Gene flow among small populations of a self-incompatible plant: an interaction between demography and genetics

We assessed the effects of population size and genetic relatedness on rates of pollen gene flow into experimental populations of the insect-pollinated, self-incompatible plant Raphanus sativus. We created synthetic populations of sizes 2, 5, 10, and 20 with three genetic structures (full siblings, half siblings, and unrelated plants). Following pollination in a natural setting, we conducted a simple paternity exclusion analysis using the allozyme genotypes of progeny to measure apparent gene flow and Monte Carlo simulations to estimate total gene flow. Estimates of apparent pollen gene flow rates ranged from 0 to 100% and were similar in rank to estimates of total gene flow. There were significant effects of population size and relatedness on the rate of apparent gene flow, and there were significant population size by relatedness interactions. Populations of size 2 had higher gene flow rates than larger populations, gene flow being negatively associated with the level of cross-compatibility (as measured by hand pollinations). Gene flow into populations of size 2 was also negatively associated with the distance to the nearest population of size 10 or 20. These results suggest that interactions among demography (population size), genetics (cross-compatibility), and ecology (pollinator behavior) are important influences on pollen gene flow rates into small plant populations.     

Nonlocal transplantation and outbreeding depression in the subshrub Lotus scoparius (Fabaceae)

The genetic background of transplants used to create or augment wild populations may affect the long-term success of restored populations. If seed sources are from differently adapted populations, then the relative performance of progeny from crosses among populations may decrease with an increase in genetic differences of parents and in the differences of parental environments to the transplant location. We evaluated the potential for such outbreeding depression by hybridizing individuals from six different populations of Lotus scoparius var. scoparius and L. s. var. brevialatus. We used allozyme data to calculate genetic distances between source populations, and compiled climatic data and measured soil traits to estimate environmental distances between source populations. We found significant outbreeding depression following controlled crosses. In the greenhouse, the success of crosses (seeds/flower × seedlings/seed) decreased with increasing genetic distance between populations revealing genetically based outbreeding depression unrelated to local adaptation. After outplanting to one native site (in situ common garden), field cumulative fitness of progeny (survival × fruit production) decreased significantly with mean environmental distance of the parental populations to the transplant site, but not with genetic distance between the crossed populations. This result is consistent with a disruption of local adaptation. At the second, ecologically contrasting common garden, where low survival reduced statistical power, field cumulative fitness (survival × progeny height) did not decrease significantly with either environmental distance or genetic distance. Overall, intervariety crosses were 40 and 50% as fit (seeds/flower × seedlings/seed × survival × fruits at the first garden or × height at the second) as intravariety crosses. These results suggest that the cumulative outbreeding depression was caused by a combination of genetically based ecological differences among populations and other genomic coadaptation. We conclude that mixing genetically differentiated seed sources of Lotus scoparius may significantly lower the fitness of augmented or restored populations. Genetic and environmental similarities of source populations relative to the transplant site should be considered when choosing source materials, a practice recommended by recent seed transfer policies. Geographic separation was not a good surrogate for either of these measures.     

THE EFFECT OF COTYLEDON EXCISION ON THE GROWTH OF PEA SEEDLINGS

Pea plants grown under controlled conditions for 28 days were sampled nine times during this growth period. On each sampling day 80 plants were harvested. Cotyledons were excised from 40 plants and the root-shoot axis allowed to continue growth. The other 40 plants were divided into roots, shoots, and cotyledons; the dry weights of alcohol-water extracts and residues from these organs were determined. Shoot lengths, numbers of nodes, buds, flowers, and fruits of 28-day-old plants, whose cotyledons were excised at different times, were determined. These data indicate that cotyledon excision after the 13th day has no apparent effect on seedling growth, whereas excision before the 13th day reduces growth. Glucose-U-C¹⁴ supplied to attached cotyledons of 6- and 16-day-old seedlings resulted in no major difference in the distribution pattern of recovered label throughout the plant even though 16-day-old cotyledons were depleted of their normal endogenous food reserve.     

