Dioecious Plant Populations Research Articles

Male-biased effective sex ratio across populations of the threatened Zamia boliviana (Zamiaceae)

Populations of dioecious plants commonly exhibit dissimilarities to the equilibrium expectation of a 1:1 sex ratio. Differential expenditure for reproduction between genders is cited as the primary mechanism responsible for a male-biased sex ratio, with increased effects on long-living species, but these mechanisms are still poorly understood. We explore the sex ratio in the endemic gymnosperm Zamia boliviana (Zamiaceae) populations from the Brazilian savanna (the Cerrado). We aim to investigate what the Z. boliviana sex ratio is, and whether population density and ecological correlates lead to variation in the sex proportion among Cerrado habitats. The study was conducted on ten in situ populations of Z. boliviana at sexual maturity (tertiary sex ratio). We estimated the populations’ sex ratio and performed a redundancy analysis to assess the relationship between biotic traits, such as sex ratio, and associated environmental features. Soil texture classes were used to classify the cycad habitats and were expressed in a ternary phase diagram. The results show a significant male-biased sex ratio in seven of the ten populations surveyed. Environmental factors did not explain the redundancy in the reproductive characteristics. However, the cycad occurs in different habitats in their endemic zone. Our study provides new biological data for Z. boliviana, suggesting that the differential reproductive expenditure of sexes in reproduction is governing the mechanisms of sex ratio variation, compared to local environmental factors in this cycad. The pattern of effective sex ratio found here improves our understanding of mechanisms causing biased sex ratios in cycads and other dioecious species from tropical ecosystems.

Male-skewed sex ratio in Myrica esculenta: a dioecious tree species

The population of M. esculenta tends to deviate from the equilibrium of expected 1:1 primary sex ratio predicted by Fisherian theory. Populations of dioecious plants may often exhibit biased sex ratios, which generally favour males. A study on reproductive potential, i.e., pollen production and female flower production, and reproductive success in Myricaesculenta was assessed in three diameter (diameter at breast height: dbh at 1.37 m) classes, i.e., small (10–20 cm), medium (20–30 cm), and large (30–40 cm). Population structure and sex ratio was determined in transects in four major associations of Myricaesculenta, i.e., (i) with mixed Oak-Rhododendron forest (ii) with Chir Pine–Oak forest, (iii) with pure Oak forest, and (iv) with pure Deodar forest. The sex ratio of trees was male-biased. Male frequency was higher in lower diameter classes and maximum on resource-poor site. More female individuals were recorded in higher-diameter classes and resource rich location, indicating that the total reproductive cost in M. esculenta is greater in female trees than that of the male trees. The population of M.esculenta tends to deviate from the equilibrium of expected 1:1 primary sex ratio predicted by Fisherian theory. The male trees demonstrated greater survival, especially in resource-poor conditions, which was reflected strongly biased sex ratios within the lower size classes. The study would be helpful for formulating appropriate silvicultural and genetic management strategies for this species.

Nutrient deficiency promotes male-biased apparent sex ratios at the ramet level in the dioecious plant Myrica gale var. tomentosa in oligotrophic environments in bogs.

In populations of dioecious plants, the differences in the cost of reproduction between male and female plants can promote a male-biased sex ratio. In this study, we examine the macronutrient levels in tissues of the dioecious wetland shrub Myrica gale to identify the cost of reproduction for male and female plants and to examine the effect of nutrients on the apparent sex ratio at the ramet level. We examined plants across 12 populations of M. gale inhabiting bogs and fens in Japan. For each population, we used line transects to estimate the apparent sex ratio and measured the concentrations of nitrogen (N), phosphorus (P), and potassium (K) in the leaves sampled from male and female plants and in the fruits from female plants. For five of the populations, we calculated the flowering frequency, mortality, and the recruitment rate (as the rate of clonal propagation). We found that the proportion of females was positively affected, and the male bias of sex ratios reduced, by increases in P concentration in leaves sampled from female plants. Neither mortality nor recruitment was affected by sex or by the nutrient concentration (P, K). The flowering frequency was not affected by sex or by K concentration, but decreased with decreases in the P concentration measured in leaves. This study confirmed that reproduction in M. gale is P-limited. We found no distinct differences in the flowering frequency, mortality, or recruitment rate between the male and female plants.

