.62)相比艾纳香的种群克隆多样性水平稍高,Simpson指数平均为0.973,基因型比率PD平均为0.800;(3)遗传一致度和遗传距离分析表明,4个艾纳香野生种群被分成两组,一组是海南的所有种群,另外一组是云南类群。     

羊草基因型多样性能增强种群对干扰的响应

研究了不同基因型多样性(1、3、6三种基因型组合)羊草种群的地上生物量、地下生物量、分蘖数、根茎芽数和根冠比5个指标对干扰强度(用不同留茬高度来模拟)的响应。结果表明:(1)基因型多样性和干扰强度对地上生物量、地下生物量、分蘖数和根茎芽数均有显著影响(P0.05),但两者的交互作用不显著(P0.05)。其中,多基因型组合(3、6基因型组合)羊草种群中这4个响应变量的值均显著高于单基因型羊草种群(P0.05);而干扰强度的增加显著降低了这4个响应变量的值(P0.05)。对于根冠比这一响应变量来说,仅干扰强度对其产生了显著地影响(P0.05)。(2)29个基因型多样性效应值中,有25个值大于0,其中12个表现为显著的基因型多样性正效应。依Loreau和Hector的方法将多样性净效应分解后发现,互补效应和选择效应共同主导12个响应指标的基因型多样性效应,而互补效应独自主导3个、选择效应独自主导5个响应指标的基因多样性效应;但对基因型多样性正效应起主要贡献的是互补效应。所得结果表明,基因型多样性能提高羊草种群的表现,能增强羊草种群对干扰的响应,不同基因型间的互补作用对这种正效应起主要贡献,这将为该物种种质资源保护和合理利用提供理论指导。     

陆地生态系统混合凋落物分解研究进展

凋落物分解在陆地生态系统养分循环与能量流动中具有重要作用,是碳、氮及其他重要矿质养分在生态系统生命组分间循环与平衡的核心生态过程。自然生态系统中,植物群落大多具有较高的物种丰富度和多样性,其混合凋落物在分解过程中也更有可能发生养分传递、化学抑制等种间互作,形成多样化的分解生境,多样性较高的分解者类群以及复杂的级联效应分解,这些因素和过程均对研究混合凋落物分解过程、揭示其内在机制形成了极大的挑战。从构成混合凋落物物种丰富度和多样性对分解生境、分解者多样性及其营养级联效应的影响等方面,综合阐述混合凋落物对陆地生态系统凋落物分解的影响,探讨生物多样性在凋落物分解中的作用。通过综述近些年的研究发现,有超过60%的混合凋落物对其分解速率的影响存在正向或负向的效应。养分含量有差异的凋落物混合分解过程中,分解者优先利用高质量凋落物,使低质量的凋落物反而具有了较高的养分有效性,引起低质量凋落物分解加快并最终使混合凋落物整体分解速率加快;而凋落物物种丰富度对土壤动物群落总多度有轻微的影响或几乎没有影响,但是对线虫和大型土壤动物的群落组成和多样性有显著影响,并随着分解阶段呈现一定动态变化;混合凋落物改变土壤微生物生存的理化环境,为微生物提供更多丰富的分解底物和养分,优化微生物种群数量和群落结构及其分泌酶的活性,并进一步促进了混合凋落物的分解。这些基于植物-土壤-分解者系统的动态分解过程的研究,表明混合凋落物分解作用不只是经由凋落物自身质量的改变,更会通过逐级影响分解者多样性水平而进一步改变分解速率和养分释放动态,说明生物多样性确实在一定程度上调控凋落物分解及其养分释放过程。     

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。     

青冈种群克隆多样性及其与环境因子的关系

 采用等位酶分析测定了6个青冈种群的克隆多样性。种群和种水平上每基因型含有的个体(克隆分株)数分别为1.270和1.508。青冈种群中克隆多样性较高,Simpson’s指数平均为0.9882,均匀度平均为0.7921。81.3％的基因型为局部分布,仅有1种(0.5％)基因型分布于75％以上的种群中。相关分析表明Simpson’s指数与总氮含量呈显著的负相关(P<0.05),与其他环境因子的相关性不显著。     

Consequences of prolonged clonal growth on local and regional genetic structure and fruiting success of the forest perennial Maianthemum bifolium

