Evolutionary increase in sexual and clonal reproductive capacity during biological invasion in an aquatic plant Butomus umbellatus (Butomaceae)

To test the hypothesis that increased allocation to reproduction is selected during biological invasion, we compared germination, survival, growth, and reproduction of native vs. introduced populations of the invasive aquatic plant Butomus umbellatus in a common greenhouse environment. Although seedling emergence and establishment did not differ consistently, survival thereafter was twice as high for eight introduced North American than eight native European populations. As predicted, introduced plants were more likely to produce sexual inflorescences and clonal asexual vegetative bulbils, and they invested much more biomass in both reproductive modes. Higher reproductive investment was due to higher proportional allocation of biomass rather than larger plant size. These results are consistent with selection for increased reproduction during range expansion. However, population genetic surveys indicate that recruitment from seed rarely occurs in introduced populations. Hence increased sexual allocation is not an adaptive response to invasion. Although increased clonal reproduction may be advantageous in expanding populations, genetic evidence from introduced populations of B. umbellatus suggests that increased clonal allocation may have arisen via stochastic processes during long-distance transport or a selective filter right at introduction, rather than incremental natural selection during range expansion.     

Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant

Clonal plants grow in diameter rather than height, and therefore competition among genets is likely to be symmetric and to result in smaller variation in size of genets than in non-clonal plants. Moreover, clonal plants can reproduce both sexually and vegetatively. We studied the effects of density on the size of rosettes and of clones, variation in the size of rosettes and of clones, and allocation to sexual and vegetative reproduction in the clonal herb Ranunculus reptans . We grew plants from an artificial population of R. reptans in 32 trays at two densities. After four months, differences in density were still apparent, although clones in the low-density treatment had on average 155% more rosettes and 227% more rooted rosettes than clones in the high-density treatment. The coefficient of variation of these measures of clone size was 15% and 83% higher, respectively, in the low-density treatment. This indicates that intraspecific competition among clones of R. reptans is symmetric and increases the effective population size. Rooted rosettes were larger and varied more in size in the low-density treatment. The relative allocation of the populations to sexual and to vegetative reproduction was 19% and 13% higher, respectively, in the high-density treatment. Moreover, seeds produced in the high-density treatment had a 24% higher mass and a 7% higher germination percentage. This suggests that with increasing density, allocation to sexual reproduction increases more than allocation to vegetative reproduction in R. reptans , which corresponds to the response of some other species with a spreading growth form but not of species with a compact growth form. We conclude that intraspecific competition is an important factor in the life-history evolution of R. reptans because intraspecific competition affects its clonal life-history traits and may affect evolutionary processes such as genetic drift and selection through its effect on the effective population size.     

草地植物种群繁殖对策研究

植物的繁殖包括有性繁殖和无性繁殖两大类型,克隆繁殖是一种较为特殊的营养繁殖方式。本文综述了草地种子植物的生殖分配及生殖投资,克隆生长以及放牧对草地植物种群繁殖的影响。植物种群生物量、能量和养分生殖分配是植物种群生殖分配的重要内容,不同植物在结实期营养元素及能量的配置上有着显著的区别,这可能是植物在长期进化过程中形成的生殖对策,是适应环境的结果。在种群水平上,中等强度以上的放牧干扰有利于植物的克隆生长,但有性生殖减弱。草原植物发达的营养繁殖或克隆生长方式是对放牧的适应性对策。     

