Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

Sex allocation under simulated herbivory in the generalist perennial herb Paeonia broteroi (Paeoniaceae)

Theories of optimal resource allocation in flowering plants postulate that allocation to sexual functions are balanced. While many studies have found such evidence in hermaphroditic species among flowers, plants or populations, or at different phenological stages, it has not been supported by other studies. This has been explained by differences among genotypes in ability to acquire resources, resource partitioning among traits unrelated to reproduction or strong selection to maintain positive genetic correlation among traits. I studied how herbivory affected resource allocation to sexual functions in the perennial herb Paeonia broteroi (Paeoniaceae) by measuring a number of floral traits in control plants and in experimental plants under simulated herbivory. The species shows very little plasticity in resource allocation between sexual structures and functions, and appears to be highly sensitive to alterations in the balance of resource acquisition and allocation, with an immediate outcome in terms of seed production and a mid-term response in terms of sprouting and flowering. Plants' ability to collect resources for growth and reproduction before their senescence at the end of the reproductive season, are compromised. This may limit their reproductive potential, the maintenance and growth of their populations, and may influence their demographic state and the plants' ability to respond to selective pressures by external biotic agents.     

Trade‐offs between life‐history traits at range‐edge and central locations

The allocation of resources to different life‐history traits should represent the best compromise in fitness investment for organisms in their local environment. When resources are limiting, the investment in a specific trait must carry a cost that is expressed in trade‐offs with other traits. In this study, the relative investment in the fitness‐related traits, growth, reproduction and defence were compared at central and range‐edge locations, using the seaweed Ascophyllum nodosum as a model system. Individual growth rates were similar at both sites, whereas edge populations showed a higher relative investment in reproduction (demonstrated by a higher reproductive allocation and extended reproductive periods) when compared to central populations that invested more in defence. These results show the capability of A. nodosum to differentially allocate resources for different traits under different habitat conditions, suggesting that reproduction and defence have different fitness values under the specific living conditions experienced at edge and central locations. However, ongoing climate change may threaten edge populations by increasing the selective pressure on specific traits, forcing these populations to lower the investment in other traits that are also potentially important for population fitness.     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively),and the balance between the two reproductive modes may vary widely between and within species.Extensive clonal growth may affect the evolution of life history traits in many ways.First,in some clonal species,sexual reproduction and sex ratio vary largely among populations.Variation in sexual reproduction may strongly affect plant's adaptation to local environments and the evolution of the geographic range.Second,clonal growth can increase floral display,and thus pollinator attraction,while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal.Geitonogamous pollination can bring a cost to plant fitness through both female and male functions.Some co-evolutionary interactions,therefore,may exist between the spatial structure and the mating behavior of clonal plants.Finally,a trade-off may exist between sexual reproduction and clonal growth.Resource allocation to the two reproductive modes may depend on environmental conditions,competitive dominance,life span,and genetic factors.If different reproductive modes represent adaptive strategies for plants in different environments,we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition,while clonal growth should be dominant in stable habitats.Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes.Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives.     

青藏高原东缘块茎堇菜鳞茎分配的个体大小依赖性

块茎堇菜(Viola tuberifera)为青藏高原特有两型闭锁花植物,属多年生草本,具独特的混和交配系统,既可通过早春开放花异花受精和夏季地上地下闭锁花自花受精有性繁殖,还可通过秋季新鳞茎无性繁殖产生后代。高山环境下,异花受精常因花粉限制而无法正常进行,自花受精和克隆繁殖成为保障植物种群正常繁衍的不二之选,而克隆繁殖更能在植株资源消耗最小的情况下保障子代的存活。该文以青藏高原东缘高寒草甸的混合繁育植物块茎堇菜为研究对象,探索其生长期内鳞茎分配的个体大小依赖性,以及植株如何权衡鳞茎的资源分配以适应个体大小的变化。结果表明：块茎堇菜生活史阶段的鳞茎分配具有个体大小依赖性,鳞茎分配与个体大小呈极显著负幂指数相关关系(P<0.01),个体越大,鳞茎分配越小;反之,个体越小,鳞茎分配越高。即块茎堇菜对鳞茎的资源投入受个体大小的制约,通过鳞茎分配比例的高低响应植株自身资源状况的变化,保障在高寒环境下植物种群的生存和繁衍。该研究结果为高山植物克隆繁殖的生活史进化提供了依据。     

Demographic consequences of greater clonal than sexual reproduction in Dicentra canadensis

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.     

