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.     

Optimum reproduction and dispersal strategies of a clonal plant in a metapopulation: a simulation study with Hieracium pilosella

Clonal spread is favoured in many plants at the expense of seed production in order to expand rapidly into open habitats or to occupy space by forming dense patches. However, for the dynamics of a population in a patchy landscape seed dispersal remains important even for clonal plants. We used a spatially explicit individual-based metapopulation model to examine the consequences of two trade-offs in Hieracium pilosella L: first, between vegetative and sexual reproduction, and second, between short and far-distance dispersal of seeds. Our main question was, what are the environmental conditions that cause a mixed strategy of vegetative and sexual reproduction to be optimal. The model was parameterised with field data on local population dynamics of H. pilosella. Patch dynamics were given firstly by disturbance events that opened patches in a matrix of a clonal grass that were colonisable for H. pilosella, and secondly by the gradual disappearance of H. pilosella patches due to the expanding grass. Simulations revealed opposing selection pressures on traits determined by the two trade-offs. Vegetative reproduction is favoured by local dynamics, i.e. the need for maintenance and expansion of established populations, whereas seed production is favoured by the necessity to colonise empty habitats. Similar pressures act on the proportion of seeds dispersed over short and far distances. Optimum reproductive and dispersal strategies depended on habitat quality (determined by seedling establishment probability), the fraction of dispersed seeds, and the fraction of seeds lost on unsuitable ground. Under habitat conditions supporting moderate to low seedling establishment, between 20% and 40% of reproductive effort in H. pilosella should be devoted to sexual reproduction with at least 10% of the seeds dispersed over distances suitable to attain empty patches. We conclude that in a spatially heterogeneous landscape sexual seed production in a clonal plant is advantageous even at the expense of local vegetative growth.     

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]     

Optimal patterns of growth and reproduction for perennial plants with persisting or not persisting vegetative parts

Summary Optimal allocation of energy to growth, reproduction and storage was considered for perennial plants differing in the proportion of vegetative structures persisting over winter and/or in the amount of resources which can be relocated to storage before abscission of some organs. It was found that for every mortality level there exists a critical proportion of persistent organs. Below this critical value it is optimal to grow without reproduction for the first years until a characteristic size is reached; afterwards, that size is maintained year after year and all extra resources are devoted to reproduction. Some storage is also necessary to maintain constant size. If the proportion of retained vegetative mass is above the critical value, the optimal strategy is gradual growth to an asymptotic size, with growth and reproduction occurring in several years following maturation. In this case real storage occurs only until maturation is reached, then storage is realized only by energy relocation from the vegetative body. Although the optimal solution changes abruptly qualitatively at a given proportion of resources saved from year to year, further growth of this proportion above the critical level brings about a greater difference between size reached at maturity and final size. The predictions of the model seem to follow the pattern of nature qualitatively.     

Recruitment trade-offs and the evolution of dispersal mechanisms in plants

In this study we place seed size vs. seed number trade-offs in the context of plant dispersal ability. The objective was to suggest explanations for the evolution of different seed dispersal mechanisms, in particular fleshy fruits, wind dispersal and the maintenance of unassisted dispersal. We suggest that selection for improved dispersal may act either by increasing the intercept of a dispersal curve (log seed number vs. distance) or by flattening the slope of the curve. 'Improved dispersal' is defined as a marginal increase in the number of recruits sited at some (arbitrary) distance away from the parent plant. Increasing the intercept of the dispersal curve, i.e. producing more seeds, is associated with a reduction in seed size, which in turn affects the recruitment ability, provided that this ability is related to seed size. If recruitment is related to seed size there will be a recruitment cost of evolving increased seed production. On the other hand, a flattening of the slope by evolving dispersal attributes is likely to be associated with a fecundity cost. An exception is wind dispersal where smaller (and hence more numerous) seeds may lead to more efficient dispersal. We derive two main predictions: If recruitment is strongly related to seed size, selection for improved dispersal acts on the slope of the dispersal curve, i.e. by favouring evolution of dispersal attributes on seeds or fruits. If, on the other hand, recruitment is only weakly related to seed size (or not related, or negatively related), selection for improved dispersal favours increased seed production. Despite its simplicity, the model suggests explanations for (i) why so many plant species lack special seed dispersal attributes, (ii) differences in dispersal spectra among plant communities, and (iii) adaptive radiation in seed size and dispersal attributes during angiosperm evolution. This revised version was published online in July 2006 with corrections to the Cover Date.     

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。     

克隆植物对异质生境的适应对策研究进展

生境异质性是自然生态系统的基本特征,植物生长的必需资源和环境胁迫因子均存在着复杂的时间和空间异质性。克隆植物是指在自然条件下具有克隆特性的植物,即可通过与母株相连的芽、根茎、分蘖或枝条等繁殖体产生无性繁殖的植物,这些繁殖体一旦定居便可成为潜在的独立个体。克隆植物具有独特的生境适应策略(如形态可塑性、克隆整合、克隆分工、觅食行为、风险分摊等),面对异质性的生境条件,它可以通过调整自身的生理和形态结构来适应异质生境。目前,对于克隆植物在异质生境适应行为的研究已有很多报道,然而系统性的归纳和总结尚有欠缺。综述了克隆植物在不同资源异质生境(光照、养分、水分)和不同胁迫生境(盐碱胁迫、风沙胁迫、重金属胁迫)下独特的适应对策。最后,针对克隆植物对异质生境的适应对策,进行了总结并对未来的重点研究方向提出建议：(1)时间异质性尺度上的考量;(2)异质性生境中生物因子的调控作用;(3)克隆植物入侵机制;(4)克隆植物在生态修复中的应用潜力。     