新疆郁金香营养生长、个体大小和开花次序对繁殖分配的影响

植物有性繁殖与资源分配的关系研究对于揭示植物生活史特征及繁育系统进化具有重要意义。新疆郁金香(Tulipa sinkiangensis)是新疆天山北坡荒漠带特有的一种多年生早春短命植物。在自然生境中,该物种仅以有性繁殖产生后代,每株能产生1-8朵花,且不同植株上的花数及果实数以及花序不同位置上的花与果实大小明显不同。本文通过对新疆郁金香有性繁殖与营养生长及植株大小的关系以及花序中不同位置花及果实间的资源分配研究,旨在揭示营养生长、个体大小及开花次序对其繁殖分配的影响。结果表明:在开花和果实成熟阶段,新疆郁金香植株分配给营养器官(鳞茎和地上营养器官)与繁殖器官的资源间均存在极显著的负相关关系(P<0.01),说明其植株的营养生长与生殖生长间存在权衡关系。多花是新疆郁金香的一个稳定性状,其植株上花数目、花生物量、果实生物量和种子数量与植株生物量间均呈极显著的正相关关系(P<0.01),说明新疆郁金香植株的繁殖分配存在大小依赖性。在具2-5朵花的新疆郁金香植株中,花序内各花的生物量、花粉数和胚珠数、结实率、果实生物量、结籽数、结籽率及种子百粒重按其开花顺序依次递减,说明花序内各花和果实的资源分配符合资源竞争假说。植株通过减少晚发育的花或果实获得的资源来保障早发育的花或果实获得较多的资源,从而达到繁殖成功。     

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.     

Differences in allocation patterns in clonal and sexual offspring in a woodland pseudo-annual

We compared the patterns of allocation to reproduction among seed-derived and clonal offspring of a woodland pseudo-annual. Pseudo-annuals are clonal plants which survive the winter only as seeds and hibernacles produced by the rhizome system. Previous studies indicate that flowering is related to the size of these hibernacles. Since seedlings do not have a hibernacle, we did not expect that these plants would reproduce sexually. Assuming a trade-off between sexual and asexual reproduction, and assuming a linear relationship between vegetative plant weight and weight of all reproductive structures (i.e., rhizomes, hibernacles, inflorescences, and seeds), we expected that seed-derived plants would have a stronger biomass allocation to rhizomes and hibernacles. Since resource supply affects plant size, and thus hibernacle and seed production, we also subjected the plants to different levels of shade. At the start of the experiment seed-derived and clonal offspring hardly differed in total fresh weight. At the final harvest in September seed-derived and clonal offspring did not differ in vegetative plant weight (i.e., leaves, stems, and roots). Only light availability significantly affected these plant structures. As predicted, seed-derived plants did not flower in either of the light treatments. Seed-derived plants allocated more biomass to rhizomes and hibernacles, but this was only significant in the highest-light treatment. This result was due only to an increase in the number of hibernacles. Dry weight of single hibernacles was not affected by plant type. The ecological implications of this allocation pattern are discussed. Received: 2 October 1997 / Accepted: 8 March 1998     

Intraspecific competition and light effect on reproduction of Ligularia virgaurea,an invasive native alpine grassland clonal herb

The relationship between sexual reproduction and clonal growth in clonal plants often shows up at the ramet level. However, only a few studies focus on the relationship at the genet level, which could finally account for evolution. The sexual reproduction and clonal growth of Ligularia virgaurea, a perennial herb widely distributed in the alpine grasslands of the Qinghai‐Tibetan Plateau of China, were studied under different competition intensities and light conditions at the genet level through a potted experiment. The results showed that: (1) sexual reproduction did not depend on density or light, and increasing clonal growth with decreasing density and increasing light intensity indicated that intraspecific competition and light intensity may affect the clonal life history of L. virgaurea; (2) both sexual reproduction and clonal growth show a positive linear relationship with genet size under different densities and light conditions; (3) a threshold size is required for sexual reproduction and no evidence of a threshold size for clonal growth under different densities and light conditions; (4) light level affected the allocation of total biomass to clonal and sexual structures, with less allocation to clonal structures and more allocation to sexual structures in full sunlight than in shade; (5) light determined the onset of sexual reproduction, and the genets in the shade required a smaller threshold size for sexual reproduction to occur than the plants in full sunlight; and (6) no evidence was found of trade‐offs between clonal growth and sexual reproduction under different densities and light conditions at the genet level, and the positive correlation between two reproductive modes indicated that these are two integrated processes. Clonal growth in this species may be viewed as a growth strategy that tends to maximize genet fitness.     