Vegetative propagation as a successful reproductive strategy in woody dioecious species in tropical coastal vegetation, southeast Brazil

Dioecious plant populations are composed of two distinct sexual morphs representing the female and male counterparts. Dioecy in restinga areas has been strongly associated with fleshy fruits, woodiness, generalist entomophily, and resource-poor environments. The ability of vegetative reproduction is another ecological trait commonly associated with dioecy, but not shown for the woody dioecious species in restingas. This study aimed to investigate whether the distribution of sexual morphs in populations is due to occurrence of vegetative propagation as a reproductive strategy among dioecious woody species on sandy coastal plains. We also intended to evaluate possible effects of sex ratio on sexual reproduction (female reproductive success). The study was conducted in the sandy coastal plains (restinga) of Marica, Rio de Janeiro State. We studied six woody dioecious species. During the reproductive period, all individuals of each species and your nearest neighbor were marked using GPS, along an area with approximately 5.1 ha. The sex of the individuals was recorded. This study provides evidence of the occurrence of vegetative propagation in the woody dioecious species studied and indicates that the ability to reproduce in this manner varies between sexual morphs, generating male-biased populations in most cases. Our data suggest that asexual reproduction by vegetative propagation of the male morphs can have an effect on sexual reproduction, as it increases the amount of staminate flowers, and therefore the supply of the pollen grains. Our results provide insights into vegetative reproduction as a strategy to increase male plant fitness and an alternative to populate environments under stress.

SEX RATIO OF RUMEX ACETOSA SEEDS OBTAINED FROM POLLINATION WITHIN AND BETWEEN POPULATIONS

Rumex acetosa L. (common sorrel) is a dioecious, wind pollinated plant with an XX/XY1Y2 sex chromosome system. Sex-ratio in the populations of dioecious plants is rarely even; generally it is skewed towards one gender. Rumex populations showed female sex-ratio bias at all of the stages of development, but the difference in sex proportion was smaller in earlier stages (Stehlik et al. 2008). Moreover, sex-ratio bias varied in different populations of the same species (Zarzycki & Rychlewski 1972). The possible causes of unequal proportion of males and females are: certation, sex-dependent germinability of seeds, and/or difference in mortality of gametophytes, embryos, and plants (BA‚ocka-Wandas et al. 2007). The plants originating from two distant populations in S Poland (Dobranowice and Wieliczka) were isolated and pollinated in a controlled condition to minimize the influence of certation. The pollinations were following: DW – ♀ from Dobranowice and ♂ from Wieliczka, WD – ♀ from Wieliczka and ♂ from Dobranowice, and DD – ♀ from Dobranowice and ♂ from Dobranowice. Seeds obtained from each experiment were collected and examined for sex by PCR. Two pairs of PCR primers that amplify sequences located in Y chromosomes were used: RAY-f and RAY-r (Korpelainen 2002), and UGR08-r and UGR08-f (Mariotti et al. 2009). To confirm the good quality of isolated DNA an amplification of autosomal sequence using R730-a and R730-b primers were carried out (Navajas-Perez et al. 2005). The same set of primers was successful in our previous studies on sorrel (Kwolek & Joachimiak 2010). Sex ratios of WD and DW seeds were similar, but clearly differed from the observed in DD seeds (Tab. 1). Progeny produced by parents from different populations showed much larger proportion of females than progeny of parents originated from the same population. The finding requires further investigations to verify the results and to explain the mechanisms underlying the observed phenomenon.

