Comparative Effects of Pollen and Seed Migration on the Cytonuclear Structure of Plant Populations. I. Maternal Cytoplasmic Inheritance

We explicitly solve and analyze a series of deterministic continent-island models to delimit the effects of pollen and seed migration on cytonuclear frequencies and disequilibria in random-mating, mixed-mating and self-fertilized populations. Given the critical assumption of maternal cytoplasmic inheritance, five major findings are (i) nonzero cytonuclear disequilibria will be maintained in the island population if and only if at least some migration occurs each generation through seeds with nonrandom cytonuclear associations; (ii) immigrant seeds with no cytonuclear disequilibria can strongly affect the genetic structure of the island population by generating significant and long-lasting transient associations; (iii) with all else being equal, substantially greater admixture disequilibria are generally found with higher rates of seed migration into, or higher levels of self-fertilization within, the island population (with the possible exception of the heterozygote disequilibrium); (iv) pollen migration can either enhance or reduce the cytonuclear disequilibria caused by seed migration, or that due to mixed-mating in the absence of seed migration, but the effect is usually small and appears primarily to make a noticeable difference in predominantly outcrossing populations; and (v) pollen migration alone cannot generate even transient disequilibria de novo in populations with completely random associations. This same basic behavior is exhibited as long as there is some random outcrossing in the island population. Self-fertilized populations represent a special case, however, in that they are necessarily closed to pollen migration, and nonzero disequilibria can be maintained even in the absence of seed migration. All of these general results hold whether the population is censused as adults or as seeds, but the ability to detect nonrandom cytonuclear associations can depend strongly on the life stage censused in populations with a significant level of random outcrossing. We suggest how these models might be used for the estimation of seed and pollen migration.     

种子与花粉的随机迁移对植物群体遗传结构分化的影响

将Ｗｒｉｇｈｔ的经典岛屿模型拓广到植物群体上,同时考虑了含有花粉和种子随机迁移的影响。并给出了３种不同遗传方式的基因（双亲遗传,父本和母本遗传）频率的期望均值和方差。理论结果证明花粉或种子的随机迁移可增加基因频率方差,其幅度取决于迁移率和迁移基因频率的方差。同绝对迁移率一样,花粉和种子的迁移率方差及迁移基因频率的方差对群体遗传结构的分化有着同样的重要。一个重要结论就是花粉或种子的随机迁移率和随机迁     

The Effects of Assortative Mating and Migration on Cytonuclear Associations in Hybrid Zones

We examine the influence of nonrandom mating and immigration on the evolutionary dynamics of cytonuclear associations in hybrid zones. Recursion equations for allelic and genotypic cytonuclear disequilibria were generated under models of (1) migration alone, assuming hybrid zone matings are random with respect to cytonuclear genotype; and (2) migration in conjunction with refined epistatic mating, in which females of the pure parental species preferentially mate with conspecific males. Major results are as follows: (a) even the slightest migration removes the dependency of the final outcome on initial conditions, producing a unique equilibrium in which both pure parental genotypes are maintained in the hybrid zone; (b) in contrast to nuclear genes, the dynamics of cytoplasmic allele frequencies appear robust to changes in the assumed mating system, yet are particularly sensitive to gene flow; (c) continued immigration can generate permanent cytonuclear disequilibria, whether mating is random or assortative; and (d) the order of population censusing (before versus after reproduction by immigrants) can have a dramatic effect on the magnitude but not the pattern of cytonuclear disequilibria. Using the maximum likelihood method, the parameter space of migration rates and assortative mating rates was examined for best fit to observed cytonuclear disequilibria data in a hybrid population of Hyla tree frogs. An epistatic mating model with a total immigration rate of about 32% per generation produces equilibrium gene frequencies and cytonuclear disequilibria consistent with the empirical observations.     