Relative effects of nocturnal vs diurnal pollinators and distance on gene flow in small Silene alba populations

Silene alba exists in natural metapopulations throughout its range and is visited by a suite of both diurnal and nocturnal pollinators. Pollen-mediated gene flow may help reduce genetic isolation of subpopulations. Here, we compared the relative effects of nocturnal vs diurnal pollinators on pollen-mediated gene flow in subpopulations separated by two distance treatments. We established populations consisting of genetically marked individuals in an old field in Tennessee (USA). Electrophoretic examination of seedlings produced by plants exposed to nocturnal, diurnal and control pollinator treatments and separated by either 20 or 80 m allowed us to directly measure pollen-mediated gene flow. Gene flow was more common between populations separated by only 20 m. Nocturnal pollinators were responsible for most gene flow between populations, regardless of distance. Diurnal pollinators played only a small role in pollen-mediated gene flow. The results suggest that nocturnal pollinators are better than diurnal pollinators at moving pollen between small S. alba subpopulations. However, their effectiveness declines as the distance between subpopulations increases, making them relatively ineffective at moving genes between isolated subpopulations.     

Long distance pollen-mediated flow of herbicide resistance genes in <Emphasis Type="Italic">Lolium rigidum</Emphasis>

Gene flow promotes genetic exchange among plant populations mediating evolutionary dynamics; yet, the importance of gene flow at distance via pollen movement is poorly understood. A field experiment at the landscape level was conducted with Lolium rigidum herbicide-susceptible individuals (population VLR1) placed into an otherwise Lolium-free bushland environment at increasing distances from adjacent large commercial crop fields infested with herbicide-resistant L. rigidum. Herbicide resistance was used as a marker to quantify the distance and the rate of pollen-mediated gene flow. About 21,245 seeds were produced on the isolated, susceptible mother plants of which 3,303 seedlings were tested for herbicide resistance and 664 seedlings were found to be resistant. Pollen-mediated gene flow occurred at 3,000 m (maximum tested distance). Both Mendelian and molecular analyses (sequencing and CAPS markers) confirmed the introgression of herbicide resistance genes. This is the first documented case of long-distance gene flow in L. rigidum. The results are important for future modeling simulations of herbicide resistance evolution and subsequent mobility. The adoption of integrated agronomic strategies, the control of potential receptor plants on fields’ margins and conservative use of herbicides can be realistic options to minimize herbicide resistance spread. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

GENETICS AND DEVELOPMENT OF AN UNSTABLE CHLOROPHYLL DEFICIENCY IN LYCOPERSICON ESCULENTUM

Rick , Charles M. (U. California, Davis.), Anson E. Thompson , and Oscar Brauer . Genetics and development of an unstable chlorophyll deficiency in Lycopersicon esculentum. Amer. Jour. Bot. 46(1) : 1-11. Illus. 1959.—A single gene gh determines a condition of variable chlorophyll deficiency in the tomato. Two independent mutations have been recorded at this locus, and both mutants exhibit the same phenotype. Linkage tests ascertained that the locus of gh lies between hl and j in group V. Graft and inoculation experiments prove that the gh phenotype is not caused by a virus. The mutant seedlings have cotyledons that can be identified by their partial chlorophyll deficiency. First true leaves may rarely have normal morphology and pigmentation; in most seedlings they show a mosaic of normal and chlorotic tissue. With subsequent growth, the plant rapidly reaches a condition of complete chlorophyll deficiency. Two constant levels of deficiency have been observed: a common white phase with very little or no chlorophyll and an infrequent yellow phase with about 5% of the normal chlorophyll content present in irregularly distributed chloroplasts. In both phases leaf lamina structure is drastically modified; petioles, stems, flowers, and fruits are much less affected. Sporadic islands of normal green coloration appear in the otherwise stable white or yellow phase. The degree of chlorophyll development is subject to great environmental modification but no evidence was obtained for genetic control in addition to the determination by gh. Depending upon environment, most gh seedlings perish before fruiting, but those with sufficient chlorophyll flower and set fruits with seeds after self-or cross-pollination. Most gh fruits remain white until mature, when the normal yellow epidermal pigment appears, the interior remaining white; occasional streaks or patches of chlorophyll that develop on immature fruits turn reddish at maturity. Seeds from such sectors, like all other seeds harvested from selfed gh plants, give rise only to gh progeny, thereby rendering improbable a mutational basis for the sporadic reversions to green. Applications of gh to further genetic and physiological studies are discussed.     