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     

Trade-offs between sexual and asexual reproduction in a monoecious species Sagittaria pygmaea (Alismataceae): the effect of different nutrient levels

Available resources could influence the trade-offs among different reproductive components in plants. Here, we created three nutrient levels to test the nutrient effects on trade-offs among sexual reproduction, clonal propagation and vegetative growth in a monoecious clonal herb Sagittaria pygmaea. The results of this study showed that the plant exhibited different trade-off patterns among different nutrient levels. When the nutrient level was low, there were weak trade-offs between sexual reproduction and vegetative growth and between clonal propagation and vegetative growth; when the nutrient level was moderate, we found a strong trade-off between sexual reproduction and clonal propagation; but when the nutrient level was high, we found no trade-offs among these three different reproductive components. These results indicated that the plant could adjust its trade-off patterns to fit the nutrient variation and suggested that trade-offs are unlikely to constrain the evolution of reproductive strategy in this species.     

SEXUAL DIMORPHISM IN BIOMASS ALLOCATION AND CLONAL GROWTH OF XANTHOXYLUM AMERICANUM

Reproduction can have a high resource cost. It has been suggested that greater investments in sexual reproduction by female dioecious plants leads to a lower rate of vegetative growth in females than in males. In this study, we investigated sexual dimorphism in biomass allocation and genet growth of the dioecious clonal shrub, northern prickly ash (Xanthoxylum americanum). The allocation of biomass over the course of one growing season to reproductive tissue, leaves, and growth of aboveground first-year wood, was compared in 18 clones growing in fields and six clones in woods in southeastern Wisconsin during 1985 and 1986. In addition, the number of shoots per clone, and weight of nonfirst-year wood (accumulated biomass) above- and below-ground were estimated. In open field sites, male clones allocated more biomass to new wood and less to reproduction than females, although males allocated more to flowers alone. Accordingly, male clones had significantly more shoots and more accumulated biomass both above- and below-ground than female clones. In the woods, where fruit set was near zero, there were few significant differences between male and female clones in either biomass allocation or accumulated biomass. These results support the hypothesis that the high resource investment in fruit production by females reduces their vegetative growth relative to males.     

Effects of light on the growth and clonal reproduction of Ligularia virgaurea

Ligularia virgaurea is a perennial herb that is widely distributed in the alpine meadow on the eastern Qinghai-Tibet plateau.We investigated the patterns of growth and reproduction of L.virgaurea under two contrasting levels of light conditions for two continuous growing seasons.Our results showed that the light affects on the maximum relative growth rate,the shoot weight ratio and the root weight ratio differed between the two growing seasons.L.virgaurea reproduced initially through rhizome in the second growing season,rather than sexual reproduction.The proportion of genets with clonal reproduction decreased under shaded conditions.A minimum genet size should be attained for clonal reproduction to begin under the shaded conditions.There was a positive linear relationship between clonal reproduction and genet size.Light level affected the allocation of total biomass to clonal structures,with less allocation under the full natural irradiance than under the shaded conditions.There seemed to be a trade-off between vegetative growth and clonal reproduction under the full natural irradiance,in terms of smaller relative growth rates of genets with clonal reproduction than those without clonal reproduction.L.virgaurea emphasized clonal reproduction under the full natural irradiance,while the plant emphasized vegetative growth under the shaded conditions.     

植物种群的繁殖对策

植物种群的繁殖对策钟章成（西南师范大学,重庆６３０７１５）ＲｅｐｒｏｄｕｃｔｉｖｅＳｔｒａｔｅｇｉｅｓｏｆＰｌａｎｔＰｏｐｕｌａｔｉｏｎｓ．￥ＺｈｏｎｇＺｈａｎｇｃｈｅｎｇ（ＳｏｕｔｈｗｅｓｔＣｈｉｎａＴｅａｃｈｅｒｓＵ－ｎｉｖｅｒｓｉｔｙ,Ｃｈｏｎ...     

Senescence in growth and reproductive allocation in a bunchgrass

• Senescence is a puzzling phenomenon. Few convincing studies of senescence in perennial herbaceous plants exist. While ramets are known to senesce, whether senescence of bunchgrasses actually occurs is not clear.
• In this study, we grew a set of plants of Elymus excelsus, a bunchgrass, to examine plant size, sexual reproduction and bud formation in individual plants in relation to their gradual ageing, in order to determine whether E. excelsus experiences senescence. We collected data in two consecutive years (2009 and 2010) from field samples of plants from 1 to 5 years old. Using regression models, we performed age‐related analyses of growth and reproduction parameters.
• Our results showed that individual plant size (diameter, individual biomass), total biomass of ramets, number and biomass of reproductive ramets, percentage of ramets that were reproductive, reproductive allocation, over‐wintering buds and juvenile ramets all declined with age. However, vegetative growth (number and biomass of vegetative ramets) did not decrease with age.
• Those plants that survived, dwindled in size as they aged. However, no plants shifted their resource allocation between growth and reproduction as they aged, so the shift in allocation did not account for the fall in size.