Comparative Biology of Cycad Pollen,Seed and Tissue - A Plant Conservation Perspective

Cycads are the most endangered of plant groups based on IUCN Red List assessments; all are in Appendix I or II of CITES, about 40% are within biodiversity ‘hotspots,’ and the call for action to improve their protection is long-standing. We contend that progress in this direction will not be made until there is better understanding of cycad pollen, seed and tissue biology, which at the moment is limited to relatively few (<10%) species. We review what is known about germplasm (seed and pollen) storage and germination, together with recent developments in the application of contemporary technologies to tissues, such as isotype labelling, biomolecular markers and tissue culture. Whilst progress is being made, we conclude that an acceleration of comparative studies is needed to facilitate the integration of in situ and ex situ conservation programmes to better safeguard endangered cycads.     

Pollen Organ Telangiopsis sp. of Late Devonian Seed Plant and Associated Vegetative Frond

Pollen organ Telangiopsis sp., associated with but not attached to vegetative fronds, has been collected from the Upper Devonian (Famennian) Wutong Formation, Dongzhi County, Anhui Province, China. Fertile axes with terminal pollen organs are dichotomous for 2–4 times and may be proximally attached by fragmentary pinnules. Pollen organs are synangiate and borne on the top of a short stalk. Synangia are radial in symmetry and each consists of 4–8 elongate microsporangia fused at base. Microsporangia have a longitudinal dehiscence line and show a tapered apex. The associated stem is spiny and bears a vegetative frond which bifurcates once at the basalmost part. Frond rachises possess one order of pinna arranged alternately. Pinnules are borne alternately, planate, highly dissected, and equally dichotomous for 2–3 times. Comparisons among Late Devonian seed plants recognize several branching patterns in the fertile fronds/axes bearing terminal pollen organs. Telangiopsis sp. reinforces that the Late Devonian pollen organs are synangiate usually with basally fused microsporangia. It is suggested that the evolutionary divergence of radial and bilateral symmetries of pollen organs may have occurred in the Famennian, when the earliest seed plants evolved planate and sometimes laminate pinnules.     

Natural Selection with Nuclear and Cytoplasmic Transmission. II. Tests with Drosophila from Diverse Populations

Observations of intraspecific variation in organelle DNA have prompted a renewed interest in the evolutionary consequences of cytoplasmically transmitted factors. Attempts to quantify the significance of cytoplasmic effects are frequently limited by the difficulty in partitioning the cause of reciprocal cross differences among a series of possibilities. In the experiment reported here the nuclear genomes of a set of six lines of Drosophila melanogaster from diverse geographic locations were replaced in a series of cytoplasms. The segregation of the SM5 balancer chromosome was scored in a factorial design, and the data allowed a partitioning of variance such that cytoplasmic effects were distinguished from maternal effects and meiotic drive. An attempt was made to avoid the confounding problem of hybrid dysgenesis by performing the entire experiment (including chromosomal extractions) in a P cytotype. Results indicated a significant contribution of cytoplasm to the variance in SM5 segregation. Error variance showed an increasing trend as the experiment proceeded, and additional tests indicated that this was due to an accumulation of chromosomal mutations. These findings are interpreted in light of the population genetic theory that addresses the maintenance of cytoplasmic polymorphism.     

Density Effects on the Size Structure of Annual Plant Populations: An Indication of Neighbourhood Competition

Size variability among plants has been observed to increasewith higher stand density, leading to the speculation that resourcedistribution among competing plants is primarily asymmetricrather than symmetric. The relationships between size variability,stand density, and type of resource distribution among competingplants were investigated using a spatially explicit, individual-plantmodel of annual plant population dynamics. When plants variedin neighbourhood competition, size variability increased withhigher stand densities whether shared resources were symmetricallyor asymmetrically distributed among competing plants. Size variabilitydid not increase with higher stand densities when neighbourhoodcompetition was constant for all plants. These simulations indicatethat increased size variability among competing plants doesnot distinguish between symmetric and asymmetric resource distribution,but rather is direct evidence for neighbourhood competition. Size hierarchy, neighbourhood competition, density effects, asymmetric competition, symmetric competition     