The color of noise and the evolution of dispersal

The process of dispersal is vital for the long-term persistence of all species and hence is a ubiquitous characteristic of living organisms. A present challenge is to increase our understanding of the factors that govern the dispersal rate of individuals. Here I extend previous work by incorporating both spatial and temporal heterogeneity in terms of patch quality into a spatially explicit lattice model. The spatial heterogeneity is modeled as a two-dimensional fractal landscape, while temporal heterogeneity is included by using one-dimensional noise. It was found that the color of both the spatial and temporal variability influences the rate of dispersal selected as reddening of the temporal noise leads to a reduction in dispersal, while reddening of spatial variability results in an increase in the dispersal rate. These results demonstrate that the color of environmental noise should be considered in future studies looking at the evolution of life history characteristics.     

Reproductive regulation in the female Japanese minnow, Pseudorasbora parva (Cyprinidae)

The Japanese minnow, Pseudorasbora parva, is a multiple spawner that lays eggs repeatedly during the spawning season. In laboratory aquaria, 19 of 63 females did not spawn, whereas 44 other females laid 167 to 6285 eggs in 1 to 14 mating sequences. Neither fish density nor sex ratio (number of males per female) affected fecundity, but the increase in fish density significantly decreased the growth rate of females. The increase in male length significantly increased fecundity in aquaria which contained only a single male, whereas it decreased female growth rate. These relationships were not evident in aquaria housing three or ten males. Since larger males were more dominant and had larger testes than smaller males, the correlation between fecundity and male size strongly suggests that individual females regulated fecundity to increase their reproductive success.     

Importance of clonal plants and plant species diversity in the Northeast China Transect

In plant communities, the internal (genet-level) control mechanisms on a spatio-temporal scale of clonal plants impose strong constraints on spatial pattern as well as on competitive relations and, thus, species coexistence. Therefore, the presence of clonal species within a plant community affects spatio-temporal dynamics and plant species diversity. We examined the distribution of plants with different clonal growth forms in the Northeast China Transect (NECT) and correlated plant species diversity with the importance of clonal plants, and the importance of phalanx and guerilla clonal plants. Phalanx clonal plants were more abundant in western communities where the altitude was higher and both the soil nitrogen contents and precipitation were relatively low. Whereas guerilla clonal plants were more abundant in the middle of the NECT where the precipitation, mean annual temperature and photosynthetically active radiation were relatively high. In the relatively productive temperate typical steppe, plant species diversity was negatively correlated with the importance of phalanx clonal plants and positively correlated with the importance of guerilla clonal plants. In relatively unproductive temperate desert steppe, plant species diversity was positively correlated with the importance of both phalanx and guerilla clonal plants.     

Ontogenetic Behavior and Migration of Dabry's Sturgeon, Acipenser Dabryanus, from the Yangtze River, With Notes on Body Color and Development Rate

We conducted laboratory experiments with Dabry's sturgeon, Acipenser dabryanus, from the upper Yangtze River to develop a conceptual model of early behavior. We daily observed fish from day-0 (embryo, first life interval after hatching) to day-30 feeding larva for preference of bright habitat and cover, swimming distance above the bottom, up- and down-stream movement, and diel activity. Hatchling to day-12 embryos and days 13–24 larvae were similar for ontogenetic behavior, i.e., neither initiated a dispersal migration, both swam within 15cm of the bottom, both preferred bright habitat, and neither strongly preferred cover or open habitat. Embryos and larvae were weakly active day and night. Days 72–76 juveniles had a weak nocturnal downstream migration, indicating wild juveniles disperse from a spawning site. In other sturgeon species yet studied representing three genera on three continents, Dabry's sturgeon is the first that does not disperse as an embryo or larva. Development of Dabry's sturgeon is slow, requiring more cumulative temperature degree days per millimeter of larvae TL than is required for other sturgeons to develop into larvae. Thus, a dispersal migration that diverts energy from development may not be adaptive. The available information suggests the initial dispersal of early life intervals is likely done by females, which spawn in a dispersed spawning style, not the usual aggregated spawning style. Juvenile migrants had a black body and tail with a light line along the lateral scutes. The color of juvenile migrants shows that a dark body and tail is characteristic of Acipenser that migrate downstream as larvae or juveniles.     

Game theory provides no explanation for seed size variation in grasslands

Game theoretical models have been suggested to explain the maintenance of a remarkable variation in seed size across species in most types of vegetation. According to these models, which are based on the existence of a trade-off between seed size and seed number; smaller-seeded species can invade any species mixture due to their numerical advantage, and larger-seeded species can invade any species mixture due to their competitive superiority over smaller-seeded species during recruitment. However, till now, there is very limited evidence for seed size effects on recruitment interactions among different species. An experiment was designed using 16 species in Swedish grasslands, varying 384-fold in seed size. Species were sown pairwise and alone in disturbed versus undisturbed small plots in grazed versus ungrazed grassland, and the resulting recruitment was recorded. Seedling densities in the experimental plots were within the natural range. Both disturbance and grazing had a positive effect on recruitment. Seed size did not affect recruitment except from a tendency that increasing seed size made recruitment less dependent on disturbance. Recruitment of smaller-seeded species was not affected by the presence of larger-seeded species. Larger-seeded species did not generally win in direct contest. These results suggest that game theoretical models do not explain maintenance of seed size variation across species in these grasslands. Alternative explanations for seed size variation are that either small-scale heterogeneity provide conditions favouring a range of different seed sizes or other attributes than seed size effectively determine recruitment.