     

Production dependence of vegetative propagation inDisporum smilacinum A. Gray

The dependence of vegetative propagation on the production of individual plants was examined inDisporum smilacinum A. Gray on the basis of shading experiments and field surveys. This species typically showed four types of reproductive behavior: sterile plants producing one plantlet (no propagation), sterile plants producing more than two plantlets (vegetative propagation), fertile plants producing one plantlet (sexual reproduction) and fertile plants producing more than two plantlets (both sexual reproduction and vegetative propagation). The propagation ofD. smilacinum was clearly related to the annual net production of each individual plant. The probability of a mother plant producing more than two vegetative propagules (plantlets) increased with net production of the plant in the current year. The number of propagules per plant and runner length increased with net production. It was possible to explain the types of reproductive behavior of this species on the basis of both the initial plant size before sprouting and its net production during the growing season. There was a critical initial plant size for sexual reproduction and a critical level of production for vegetative propagation.     

Temporal variation of resource allocation between sexual and asexual structures in response to nutrient and water stress in a floating-leaved plant

Aims Adaptive plasticity of biomass allocation to different environmental stressors enables plants to maintain functional relationships among reproductive structures. In freshwater systems, water depth and nutrient content of sediments can have a major effect on biomass allocation in aquatic macrophytes. However, the relative importance of these two stressors is unknown as it is the temporal variation of biomass allocation to the stressors during the growing period. This information may be critical for understanding the tolerance of a plant to environmental conditions.Methods Here, we used four levels of environmental stressors generated by deep and shallow water and high or low sediment nutrient content in a factorial experiment to investigate the biomass allocation responses of a floating macrophyte, Trapella sinensis, during the growing period.Important findings The results showed that the lower sediment nutrient content inhibited biomass increase, whereas the lower water depth increased the sexual reproduction of the plants. The lower sediment content also led to a delay in flowering and compensated trade-offs among sexual reproduction and elongation and clonal reproduction during the growing period. These results indicated that water depth affected the ratio of biomass allocation, whereas the sediment nutrient content affected biomass accumulation when the plants faced these two environmental factors simultaneously. The temporal changes in allocation under lower sediment nutrient content underscored the importance of collecting data at different stages of growth when trying to interpret resource allocation, especially in resource-limited environments.     

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.     

Size-dependent resource allocation among vegetative propagules and male and female functions in the forest herb Laportea bulbifera

Reproductive resource investment among vegetative propagules and male and female sexual function and their size-dependence were investigated in a perennial forest herb, Laportea bulbifera . A theoretical model based on fitness gain curves predicts that optimal investments in three reproductive modes will increase with plant size if fitness returns in all three modes increase but become saturated with investment. In a field population, large plants of L. bulbifera produced both male and female inflorescences with propagules, while small plants produced only vegetative propagules. Biomass of propagules, male inflorescences, and infructescences with achenes were all positively correlated with plant size. The increase in investment with plant size was larger for propagule production than for sexual reproduction. The relationship between propagule biomass and plant size was constant irrespective of year, while the relationship between the biomass of sexual reproductive organs and plant size differed between two successive years. Annual change of individual sex expression was investigated for 25 transplanted plants. Although each plant changed its sex expression variously among male, female and bisexual from year to year, 23 out of 25 plants produced both male and female inflorescences in at least one year. The number of viable (germinated and survived) offspring from seeds was not significantly different from the number from propagules. The production cost of a propagule was higher than that of a seed. Resource allocation theory does not seem to be applicable to size-dependent resource allocation, especially the allocation between seeds and propagules in this species.     