The balance between clonal propagation and sexual reproduction varies among species. Although theory predicts an impact of clonal growth on both‐ within‐ and between population genetic structure, most empirical evidence available to date does not reveal sharp differences between sexually reproducing and clonal species. This has been attributed mainly to the fact that even low levels of sexual recruitment can maintain high levels of genetic diversity. Here we study the effects of prolonged clonal growth and very low rates of sexual recruitment on the genetic structure of the perennial Maianthemum bifolium, an outcrossing understorey species of temperate forests. Average genotypic diversity (0.37) of the populations, as revealed by AFLP, was above the average values reported for species of similar characteristics, but some populations were extremely poor in genotypes. Fruiting success was positively correlated with genotypic diversity, probably as a result of shortage in mating types and compatible pollen in populations poor in genotypes. This was confirmed by a pollination experiment. Fruiting success increased by a factor three when individuals were hand‐pollinated with pollen from a nearby population compared to hand‐pollinations with pollen from the own population. Furthermore, the fruiting success after natural pollination (control individuals) was positively related to number of nearby populations which could act as pollen sources. Given the limited colonization capacity of the species (no seed flow), and the long time since fragmentation of the forest fragments studied, between‐population genetic differentiation was relatively low (Φ_st=0.14). Lack of genetic drift due to long generation times and very limited sexual recruitment is probably responsible for this. Our results show that prolonged clonal growth and lack of sexual recruitment may affect within‐ and between‐ population genetic structure and the capability for sexual reproduction.     

Tropical Understory Piper Shrubs Maintain High Levels of Genotypic Diversity Despite Frequent Asexual Recruitment

Many plant species have the capacity to regenerate asexually by resprouting from stem and leaf fragments. In the pan‐tropical shrub genus Piper, this tendency is thought to be higher in shade‐tolerant than light‐demanding species, and to represent a trade‐off with annual seed production. Here we use molecular markers to identify clones in five Piper species varying in light requirements. We test predictions that (i) asexual recruitment success is highest in shade‐tolerant species, and (ii) that consequently, shade‐tolerant species are characterized by lower genotypic diversity than light‐demanding Piper. We found that two shade‐tolerant Piper species recruited asexually more frequently (36–42% of sampled shoots were of asexual origin) than, two light‐demanding and one shade‐tolerant species (0–26%). Furthermore, as predicted, genotypic diversity was negatively correlated with the frequency of asexual recruitment in the population. Nonetheless, genotypic diversity of Piper was high compared with other clonal plants. The proportion of unique genotypes found per population ranged from 0.58 to 1.0 and the genotypic Simpson's diversity ranged from 0.93 to 1.0 for all five species. Our results suggest that even though asexual reproduction plays an important role in maintaining local populations of Piper in the understory, it does not seem to reduce genotypic diversity to levels that will threaten these species ability to respond to environmental change. Abstract in Spanish is available in the online version of this article.     

The loss of sex in clonal plants

Most plants combine sexual and clonal reproduction, and the balance between the two may vary widely between and within species. There are many anecdotal reports of plants that appear to have abandoned sex for clonal reproduction, yet few studies have quantified the degree of sexual variation in clonal plants and fewer still have determined the underlying ecological and/or genetic factors. Recent empirical work has shown that some clonal plants exhibit very wide variation in sexual reproduction that translates into striking variation in genotypic diversity and differentiation of natural populations. Reduced sexual reproduction may be particularly common at the geographical margins of species' ranges. Although seed production and sexual recruitment may often be limited by biotic and abiotic aspects of the environment in marginal populations, genetic factors, including changes in ploidy and sterility mutations, may also play a significant role in causing reduced sexual fertility. Moreover, environmental suppression of sexual recruitment may facilitate the evolution of genetic sterility because natural selection no longer strongly maintains the many traits involved in sex. In addition to the accumulation of neutral sterility mutations in highly clonal populations, the evolution of genetic infertility may be facilitated if sterility is associated with enhanced vegetative growth, clonal propagation or survival through either resource reallocation or pleiotropy. However, there are almost no experimental data with which to distinguish among these possibilities. Ultimately, wide variation in genotypic diversity and gene flow associated with the loss of sex may constrain local adaptation and the evolution of the geographical range limit in clonal plants.     