Using genetic markers to directly estimate gene flow and reproductive success parameters in plants on the basis of naturally regenerated seedlings

Estimating seed and pollen gene flow in plants on the basis of samples of naturally regenerated seedlings can provide much needed information about "realized gene flow," but seems to be one of the greatest challenges in plant population biology. Traditional parentage methods, because of their inability to discriminate between male and female parentage of seedlings, unless supported by uniparentally inherited markers, are not capable of precisely describing seed and pollen aspects of gene flow realized in seedlings. Here, we describe a maximum-likelihood method for modeling female and male parentage in a local plant population on the basis of genotypic data from naturally established seedlings and when the location and genotypes of all potential parents within the population are known. The method models female and male reproductive success of individuals as a function of factors likely to influence reproductive success (e.g., distance of seed dispersal, distance between mates, and relative fecundity--i.e., female and male selection gradients). The method is designed to account for levels of seed and pollen gene flow into the local population from unsampled adults; therefore, it is well suited to isolated, but also wide-spread natural populations, where extensive seed and pollen dispersal complicates traditional parentage analyses. Computer simulations were performed to evaluate the utility and robustness of the model and estimation procedure and to assess how the exclusion power of genetic markers (isozymes or microsatellites) affects the accuracy of the parameter estimation. In addition, the method was applied to genotypic data collected in Scots pine (isozymes) and oak (microsatellites) populations to obtain preliminary estimates of long-distance seed and pollen gene flow and the patterns of local seed and pollen dispersal in these species.     

POLLEN-MEDIATED GENE FLOW IN CUCURBITA FOETIDISSIMA (CUCURBITACEAE)

Pollen-mediated gene flow along a linear series of patches of the functionally gynodioecious, bee-pollinated Cucurbita foetidissima was assessed using electrophoretic analysis of seed allozymes. Gene flow was documented at distances up to 0.7 km. For the 13 patches examined, interpatch pollen must have sired from 0% to 48.3% (X̄ = 8.5%) of the seeds of monoecious plants (hermaphrodites). Rates of interpatch siring of seeds of pistillate plants (females) averaged 20.4% (range 8.6%-40%) for the three patches examined. Heterogeneity among fruits in seeds sired by interpatch pollen indicates that the arrival of interpatch pollen is clumped with respect to stigmas. Within patches, plants of the same sex type usually shared identical five-locus genotypes, suggesting that clonal propagation predominates. Since approximately 90% of seeds are sired by intrapatch pollen, seeds of monoecious plants appear to result primarily from geitonogamous (self-) fertilization. This may help explain the existence of female plants in natural populations, since self-fertilization has been shown to severely reduce the survival of seedlings in this species.     

Variation in horizontal and vertical transmission of the endophyte Epichloë elymi infecting the grass Elymus hystrix

Systemic fungal endophytes (Clavicipitaceae) of grasses reproduce sexually when the fungus forms stromata and contagious ascospores, or asexually by vertical transmission of hyphae into seeds and seedlings. Vertical transmission is predicted to favor reduced virulence compared with horizontal transmission in systems with both types of transmission. Here, variation in vertical and horizontal transmission and its potential heritability in a host grass-endophyte interaction, Elymus hystrix infected with Epichlo? elymi, were examined in natural populations and two common garden experiments using field-collected host tillers and seed progeny of maternal plants with known infection phenotypes. Transmission mode exhibited year-to-year variation in field and common garden environments. In the common garden there were consistent differences among maternal plant families in stroma production and significant correlations between stroma production in the common garden and in natural populations. Transmission mode differed among maternal families, spanning a continuum from pure vertical transmission to a high proportion of stroma production and horizontal transmission potential. Vertical transmission to seeds occurred at high rates in all maternal families regardless of their stroma production. Observed patterns of variation indicate that endophyte transmission mode and correlated changes in virulence can respond to selection by biotic and abiotic factors.     

DISPERSION OF SEEDLINGS OF THE PRAIRIE COMPASS PLANT,SILPHIUM LACINIATUM (ASTERACEAE)

Although the dispersal of seeds around individual plants (the seed shadow) has frequently been characterized, the dispersion of seedlings around plants (the seedling shadow) has rarely been examined. We mapped 101 and 149 seedlings of the prairie compass plant (Silphium laciniatum) that appeared in our study area in 1987 and 1990 following mass flowering in 1986 and 1989. We also mapped the locations of flowering stems which appeared in 1986 and 1989 and recorded the number of flowerheads at each stem location. The frequency distributions of distance between a seedling and the nearest flowering stem were identical in the 2 years, with a median distance of 1.0 m. The large size and lack of wind-dispersal structures of compass plant seeds (achenes) are responsible for their limited dispersal. From estimates of the total seed production in the study area in 1986 and 1989, we calculated that about 1% of seeds became seedlings in each year. Flowering stem locations with a higher number of flowerheads had a significantly higher density of seedlings around them. This indicates that recruitment to compass plant populations is not a “lottery”; individual plants that produce more seeds produce, on average, more seedlings.     