不同生境中克隆植物的繁殖倾向

对克隆植物在林下、草原、荒漠、冻原、农田和水域等6种不同类型生境中的繁殖对策及其生态适应意义进行了归纳和总结。对克隆植物有性繁殖与无性繁殖之间的权衡关系及相对重要性随水分、光照和温度的变化规律进行了阐述。分析表明,在环境条件严酷地区,克隆植物的无性繁殖在种群更新中的相对重要性有所增加。对两种繁殖方式的生态功能特性进行比较,综述了有关二者权衡关系的研究成果及影响二者权衡关系的部分因素。从研究手段上对克隆植物繁殖对策的研究进行了思考与展望。     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively), and the balance between the two reproductive modes may vary widely between and within species. Extensive clonal growth may affect the evolution of life history traits in many ways. First, in some clonal species, sexual reproduction and sex ratio vary largely among populations. Variation in sexual reproduction may strongly affect plant’s adaptation to local environments and the evolution of the geographic range. Second, clonal growth can increase floral display, and thus pollinator attraction, while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal. Geitonogamous pollination can bring a cost to plant fitness through both female and male functions. Some co-evolutionary interactions, therefore, may exist between the spatial structure and the mating behavior of clonal plants. Finally, a trade-off may exist between sexual reproduction and clonal growth. Resource allocation to the two reproductive modes may depend on environmental conditions, competitive dominance, life span, and genetic factors. If different reproductive modes represent adaptive strategies for plants in different environments, we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition, while clonal growth should be dominant in stable habitats. Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes. Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives. Translated from Acta Phytoecologica Sinica, 2006, 20(1): 174–183 [译自: 植物生态学报]     

Interpreting reproductive allometry: Individual strategies of allocation explain size-dependent reproduction in plant populations

Size-dependent or allometric relationships between reproductive and vegetative size are extremely common in plant populations. Reproductive allometry where plant size differences are due to environmental variability has been interpreted both as an adaptive strategy of plant growth and allocation, and as the product of fixed developmental constraints. Patterns of development are crucial in defining reproductive allometry but development is not fixed across individuals. For example, environmental adversity (e.g. resource impoverishment) tends to favor reproduction at relatively small sizes – an adaptive response to environmental adversity. While small individuals may have lower reproductive output than large individuals, all plants should maximize their reproductive output and relative allocation to reproduction may be constant across sizes. Thus, where individual plants within a population initiate reproduction at different sizes, no significant reproductive allometry is an appropriate null expectation. Reproductive allometry occurs in plant populations where initiating reproduction at small sizes yields relatively high or low reproductive size at final development. Both of these outcomes are common in plant populations. Our interpretation of reproductive allometry combines previous adaptive and developmental constraint interpretations, and is the first to successfully explain the range of relationships in plant populations where relative allocation has been observed to increase, decrease or remain constant will increasing plant size.     

EFFECTS OF LIGHT AND DENSITY ON RESOURCE ALLOCATION IN A FOREST HERB,ASTER ACUMINATUS (COMPOSITAE)

Recent studies show that forest herbs have low sexual reproductive efforts (SRE) and high vegetative reproductive efforts (VRE) and that a variety of factors may affect the levels of reproduction and the distribution patterns of forest species. Aster acuminatus is a micro-successional forest herb common in northern New England. Resource allocation patterns were determined for seven aster patches varying in density and light level. Plant density affects neither plant size nor any aspect of resource allocation, and VRE remains constant over all density and light levels. Light level is, however, significantly correlated with patch location, average plant size and average patch SRE. Only large plants produce flowers suggesting that light affects sexual reproduction indirectly through effects on plant size. The high rhizome and low seed survivorship of this species may partly explain the constant VRE and varying SRE levels, although the resource allocation patterns observed do not correspond well with the predictions of certain theoretical models.     