High genotypic diversity and strong spatial structure in populations of Trifolium alpestre with low seed production

Studies on clonal plants indicate that the proportion between clonal and sexual reproduction can be variable, depending on local habitat conditions and the biological characteristics of the species. In the present study, we assessed this question in Trifolium alpestre by assaying genetic diversity and spatial genotypic structure of natural populations with the use of allozyme markers. Populations revealed high genetic diversity as well as strong spatial structure of multilocus genotypes. The values of genetic diversity were moderately high. Spatially aggregated, identical genotypes spread up to 15 m along linear transects and across 4‐m² plots indicate extensive clonal propagation within populations. However, the existence of numerous unique and small‐sized clones reflects significant contribution from sexual reproduction. Spatially and temporarily stochastic soil disturbances have evidently opened new opportunities for the successful sexual recruitment from the permanent soil seed bank and thus counteracted losses of genetic and genotypic diversity. Seed production in all populations during the three study years was low, in average up to 1.5–2.4 seeds per shoot. The almost total lack of seed set for 57 bagged flower heads on genotypes grown in a common garden indicates that T. alpestre needs pollinators for seed production.     

The population genetics of clonal and partially clonal diploids

The consequences of variable rates of clonal reproduction on the population genetics of neutral markers are explored in diploid organisms within a subdivided population (island model). We use both analytical and stochastic simulation approaches. High rates of clonal reproduction will positively affect heterozygosity. As a consequence, nearly twice as many alleles per locus can be maintained and population differentiation estimated as F(ST) value is strongly decreased in purely clonal populations as compared to purely sexual ones. With increasing clonal reproduction, effective population size first slowly increases and then points toward extreme values when the reproductive system tends toward strict clonality. This reflects the fact that polymorphism is protected within individuals due to fixed heterozygosity. Contrarily, genotypic diversity smoothly decreases with increasing rates of clonal reproduction. Asexual populations thus maintain higher genetic diversity at each single locus but a lower number of different genotypes. Mixed clonal/sexual reproduction is nearly indistinguishable from strict sexual reproduction as long as the proportion of clonal reproduction is not strongly predominant for all quantities investigated, except for genotypic diversities (both at individual loci and over multiple loci).     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

The effects of sexual and asexual reproduction on geographic variation in the sea anemone Actinia tenebrosa

Allelic and genotypic frequencies were determined for samples from 35 widely distributed Australasian colonies of Actinia tenebrosa and 2 South African colonies of A. equina. These data provided no evidence of gene flow between Australisian and South African Actinia colonies and indicated that there may be some restriction of gene flow between widely separated Australasian colonies.Both species are viviparous, and brooded A. tenebrosa are known to be produced asexually. The present data indicate that, within both species, almost all genotypic diversity is generated by sexual reproduction with recombination. Sexually produced juveniles appear to be widely dispersed and panmixis may occur over thousands of kilometres. However, successful sexual recruitment must be episodic or rare. Colonies on stable shores displayed relatively low levels of genotypic diversity, as compared with expectations for sexually reproducing populations, indicating strong local effects of asexual recruitment. Clonal genotypes may be spread over hundreds of metres of shore, but are typically restricted to discrete colonies. Asexual recruitment is highly localised and asexual dispersal appears to be limited by lengths of shore (500 m) which are unsuitable for colonization. Colonies on unstable shores are significantly more diverse genotypically and show little evidence of clonal proliferation.     