Changes in sex ratios of a dioecious grass with grazing intensity: the interplay between gender traits, neighbour interactions and spatial patterns

Summary Dioecious plants frequently face sex‐related resource allocation trade‐offs. Differential traits of male and female plants can have important consequences that influence their direct and indirect interactions with neighbours and herbivores. Gender differences in traits have been considered to be inherently linked to sex‐biased ratios along environmental stress gradients. However, the mechanisms behind the variation in sex ratios with increasing biotic stress remain elusive. We linked the results of process‐based field experiments with fine‐scale spatial patterns of naturally established plants (both sexes) at different grazing intensities. We demonstrated that slow‐growing females were better defended against insects and small mammals but were more susceptible than males to competition from neighbours. In large‐herbivore exclosures, we found that sex‐biased spatial patterns at the microsite scale can be predicted from gender‐specific traits, with females more segregated from competitors (unpalatable grasses) than males. Traits related to growth/defence trade‐off in females, coupled with their higher competition costs than males, suggest that gender dimorphism in traits can directly influence species interactions and spatial organization. In the presence of large herbivores, female individuals were preferentially browsed despite their larger investment in antiherbivore compounds. The greater distance of females from unpalatable competitors was the main determinant of female‐biased consumption by domestic herbivores. Females growing close to neighbours successfully eluded browsing but also faced competition. Scaling up to a population level, population sex ratios drift from female‐ to male‐bias with increasing domestic grazing intensity. Synthesis: The sexual dimorphism in traits of a dioecious bunchgrass results in differential herbivory by natural enemies, differential competitive ability and differential spatial distribution of sexes. Domestic grazers disrupt this balance by shifting species interactions and their spatial organization. Changes in dioecious plant populations towards male‐biased sex ratios with grazing intensity could not be predicted solely by gender‐specific traits.

Ecological context and metapopulation dynamics affect sex-ratio variation among dioecious plant populations.

Populations of dioecious flowering plants commonly exhibit heterogeneity in sex ratios and deviations from the equilibrium expectation of equal numbers of females and males. Yet the role of ecological and demographic factors in contributing towards biased sex ratios is currently not well understood. Species-level studies from the literature were analysed to investigate ecological correlates of among-population sex-ratio variation and metapopulation models and empirical data were used to explore the influence of demography and non-equilibrium conditions on flowering sex ratios. The survey revealed significant among-population heterogeneity in sex ratios and this was related to the degree of sampling effort. For some species, sex-ratio bias was associated with the proportion of non-reproductive individuals, with greater male bias in populations with a lower proportion of individuals that were flowering. Male-biased ratios were also found at higher altitudes and latitudes, and in more xeric sites. Simulations and empirical data indicated that clonal species exhibited greater heterogeneity in sex ratios than non-clonal species as a result of their slower approach to equilibrium. The simulations also indicated the importance of interactions between reproductive mode and founder effects, with greater departures from equilibrium in clonal populations with fewer founding individuals. The results indicate that sex-based differences in costs of reproduction and non-equilibrium conditions can each play important roles in affecting flowering sex ratios in populations of dioecious plants.

Does Landscape Fragmentation Influence Sex Ratio of Dioecious Plants? A Case Study of Pistacia chinensis in the Thousand-Island Lake Region of China

The Thousand-Island Lake region in Zhejiang Province, China is a highly fragmented landscape with a clear point-in-time of fragmentation as a result of flooding to form the reservoir. Islands in the artificial lake were surveyed to examine how population sex ratio of a dioecious plant specie Pistacia chinensis B. was affected by landscape fragmentation. A natural population on the mainland near the lake was also surveyed for comparison. Population size, sex ratio and diameter at breast height (DBH) of individuals were measured over 2 years. More than 1,500 individuals, distributed in 31 populations, were studied. Soil nitrogen in the different populations was measured to identify the relationship between sex ratio and micro-environmental conditions. In accordance with the results of many other reports on biased sex ratio in relation to environmental gradient, we found that poor soil nitrogen areas fostered male-biased populations. In addition, the degree of sex ratio bias increased with decreasing population size and population connectivity. The biased sex ratios were only found in younger individuals (less than 50 years old) in small populations, while a stable 1∶1 sex ratio was found in the large population on the mainland. We concluded that the effects of landscape fragmentation on the dioecious population sex ratio were mainly achieved in relation to changing soil nitrogen conditions in patches and pollen limitation within and among populations. Large populations could maintain a more suitable environment in terms of nutrient conditions and pollen flow, subsequently maintaining a stable sex ratio in dioecious plant populations. Both micro-environmental factors and spatial structure should be considered in fragmented landscape for the conservation of dioecious plant species.