Variation in reproductive modes and population genetic structures of a monocarpic perennial herb,Cardiocrinum cordatum,in relation to habitat fragmentation

Reproductive systems are recognized as having a profound influence on the extent and structure of genetic variation in plant populations. To investigate the spatiotemporal variation in the reproductive modes (sexual and vegetative reproduction) and population genetic structures of a monocarpic perennial herb, Cardiocrinum cordatum (liliaceae), we selected a variety of habitats (e.g. large forested area including primeval forest, small fragmented secondary forest, and so on) around Sapporo City, Japan. We conducted breeding experiments, monitored the fate and growth of marked individuals for 3 years, and also analyzed the spatiotemporal genetic variation of flowering plants within the populations using allozyme variation. Plants emasculated prior to anthesis produced mature fruits in all populations examined. However, seed production was significantly lower in the small fragmented populations, possibly because of the low availability of pollinators and subsequent pollen limitation. In these fragmented populations, the mature flowering plants tended to be more dependent on vegetative reproduction for their recruitment, because they can only produce flowers once in their lifetime. Genetic diversity using samples from mature flowering plants in each population was lower in the small fragmented populations than in the large populations. In addition, although genotypic compositions in the fragmented populations were more or less similar during the 3 years of the study, the dominant genotypes changed temporally and spatially every year in the large populations. The present study demonstrated that the reproductive features of C. cordatum can be altered in various environmental conditions, such as habitat fragmentation, and these changes considerably affected the population genetic structures and vice versa.     

Cost of reproduction in the perennial herb Asphodelus albus (Liliaceae)

The cost of reproduction has been studied in two populations of the polycarpic herb Asphodelus albus under natural conditions The percentage of plants with flowers was determined in four sites and varied markedly among them The occurrence of reproduction was size-dependent, increasing flowering probability with plant size The cost of reproduction was assessed in terms of modular growth in reproductive plants relative to modular growth in vegetative ones I compared the modular growth of vegetative and reproductive plants considering two different densities m each of two populations Neither incidence of flowering nor modular growth were affected by density Flowering plants exhibited a withinramet demographic cost (in terms of modular growth) relative to non-flowering ramets in one population but not in the other This cost was greater in larger plants These results were concordant with the occurrence of flowering at both sites Both populations exhibited size-dependent patterns of allocation to reproduction, but no significant relationships were found between allocation to reproduction and cost of reproduction The data presented demonstrate differences in the cost of reproduction within a species This cost might determine whether a plant begins the reproduction, but probably have no effect on the reproductive allocation since the weight of the reproductive structures was not related to modular growth     

Variation and covariation among life-history traits in Rumex acetosella from a successional old-field gradient

In this study morphological variation and the potential for competition to affect biomass and seedling selection of the families of five populations of Rumex acetosella L. sampled along a successional old-field gradient have been investigated. Seeds from 25 families were submitted to four competitive regimes: no competition (one plant per pot), medium competition (two plants/ pot taking plants from the same population), high within-population competition (four individuals from the same population in a pot) and high between-population competition (four individuals from two different populations in a pot). Eight traits were analysed after 3 months of growth for variation among families within populations. A significant difference among families within the two older populations was recorded for sexual biomass and related components. High sensitivity of these traits to density was observed in all populations except the youngest, suggesting specialization to particular environmental conditions in late successional populations, and a good adaptive capacity to buffer environmental variation in the pioneer population. Little significant interaction between competitive regimes and families within populations was found, i.e. genotypes within each population showed little variation in their response to environmental variation. Genotypic variance decreased with increasing competitive conditions for the majority of the traits. However, the percentage of variance in sexual reproduction explained by family was stable among treatments. Tradeoffs between vegetative reproduction and sexual reproduction were recorded at the population level along the successional gradient, with increasing competitive conditions. As succession proceeds, we observed a decrease in sexual reproduction and an increase in vegetative reproduction. At the family level, correlation among traits were similar when plants were grown in the absence of competition and at high density, with a significant negative correlation between sexual reproduction and vegetative reproduction. For both sprout number and sexual biomass, the performance of families grown under all the treatments was positively correlated. Together these results indicate allocational constraints on the reproductive biology of R. acetosella that may be favoured by natural selection and have influenced population differentiation along the successional gradient. However, they also revealed that the potential exists for evolutionary specialization through plasticity, in response to variation in environmental conditions.     