High Rates of Gene Flow by Pollen and Seed in Oak Populations across Europe

Gene flow is a key factor in the evolution of species, influencing effective population size, hybridisation and local adaptation. We analysed local gene flow in eight stands of white oak (mostly Quercus petraea and Q. robur, but also Q. pubescens and Q. faginea) distributed across Europe.Adult trees within a given area in each stand were exhaustively sampled (range [239, 754], mean 423), mapped, and acorns were collected ([17,147], 51) from several mother trees ([3], [47], 23). Seedlings ([65,387], 178) were harvested and geo-referenced in six of the eight stands. Genetic information was obtained from screening distinct molecular markers spread across the genome, genotyping each tree, acorn or seedling. All samples were thus genotyped at 5–8 nuclear microsatellite loci. Fathers/parents were assigned to acorns and seedlings using likelihood methods. Mating success of male and female parents, pollen and seed dispersal curves, and also hybridisation rates were estimated in each stand and compared on a continental scale.On average, the percentage of the wind-borne pollen from outside the stand was 60%, with large variation among stands (21–88%). Mean seed immigration into the stand was 40%, a high value for oaks that are generally considered to have limited seed dispersal. However, this estimate varied greatly among stands (20–66%). Gene flow was mostly intraspecific, with large variation, as some trees and stands showed particularly high rates of hybridisation.Our results show that mating success was unevenly distributed among trees. The high levels of gene flow suggest that geographically remote oak stands are unlikely to be genetically isolated, questioning the static definition of gene reserves and seed stands.     

Preliminary Studies on Cytoplasmic Inheritance and Variation of Azolla

By investigating the variance of Azolla leaf colour of F1 generation obtained from negative and positive crossing of Azolla between two species (Azolla filiculoides × A. microphylla, A. filiculoides × A. mescicana) and two subgenus (A. filiculoides × A. imbricata), it was revealed that the albinism of the hybrid F1 generation was variation resulting from maternal cytoplasmic inheritance, when A. filiculoide was used as female parent. Electron microscopic observation demonstrated abnormal development of plastid in the albino sporeling. The cell of light green seedling contained both normal and abnormal plastid. Both were probably related to variation in the plastid genotype. Significant difference occurred in the degree and freguency of albinism from various crossing forms, and such change had its vegularity in accord with the variation of the nuclear genotype. The results speculated that albinism were also closely related to the nuclear genotype.     

Cytological Studies of the Cytoplasmic Inheritance in Lilium regale and L. davidii

The distribution and characteristics of plastids and mitochondria in the generative and sperm cells of Lilium regale Wils. and L. davidii Duch. were described. In L. regale there were few plastids and abundant mitochondria in the newly formed generative cell. When the generative cell became free in the vegetative cytoplasm, the plastids degenerated completely within the generative cell. It was further proved by DAPI fluorescent technique that there was no organell DNA in the generative cell within the mature pollen grain or the pollen tube. However, distribution of the plastids was strictly polarizable during the division of the micmspore in L. davidii, resulting the lack of plastids in the newly formed generative cell. Data of RFLP analysis comparable between L. davidii, L. longifiorum and their interspecific hybrid have also proved the plastid inheritance in L. davidii to be of uniparental maternal transmission. Although the mitoehondria were observed both in the generative and sperm cells of L. regale and L. davidii but their DNA was decomposed in the male gametophyte stage. Therefore the mitochondda in the sperm cell could not be transmitted into the offspring. The results provided the detail, cytological evidence that organelles in the microgametophyte are incapable of genetic transmission in the two species of Lilium.     