不同高程短尖苔草对水位变化的生长及繁殖响应

在淡水湿地生态系统中,水位通常是制约植被生长和繁殖动态的关键因素,进而对物种组成、群落演替和植被分布格局产生决定性影响。无性繁殖是洞庭湖湿地克隆植物适应环境胁迫的重要策略之一,以洞庭湖湿地典型克隆植物-短尖苔草(Carex brevicuspis C.B.Clarke)为对象,研究了不同分布高程(23.7 m和25.8 m)的植物对水位变化(0 cm,-15 cm,-30 cm)的生长和繁殖特征响应。结果表明:水位变化对不同分布高程分布短尖苔草的生长和克隆繁殖特征均产生显著影响(P0.05)。对高程区的短尖苔草而言,克隆繁殖特征如分株数、分株总生物量、芽数和芽生物量随水位的降低而增加,而对生长特征(株高及总生物量)无显著影响(P0.05),表明适当干旱有利于高程区苔草的克隆繁殖。对于低程区分布短尖苔草而言,水位变化对其生长特征有显著影响(P0.05),如株高和总生物量随着水位的降低而增加;分株数和总芽生物量等克隆繁殖特征则随水位的降低而减少,而水位对低程区短尖苔草的分株总生物量和总芽数影响不显著(P0.05)。因此,短尖苔草的克隆繁殖特征除受到水位的影响外,还受其分布高程的影响。可见,同一种短尖苔草因长期适应于不同生境而对相同的环境胁迫表现出了不同的生长繁殖策略,     

展毛翠雀花期繁殖特征和生物量分配对放牧的响应

生物量分配影响植物生长和繁殖,是植物生活史研究的重要内容。为了了解植物生活史性状对放牧的响应,该研究以青藏高原高寒草甸毒杂草展毛翠雀为对象,分析了放牧干扰对展毛翠雀的花期繁殖分配和性分配的影响。结果表明:放牧显著降低了展毛翠雀的总生物量、个体大小和繁殖投入; 放牧未改变展毛翠雀的营养部分与繁殖部分的等速生长关系,但显著增加了繁殖部分的生物量分配和总花数; 展毛翠雀的个体大小与总花数呈显著的正相关关系,但与性分配呈显著的负相关关系; 展毛翠雀的总花数与单花大小、单花的花瓣比例均表现出负相关关系,表明总花数与单花大小之间、总花数与单花的花瓣比例之间均存在权衡。因此,在放牧条件下,展毛翠雀的繁殖分配和性分配均表现出显著的可塑性。     

Two fitness measures for clonal plants and the importance of spatial aspects

The capability of clonal plants to reproduce both sexually and vegetatively and their resulting hierarchical organisation into ramets and genets pose problems for the determination of fitness. We develop ramet-based measures of clonal plant fitness including both modes of reproduction. To this end we use simple population-dynamic equations accounting for limited space, limited dispersal, and disturbance. Neglecting interactions among ramets (r-selection regime) we derive an expression for initial growth rate r as a fitness measure. At higher densities interactions among ramets lead to density control (K-selection regime) and competitive exclusion of genotypes or species may occur. In this case we apply an invasibility criterion to derive an abundance fitness measure: competitiveness C. The optimum of C corresponds to an evolutionary stable strategy. C increases with the proportion of reproductive adults, with inverse module mortality, and with the sum of sexual and vegetative reproduction. The latter are defined as the product of the rate of module production and two correction factors accounting for juvenile mortality before establishment and for the efficiency of space exploitation by propagules. Thus C is equivalent to potential life-time offspring production, in contrast to realized life-time offspring production R₀. Because the correction factor for space exploitation cannot be expressed analytically we obtain it from complementary spatially-explicit individual-based simulations. To illustrate an application of the fitness measure C we predict optimal allocation to vegetative and sexual reproduction. In a homogeneous habitat an intermediate allocation may maximize fitness only if there is a non-linear trade-off between the modes of reproduction. However even if this trade-off is linear, spatially heterogenous disturbances can lead to an intermediate optimum of allocation because seed dispersal with subsequent vegetative spread improves the utilization of available space for recruitment if the spatial extent of single disturbances is much larger than the distance of vegetative dispersal. Thus, our study underlines the importance of spatial features for fitness measures especially of clonal plants.