The relative importance of sexual and clonal reproduction for population growth in the perennial herb Fragaria vesca

The relative importance of sexual and clonal reproduction for population growth in clonal plants is highly variable. Clonal reproduction is often more important than sexual reproduction but there is considerable interspecific variation and the importance of the two reproductive modes can change with environmental conditions. We carried out a demographic study on the woodland strawberry (Fragaria vesca), a widespread clonal herb, at 12 sites in Switzerland during 2 years. Study sites were selected in two different habitats, i.e., forest and forest edge. We used periodic matrix models to estimate annual population growth rates and carried out prospective analyses to identify life cycle components that influence population growth rates most. Retrospective analyses were applied to study how the two different habitats affected population dynamics. Furthermore, we tested whether trade-offs between sexual and clonal reproduction occurred. There were large differences in annual population growth rates between sites and large within-site differences between years. Results of the prospective analyses clearly indicate that clonal reproduction is the dominant reproductive pathway whereas sexual reproduction is rather insignificant for population growth. Compared to forest habitats, forest edge habitats had higher population growth rates in the first year but smaller growth rates in the second year. We attribute these differing habitat effects to different water availabilities during consecutive years. No trade-offs between sexual and clonal reproduction were found. In conclusion, population growth of F. vesca relies heavily on clonal reproduction. Furthermore, reproduction and survival rates of F. vesca depend highly on spatio-temporal variation of environmental conditions.     

Optimal resource allocation to seeds and vegetative propagules under density-dependent regulation in Syneilesis palmata (Compositae)

Syneilesis palmata reproduces by both seeds and vegetative propagules (short rhizomes). The latter result in the production of new plants that are larger in size and hence have a higher survival probability and a higher growth rate than seeds. A previous study predicted that the optimal reproductive strategy, in terms of maximizing population growth rate (a fitness measure under no density regulations), was pure vegetative reproduction. However, high resource investment to vegetative propagules can cause local crowding resulting in reduced demographic performances of the plants, because the vegetative propagules of Syneilesis are produced close to one another. We examined, in this situation, the impact of allocating a certain proportion of reproductive resource to seeds with relatively greater capacity for dispersal. We simulated dynamics of hypothetical Syneilesis populations with various reproductive resource allocation balances (from pure seed to pure vegetative reproduction), using a density-dependent matrix model. In the model, it was assumed that plants from vegetative propagules experienced density-dependent reduction in their survival probabilities, but this was not the case for plants originating from seeds. Each allocation strategy was evaluated based on an equilibrium population density, a fitness measure under density-dependent regulations. The optimal reproductive strategy predicted was pure vegetative reproduction. Unrealistic conditions were required for seed reproduction to be favoured, such as the production of seeds one hundred times the normal number per unit resource investment. However, the conditions were fairly relaxed compared with those required in the model where no density effects were incorporated. This indicates that escape from local crowding is likely to be one of the roles of seed production in Syneilesis.     

Optimal allocation to vegetative and sexual reproduction in plants: the effect of ramet density

Vegetative reproduction is a very common alternative by which plants can contribute to the next generations. There are many considerations predicting which mode of reproduction, vegetative or sexual, should be favored and numerous experimental studies to verify them. However, the results are inconsistent especially when the effect of plant density is considered. I apply here a dynamic optimization model to predict the rate of vegetative and sexual reproduction in plants as a response to changes in the local plant density. The population is assumed to occupy a heterogeneous environment consisting of patches in which growth and reproduction of plants are possible and unfavorable space between them. As the environment is globally stable, the seeds, which can disperse without restriction, exhibit a constant recruitment rate. The ramets are assumed to settle only within the patch of the mother plant. The rate of ramet production effects local density, which in turn determines ramet recruitment. The optimal strategy maximizes the expected lifetime genetic contribution, realized via both vegetative and sexual reproduction. The solutions obtained under these assumptions are dualistic. The model predicts that different approaches applied in studying the effect of ramet density should give opposite outcomes. When the comparison is between patches in natural populations, a positive relationship between relative ramet allocation and density is expected. When the density is experimentally manipulated or its effect is analyzed across different successional stages, a negative relationship should be found. The results seem to be confirmed by empirical studies.     