Effects of Differential Selection in the Sexes on Cytonuclear Polymorphism and Disequilibria

We develop a series of models that examine the effects of differential selection between the sexes on cytonuclear polymorphism and disequilibria. A detailed analysis is provided for populations under constant fertility or viability selection censused at life stages without frequency differences in the sexes. We show analytically that cytonuclear disequilibria can be generated de novo if the cytoplasmic and nuclear loci each affect female fitness and there is a nonmultiplicative fitness interaction between them. While computer simulations demonstrate that the majority of disequilibria produced by random selection are transient and small in magnitude, measurable permanent disequilibria can result from selective differences both within and between the two sexes. We derive analytic conditions for a protected cytonuclear polymorphism and use numerical simulations to quantitate the likelihood of obtaining permanent nuclear, cytoplasmic, and cytonuclear variation under various patterns of selection. The numerical analysis identifies special selection regimes more likely to generate disequilibria and maintain cytonuclear polymorphism and reveals a direct correlation to the strength of selection. As a byproduct, our models also provide the first decomposition of the different parental contributions to cytonuclear dynamics and the analytic conditions under which selection can cause cytoplasmic frequency changes or a cytonuclear hitchhiking effect.     

Density Effects on Plant Height Growth and Inequality in Sunflower Populations

Comparisons between competing and non-competing sunflower （Helianthus annuus L.） populations demonstrate pronounced effects of density on plant height growth, height-to-crown width ratio, and s popuiaUon＇s height inequality. In the present study, non-destructive measurements of height and the prolected crown area of sunflower plants were taken at seven times from emergence to fruit maturation in even-aged monospeclflc stands with initial densities of 1, 4, 16, and 64 plants/m^2. The mean height of populations Increased and then decreased with increasing population density; the height Inequalities of uncrowded populations decreased during stand growth, whereas the height inequaiiUes of crowded popuisUons decreased first and then increased during stand development. The interindlvidual relationships between the relative height growth rate and height within uncrowded populations became significantly negative during population growth, whereas these relationships were negative first and then became positive during the development of crowded populations. In the uncrowded populations, the static Interindlvldual relationship between height-to-crown width ratio and volume was positive, whereas for the crowded population these relationships became negative with increasing competition for light. The data suggest that the plastic responses of plant height and height-to-crown width ratio to light competition will become more Intense with increasing competition Intensity. The results of the present study argue strongly for the Importance of size-dependent Individual-level plastic responses due to size-asymmetric light competition In generating the variations in population height inequality.     

Effects of Physiological and Environmental Variations on Size-Structure Dynamics in Plant Populations

Sensitivity analysis was conducted, based on the canopy photosynthesisand continuity equation models which were developed in a previouspaper (Yokozawa and Hara, 1992), to investigate effects of variationin physiological parameters (maximal photosynthetic rate perunit leaf area, respiration rate per unit leaf area, maintenancerespiration rate per unit weight, growth respiration rate perunit weight, light extinction coefficient of the canopy, etc.)on the size-structure dynamics in plant populations. As thedegree of asymmetry in competition between individuals increased,effects of variation in physiological parameters diminished.Therefore, a population undergoing one-sided competition (mostasymmetric competition) is a stable system, little affectedby temporal and spatial variations in the environmental conditionswhich lead to variation in physiological parameters, whereasa population undergoing symmetric two-sided competition is sensitiveto these fluctuations. It was also shown by simulation thatthe degree of asymmetry in competition decreases (through effectson canopy photosynthesis) as nutrient level in the soil is reduced.It is suggested that symmetric two-sided competition is associatedwith non-transitivity of competition between species (i.e. frequentreversals of rank order of species), and hence with speciesdiversity. Several other ecological phenomena are discussedin relation to allometry (i.e. allocation-growth pattern) andthe degree of asymmetry in competition.Copyright 1994, 1999Academic Press Allometry, canopy photosynthesis, competition mode, continuity equation, parameter sensitivity, stability of stand structure     

Restoring Populations of the Endangered Plant Scorzonera humilis: Influence of Site Conditions,Seed Source,and Plant Stage