GENETIC ISOLATION BY ENVIRONMENT OR DISTANCE: WHICH PATTERN OF GENE FLOW IS MOST COMMON?

Gene flow among populations can enhance local adaptation if it introduces new genetic variants available for selection, but strong gene flow can also stall adaptation by swamping locally beneficial genes. These outcomes can depend on population size, genetic variation, and the environmental context. Gene flow patterns may align with geographic distance (IBD—isolation by distance), whereby immigration rates are inversely proportional to the distance between populations. Alternatively gene flow may follow patterns of isolation by environment (IBE), whereby gene flow rates are higher among similar environments. Finally, gene flow may be highest among dissimilar environments (counter‐gradient gene flow), the classic “gene‐swamping” scenario. Here we survey relevant studies to determine the prevalence of each pattern across environmental gradients. Of 70 studies, we found evidence of IBD in 20.0%, IBE in 37.1%, and both patterns in 37.1%. In addition, 10.0% of studies exhibited counter‐gradient gene flow. In total, 74.3% showed significant IBE patterns. This predominant IBE pattern of gene flow may have arisen directly through natural selection or reflect other adaptive and nonadaptive processes leading to nonrandom gene flow. It also precludes gene swamping as a widespread phenomenon. Implications for evolutionary processes and management under rapidly changing environments (e.g., climate change) are discussed.     

植物的基因流及其在濒危植物保护中的作用

基因流是影响植物种群遗传结构的重要因子,在濒危植物的保护中也有重要的意义。本文介绍了基因流的测定方法,分析了基因流的格局及其与种群遗传分化的关系,并介绍了基因流在濒危植物保护中的应用前景及途径     

The Wilhelmine W. Key 2002 Invitational Lecture. Phylogeography,haplotype trees,and invasive plant species

The distribution of genetic variants in plant populations is strongly affected both by current patterns of microevolutionary forces, such as gene flow and selection, and by the phylogenetic history of populations and species. Understanding the interplay of shared history and current evolutionary events is particularly confounding in plants due to the reticulating nature of gene exchange between diverging lineages. Certain gene sequences provide historically ordered neutral molecular variation that can be converted to gene genealogies which trace the evolutionary relationships among haplotypes (alleles). Gene genealogies can be used to understand the evolution of specific DNA sequences and relate sequence variation to plant phenotype. For example, in a study of the RPS2 gene in Arabidopsis thaliana, resistant phenotypes clustered in one portion of the gene tree. The field of phylogeography examines the distribution of allele genealogies in an explicit geographical context and, when coupled with a nested clade analysis, can provide insight into historical processes such as range expansion, gene flow, and genetic drift. A phylogeographical approach offers insight into practical issues as well. Here we show how haplotype trees can address the origins of invasive plants, one of the greatest global threats to biodiversity. A study of the geographical diversity of haplotypes in invasive Phragmites populations in the United States indicates that invasiveness is due to the colonization and spread of distinct genotypes from Europe ( Saltonstall 2002). Likewise, a phylogeographical analysis of Tamarix populations indicates that hybridization events between formerly isolated species of Eurasia have produced the most common genotype of the second-worst invasive plant species in the United States.     

Obviation of wheat resistance to the Hessian fly through systemic induced susceptibility

Unlike most documented plant-insect interactions, Hessian fly-resistance [Mayetiola destructor (Say)] in wheat (Triticum aestivum L.) is initiated by a gene-for-gene recognition event in which plants carrying a specific R gene recognize salivary effectors encoded by a corresponding larval avirulence gene. However, dual infestation resulting from oviposition by virulent insects from 5 d before to 3 d after oviposition by avirulent insects on the same host plant, lead to systemic induced susceptibility, obviation of resistance, and ultimately the survival of both virulent and genetically avirulent progeny to adulthood. Simultaneous oviposition allowed greater survival of avirulent progeny than ovipositions separated by larger intervals. Because of the induction of plant resistance, hatch of avirulent larvae before virulent was more detrimental to rate of development than hatch of virulent before avirulent larvae. Obviation of resistance was not localized to the leaf being attacked by the virulent larvae, but also functioned across spatial distance into younger leaves. This research suggests that virulent Hessian fly larvae directly suppress the defense response of wheat, thus providing a refuge for avirulent genotypes, preserving diversity in field populations and increasing durability of deployed resistance genes.     