CONTRIBUTIONS OF SEXUAL AND ASEXUAL REPRODUCTION TO POPULATION STRUCTURE IN THE CLONAL SOFT CORAL,ALCYONIUM RUDYI

Numerous studies of population structure in sessile clonal marine invertebrates have demonstrated low genotypic diversity and nonequilibrium genotype frequencies within local populations that are monopolized by relatively few, highly replicated genets. All of the species studied to date produce planktonic sexual propagules capable of dispersing long distances; despite local genotypic disequilibria, populations are often panmictic over large geographic areas. The population structure paradigm these species represent may not be typical of the majority of clonal invertebrate groups, however, which are believed to produce highly philopatric sexual propagules. I used allozyme variation to examine the population structure of the temperate soft coral, Alcyonium rudyi, a typical clonal species whose sexually produced larvae and asexually produced ramets both have very low dispersal capabilities. Like other clonal plants and invertebrates, the local population dynamics of A. rudyi are dominated by asexual reproduction, and recruitment of new sexually produced genets occurs infrequently. As expected from its philopatric larval stage, estimates of genetic differentiation among populations of A. rudyi were highly significant at all spatial scales examined (mean θ = 0.300 among 20 populations spanning a 1100-km range), suggesting that genetic exchange seldom occurs among populations separated by as little as a few hundred meters. Mapping of multilocus allozyme genotypes within a dense aggregation of A. rudyi ramets confirmed that dispersal of asexual propagules is also very limited: members of the same genet usually remain within < 50 cm of one another on the same rock surface. Unlike most previously studied clonal invertebrates, populations of A. rudyi do not appear to be dominated by a few widespread genets: estimates of genotypic diversity (G_o) within 20 geographically distinct populations did not differ from expectations for outcrossing, sexual populations. Despite theoretical suggestions that philopatric dispersal combined with typically small effective population sizes should promote inbreeding in clonal species, inbreeding does not appear to contribute significantly to the population structure of A. rudyi. Genet genotype frequencies conformed to Hardy-Weinberg expectations in all populations, and inbreeding coefficients (f) were close to zero. In general, the population structure of A. rudyi did not differ significantly from that observed among outcrossing sexual species with philopatric larval dispersal. Age estimates suggest, however, that genets of A. rudyi live for many decades. Genet longevity may promote high genotypic diversity within A. rudyi populations and may be the most important evolutionary consequence of clonal reproduction in this species and the many others that share its dispersal characteristics.     

Demographic and random amplified polymorphic DNA analyses reveal high levels of genetic diversity in a clonal violet

We performed demographic and molecular investigations on woodland populations of the clonal herb Viola riviniana in central Germany. We investigated the pattern of seedling recruitment, the amount of genotypic (clonal) variation and the partitioning of genetic variation among and within populations. Our demographic study was carried out in six violet populations of different ages and habitat conditions. It revealed that repeated seedling recruitment takes place in all of these populations, and that clonal propagation is accompanied by high ramet mortality. Our molecular investigations were performed on a subset of three of these six violet populations. Random amplified polymorphic DNA analyses using six primers yielded 45 scorable bands that were used to identify multilocus genotypes, i.e. putative clones. Consistent with our demographic results and independent of population age, we found a large genotypic diversity with a mean proportion of distinguishable genotypes of 0.93 and a mean Simpson's diversity index of 0.99. Using AMOVA we found a strong genetic differentiation among these violet populations with a PhiST value of 0.41. We suggest that a high selfing rate, limited gene flow due to short seed dispersal distances and drift due to founder effects are responsible for this pattern. Although Viola riviniana is a clonal plant, traits associated with sexual reproduction rather than clonality per se are moulding the pattern of genetic variation in this species.     

The effect of herbivores on genotypic diversity in a clonal aquatic plant

In clonal plants, vegetative parts may outcompete seeds in the absence of disturbance, limiting the build‐up of genotypic diversity through repeated seedling recruitment (RSR). Herbivory may provide disturbance and trigger establishment of strong colonizers (seeds) at the expense of strong competitors (clonal propagules). In the clonal aquatic fennel pondweed Potamogeton pectinatus, two distinct herbivore guilds may modify the dynamics of propagation. In winter, Bewick's swans may deplete patches of tubers, promoting seedling establishment in spring. In summer, seed consumption by waterfowl can reduce the density of viable seeds but grazing may also reduce tuber production and hence facilitate seedling establishment. This study is among the first to experimentally test herbivore impact on plant genotypic diversity. We assess the separate and combined effects of both herbivore guilds on genotypic diversity and structure of fennel pondweed beds. Using microsatellites, we genotyped P. pectinatus from an exclosure experiment and assessed the contribution of herbivory, dispersal and sexual reproduction to the population genetic structure. Despite the predominance of clonal propagation in P. pectinatus, we found considerable genotypic diversity. Within the experimental blocks, kinship among genets decreased with geographic distance, clearly identifying a role for RSR in the maintenance of genotypic diversity within the fennel pondweed beds. However, over a period of five years, none of the herbivory treatments affected genotypic diversity. Hence, sexual reproduction on a local scale is important in this putatively clonal plant and possibly sufficient to ensure a relatively high genotypic diversity even in the absence of herbivores. Although we cannot preclude a role of herbivory in shaping genotypic diversity of a clonal plant, after five years of exclusion of the two investigated herbivore guilds no measurable effect on genotypic diversity was detected.