Changes in land‐use have resulted in the decline of many formerly common plants of nutrient‐poor grasslands in Europe. Recently, extensification schemes have been applied at sites in order to restore former habitat conditions. However, the establishment of rare and endangered plants is often severely limited by the lack of propagules both in the seed bank and in the surrounding landscape. For such species deliberate introductions may be necessary to overcome these limitations. In a 7‐year study, we assessed the importance of gaps created by sod cutting, of plant stage, and of plant origin for the restoration of populations of Scorzonera humilis, a threatened long‐lived plant of nutrient‐poor, wet grasslands. The effect of gaps on seedling emergence and survival varied strongly among the 12 sites. Gaps increased survival at nutrient‐rich, but reduced it at nutrient‐poor sites. Remarkably, young plants grown for only 5 weeks in the laboratory and transplanted into the same sites had much higher survival than seedlings from seeds sown and there were no differences in survival between nutrient‐rich and nutrient‐poor sites. The field performance of the plants from the various populations of origin varied depending on the site into which they were transplanted, indicating genotype by environment interactions and genetic differentiation among populations, but there was no home‐site advantage. While sowing only succeeded in producing adult plants in five sites, transplanting succeeded at 10 sites. Our results suggest that transplanting young plants could be a much more effective and faster way to establish new populations than sowing seeds.     

Effects of Density and Extinction Coefficient on Size Variability in Plant Populations

The effects of density and extinction coefficient on size variability,as measured by the coefficient of variation of plant weightin even-aged monocultures, were investigated theoretically usinga diffusion model of growth and size distribution and a canopyphotosynthesis model over the range of densities at which self-thinning(size-dependent mortality) does not occur. Size inequality (thecoefficient of variation of plant weight) increases with increasingdensity or leaf area index at each growth stage. Plants witherect leaves are prone to lower size inequality than plantswith horizontal leaves. These results agree well with existingobservations on even-aged plant monocultures and suggest thatcompetition between plants is mainly one-sided (competitionfor light). One sided competition affects size variability througha G(t, x) function (mean growth of plants of size x at timet per unit time). Two-sided competition (including competitionfor nutrients) affects size variability through a D(t, x) function(variance of growth of plants of size x at time t per unit time).In this case, size inequality decreases with increasing density.The importance of studying size variability is emphasized. Helianthus annus L., size variability, size inequality, coefficient of variation, competition, density effect, extinction coefficient, diffusion model, canopy photosynthesis model     

Comparative histochemical study of prosimian primate hindlimb muscles. II. Populations of fiber types

The populations of fiber types in hindlimb muscles of the tree shrew (Tupaia glis), lesser bushbaby (Galago senegalensis), and the slow loris (Nycticebus coucang) were described and an attempt was made to correlate populations of fiber types and locomotor patterns. Muscle fibers were assigned to one of the following groups: fast-twitch glycolytic (FG), fast-twitch oxidative-glycolytic (FOG), and slow-twitch oxidase (SO). Histochemical techniques for the demonstration of alkaline- and acid-stable ATPase, succinic dehydrogenase, and mitochondrial alpha-glycerophosphate dehydrogenase were used in the classification of muscle fibers. Results indicated that the FG fiber type is the predominant fiber type in muscles used for jumping, the FOG fiber type is predominant in muscles used for running, and the SO fiber type occurs in high percentages in postural muscles. The SO fiber was also the most common fiber in muscles of the slow loris-a species that exhibits a slow, deliberate, sustained locomotor pattern. Intramuscular regional variations in populations were seen in some larger muscles of the tree shrew, but not in the lesser bushbaby and slow loris. Our results did not support the contentions of others that analogous muscles in different species have similar populations of fiber types.     

高黎贡山南北段种子植物区系比较研究

为了解高黎贡山的种子植物资源,采用植物区系研究原理和方法,将高黎贡山南段与北段种子植物区系进行对比分析。结果表明,南段有192科878属2807种植物,种可分为15个分布类型9亚型14个变型;北段有172科778属2816种植物,种可分为15个分布类型9亚型20个变型。通过比较科属分化程度、数量构成及古热带成分比例等,南段区系具有较为古老和保守的特点,而北段植物区系具有较为年青和新生的特点。回归分析显示南北段区系地理成分相似度较高,而科属相似性却因分化在逐级降低。