Competition alters life history and increases the relative fecundity of crop-wild radish hybrids (Raphanus spp.)

The evolutionary impact of crop-to-wild gene flow depends on the fitness of hybrids under natural, competitive conditions. Here, we measured the performance of third-generation (F3) radish hybrids (Raphanus raphanistrum x Raphanus sativus) and weedy R. raphanistrum to understand how competitive interactions affect life history and relative fecundity. Three wild and three F1 crop-wild hybrid radish populations were established in semi-natural, agricultural conditions in Michigan, USA. The effects of competition on life-history traits and fecundity of F3 progeny were measured 2 yr later in a common garden experiment. Third-generation hybrid plants generally produced fewer seeds per fruit and set fewer fruits per flower than wild plants, resulting in lower lifetime fecundity. With increasing competition, age at reproduction was delayed, the relative number of seeds per fruit was reduced in wild plants and differences between hybrid and wild fecundity diminished. Competition may enhance the fecundity of advanced-generation hybrids relative to wild plants by reducing differences in life history, potentially promoting the introgression of crop alleles into weed populations.     

An assessment of gene transfer by pollen from field-grown transgenic potatoes to non-transgenic potatoes and related species

Information on the extent of transgene dispersal by pollen to adjacent potato plots and to related weed species is an important requisite for risk assessment; a procedure followed before novel transgenic plants are evaluated under field conditions. The purpose of the investigation was to determine the frequency of cross-pollination between potato (Solanum tuberosum) plants at different distances, using a kanamycin resistnace transgene (nptII) as a selectable marker. All potato plants were from the variety Désirée. Non-transgenic potato plants, used as potential recipients of transgene-containing pollen, were planted in 12 sub-plots, at distances of 0–20 m from the nearest transgenic potato plants. Seeds harvested from the non-transgenic plants were screened for resistance to kanamycin, and molecular methods were used to confirm that resistant progeny contained thenptII gene. Where transgenic and non-transgenic potato plants were in alternate rows (leaves touching), 24% of seedlings from the non-transgenic parent plants were kanamycin-resistant. Comparable seedlings from plants at up to 3 m distance had a resistance frequency of 2%, at 10 m the frequency was 0.017% and at 20 m no resistant progeny were observed. Plants of the weed speciesS. dulcamara andS. nigrum were also planted close to the transgenic potatoes to test for evidence of hybridization, and no kanamycin-resistant seedlings were observed among progeny fromS. dulcamara andS. nigrum. This investigation provided evidence that the extent of gene dispersal from transgenic potatoes to non-transgenic potatoes falls markedly with increasing distance, and is negligible at 10 m. There was, also, no evidence of transgene movement from potato toS. dulcamara andS. nigrum under field conditions. These data will be valuable in defining genetic isolation procedures for the early field evaluation and the use of novel transgenic potato genotypes.     

Ecological genetics of vernalization response in Bromus tectorum L. (Poaceae)

BACKGROUND AND AIMS: Bromus tectorum (cheatgrass or downy brome) is an exotic annual grass that is dominant over large areas of former shrubland in western North America. To flower in time for seed production in early summer, B. tectorum plants generally require vernalization at winter temperatures, either as imbibed seeds or as established seedlings. METHODS: Variation in response to increasing periods of vernalization as seeds or seedlings for progeny of ten full-sib families from each of four B. tectorum populations from contrasting habitats was studied. KEY RESULTS: As vernalization was increased from 0 to 10 weeks, the proportion of plants flowering within 20 weeks increased, weeks to initiation of flowering decreased, and seed yield per plant increased, regardless of whether plants were vernalized as seeds or seedlings. Most of the variation was accounted for by differences among populations. Plants of the warm desert population flowered promptly even without vernalization, while those of the cold desert, foothill and montane populations showed incremental changes in response variables as a function of vernalization period. Populations differed in among-family variance, with the warm desert population generally showing the least variance and the cold desert population the most. Variation among populations and among families within populations decreased as vernalization period increased, whereas the non-genetic component of variance showed no such pattern. CONCLUSIONS: Variation in vernalization response was found to be adaptively significant and apparently represents the result of contrasting selection regimes on a range of founder genotypes.     