Trade-offs between sexual and clonal reproduction in an aquatic plant: experimental manipulations vs. phenotypic correlations

That trade-offs result from the allocation of limited resources is a central concept of life history evolution. We quantified trade-offs between sexual and clonal reproduction in the aquatic plant, Butomus umbellatus, by experimentally manipulating sexual investment in two distinct nutrient environments. Increasing seed production caused a significant but nonlinear trade-off. Pollinating half of all flowers strongly reduced clonal bulbil production, but pollinating the remaining flowers did not cause any further trade-off. Trade-offs were not stronger under low nutrient conditions that clearly limited plant growth. Experimentally induced trade-offs were not reflected in negative phenotypic correlations between sexual and clonal allocation among plants within eight populations grown in a uniform greenhouse environment. Diminishing effects of increased sexual allocation plus a lack of accord between experimental manipulations and phenotypic correlations suggest that trade-offs between sexual and clonal reproduction are unlikely to constrain the evolution of reproductive strategy in this species.     

The role of sexual and vegetative reproduction in the population maintenance of a monocarpic perennial herb,Cardiocrinum cordatum var. glehnii

Because monocarpic perennial plants have only one reproductive opportunity in their entire life, they need to ensure offspring production. Some plants reproduce both sexually and vegetatively, and vegetative reproduction could possibly compensate for seed production. Therefore, the role and significance of these reproductive modes is likely to differ between monocarps and polycarps, which can reproduce many times. Cardiocrinum cordatum var. glehnii is a monocarpic perennial that reproduces both sexually and vegetatively (bulblet formation). Here, we investigated the characteristics and contribution to population maintenance of sexual and vegetative reproduction to reveal the significance of these two reproductive modes in this species. First, we found that bulblet formation occurred in plants after the three‐leaved rosette stage. Second, resource allocation experiments revealed that although resources were mainly invested in fruit maturation after the flowering season, resource allocation was switched from sexual reproduction to vegetative reproduction if seed production was insufficient. Third, the outcrossing rate in this species varied greatly according to the environment surrounding the population. However, reproductive assurance by selfing kept seed production stable even if flowers did not receive sufficient pollen for full seed set via outcross pollination, and moreover, there was no intensive inbreeding depression. Finally, genotypic identification of ramets suggested that daughter ramets derived from vegetative reproduction received the space that the mother flowering ramet had occupied until the previous year.     

Environmental heterogeneity generates fluctuating selection on a secondary sexual trait

In any population in which resources are limiting, the allocation of resources toward increased reproductive success may generate costs to survival [1-8]. The relationship between a sexually selected trait and fitness will therefore represent a balance between its relative associations with fecundity versus viability [3, 6, 7]. Because the risk of mortality in a population is likely to be heavily determined by ecological conditions, survival costs may vary as a function of the prevailing environment [7]. As a result, for populations experiencing heterogeneous ecological conditions, there may not be a single optimal level of allocation toward reproduction versus survival [9]. Here, we show that early viability and fecundity selection act in opposing directions on a secondary sexual trait and that their relative magnitude depends upon ecological conditions, generating fluctuating selection. In a wild population of Soay sheep (Ovis aries), phenotypic and genetic associations between male horn growth and lifetime reproductive success were positive under good environmental conditions (because of increased breeding success) and negative under poor environmental conditions (because of reduced survival). In an unpredictable environment, high allocation to early horn growth is a gamble that will only pay off if ensuing conditions are favorable. Such fluctuating selection may play an important role in preventing the erosion of genetic variance in secondary sexual traits.     

四合木种群生殖对策的研究

 通过研究四合木(Tetraena mongolica Maxim)种群的生殖对策,发现四合木既可进行有性生殖,又可进行营养繁殖,以有性生殖为其主要的生殖方式。有性生殖过程中具以下特征：1)具有自交和异交相结合的交配系统,异交比例(50％)高于自交比例(15％);2)具有花多果少的生殖格局,其开花率与生境、降水、植株年龄关系密切,不同种群的开花率有一定差异,种群的年龄结构对开花率有较大影响。在3～5月降水丰富的情况下,不仅开花比例高(40％～80％),且每一植株的花量很大。不同年份种群结实率普遍偏低,3～5月降雨较少的1996年平均结实率为19.8％;3～5月降雨较多的1998年为12.2％。只是由于开花基数大,对四合木种群种子的总产量不会造成较大威胁;3)胚胎发育过程中败育比率极高,只有约10％～20％的胚能发育成熟。