The Detection,Rate and Manifestation of Residual Sexuality in Apomictic Populations of Pilosella (Asteraceae,Lactuceae)

The effect of maternal, facultatively apomictic plants on population diversity was evaluated in seven hybridizing polyploid Pilosella populations, where apomictic (P. bauhini or P. aurantiaca) and sexual (P. officinarum) biotypes coexist. The ploidy level, reproductive system, morphology, clonal structure and chloroplast DNA haplotypes were used to characterize these plants and their hybrids. The reproductive origins of the progeny were assessed through either a flow cytometric seed screen and/or a comparison between the ploidy level of progeny embryos/seedlings and the maternal ploidy level. The cultivated progeny derived from residual sexuality in maternal apomicts were also identified based on their morphology and reproductive behaviour. The progeny different from their maternal parents (0.6?92.3 % of progeny embryos and 0?100 % of progeny seedlings) originated either sexually or via haploid parthenogenesis. Comparing the facultatively apomictic and sexual mothers, the progeny arrays generated in the field showed that apomictic mothers produce progeny that is more variable in ploidy level. This effect was demonstrated at both the embryonic and seedling stages of progeny development. Residual sexuality in apomicts was also effective in experimental crosses, generating progeny similar to spontaneous hybrids in the field. The 2n + n hybrids produced from an apomictic and a sexual parent displayed similar reproductive behaviour, producing polyhaploid, sexual and apomictic progeny in variable ratios. Repeated hybridizations between parental species and/or multi-step crosses can result in hybrid swarms rich in cytotypes and morphotypes. The variation recorded in these populations suggests prevailing introgressive hybridization towards the sexual species P. officinarum.     

THE EFFECTS OF THE RELATIVE GEOGRAPHIC SCALES OF GENE FLOW AND SELECTION ON MORPH FREQUENCIES IN THE WALKING-STICK TIMEMA CRISTINAE

Gene frequencies in large populations are determined by a balance between selection and gene flow between neighborhoods of different selection regimes. This balance is affected by the area of the patches of a given selection regime relative to the gene-flow distance. If patches are small relative to gene-flow distance, similarity in the total area occupied by different patch types is a crucial condition for the stability of polymorphisms. However, if patches are larger than the gene-flow distance, then the relative area of different patch types is less important because of reduced gene flow resulting from isolation by distance. Two morphs (striped and unstriped) of the walking-stick Timema cristinae were each strongly associated with patches of distinct species of food plants on which they are most cryptic. The frequency of a morph was high on the plant on which it is most cryptic when either: (1) the area occupied by the food plant (patch) was very large; (2) the patch was completely isolated from other patches; or (3) the patch was larger than adjacent patches. Results (1) and (2) are consistent with isolation-by-distance models, and result (3) is consistent with Levene's multiple-niche polymorphism model.     

EFFECTS OF PLANT MORPHOLOGY AND EMERGENCE TIME ON SIZE HIERARCHY FORMATION IN EXPERIMENTAL POPULATIONS OF TWO VARIETIES OF CULTIVATED PEAS (PISUM SATIVUM)

The effects of plant form and emergence time on size hierarchy formation in populations of two morphologically and genetically distinct varieties of peas (leafless and leafed) were studied. There were no significant differences in germinability between the two varieties, although leafless peas imbibed more rapidly than the leafed ones did. Monocultures of leafed and leafless peas were established at two densities: plants grown alone in small pots and plants grown at 576 m -. Time emergence was noted, and plant shape, biomass and seed production were measured at two-week intervals for ten weeks. Seedlings emerged continually over an eight-day period, and two cohorts of seedlings were distinguished (seedlings emerging 6–7 days after planting, and seedlings emerging > 7 days after planting). Dominance and suppression were observed in the high-density populations, and early-emerging plants had less hierarchical biomass distributions than did late-emerging ones. Although leafless peas were larger and suffered less mortality than leafed ones did at identical densities, there were no differences in the degree of size inequality between the two genotypes (emergence cohorts pooled), or within emergence cohorts between genotypes. The degree of size inequality increased with time among dominant individuals and decreased with time among suppressed individuals. These results broadly support Weiner and Thomas's (1986) hypothesis that plant form may affect the extent but not the existence of competitive asymmetry in plant populations.