Sex ratio, size distribution and nitrogen resorption in the dioecious tree species Bursera morelensis (Burseraceae)

Dioecious plant populations have generally been considered to maintain sex ratios of 1:1 (Allen & Antos 1993, Carroll & Mulcahy 1993, Charnov 1982). The sex ratio does, however, change if the plants are growing in a stressful environment, often being male-biased under these conditions (Ortiz et al. 1998, 2002). This pattern has been explained by differences in how the sexes resolve their respective energy requirements (Obeso et al. 1998). Females generally assign more resources to reproduction in comparison to growth and maintenance than do males, and they are smaller and longer-lived (Allen & Antos 1993, Nicotra 1999, Obeso et al. 1998, Willson 1983). Thus their higher reproductive costs take a toll on females, modifying the population sex ratio in favour of males (Allen & Antos 1993, Lovett-Doust & Lovett-Doust 1988). However, in order to determine the reproductive cost it is necessary to consider the compensatory mechanisms (Obeso 2002). These reduce the reproductive cost and include plastic response in terms of plant architecture and plant physiology. For example, nutrient resorption is a process that may help decrease dependence on nutrient intake, thereby constituting an important nutrient economy mechanism that can also reduce the reproductive cost if a significant proportion of the nutrients had been reassigned to reproduction (Killingbeck 1986, Obeso 2002).

MECHANISMS GOVERNING SEX-RATIO VARIATION IN DIOECIOUS RUMEX NIVALS

Sex ratios of flowering individuals in dioecious plant populations are often close to unity, or are male biased owing to gender-specific differences in flowering or mortality. Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes. Two main hypotheses have been proposed to account for female bias: (1) selective fertilization resulting from differential pollen-tube growth of female- versus male-determining microgametophytes (certation); (2) differences in the performance and viability of the sexes after parental investment. Here we investigate these hypotheses in Rumex nivalis (Polygonaceae), a European alpine herb with female-biased sex ratios in which females possess XX, and males XY1Y2, sex chromosomes. Using field surveys and a glasshouse experiment we investigated the relation between sex ratios and life-history stage in 18 populations from contrasting elevations and snowbed microsites and used a male-specific SCAR-marker to determine the sex of nonflowering individuals. Female bias among flowering individuals was one of the highest reported for populations of a dioecious species (mean female frequency = 0.87), but males increased in frequency at higher elevations and in the center of snowbeds. Female bias was also evident in nonflowering individuals (mean 0.78) and in seeds from open-pollinated flowers (mean 0.59). The female bias in seeds was weakly associated with the frequency of male flowering individuals in populations in the direction predicted when certation occurs. Under glasshouse conditions, females outperformed males at several life-history stages, although male seeds were heavier than female seeds. Poor performance of Y1Y2 gametophytes and male sporophytes in R. nivalis may be a consequence of the accumulation of deleterious mutations on Y-sex chromosomes.

MECHANISMS GOVERNING SEX-RATIO VARIATION IN DIOECIOUS RUMEX NIVALIS

Sex ratios of flowering individuals in dioecious plant populations are often close to unity, or are male biased owing to gender-specific differences in flowering or mortality. Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes. Two main hypotheses have been proposed to account for female bias: (1) selective fertilization resulting from differential pollen-tube growth of female- versus male-determining microgametophytes (certation); (2) differences in the performance and viability of the sexes after parental investment. Here we investigate these hypotheses in Rumex nivalis (Polygonaceae), a European alpine herb with female-biased sex ratios in which females possess XX, and males XY1Y2, sex chromosomes. Using field surveys and a glasshouse experiment we investigated the relation between sex ratios and life-history stage in 18 populations from contrasting elevations and snowbed microsites and used a male-specific SCAR-marker to determine the sex of nonflowering individuals. Female bias among flowering individuals was one of the highest reported for populations of a dioecious species (mean female frequency = 0.87), but males increased in frequency at higher elevations and in the center of snowbeds. Female bias was also evident in nonflowering individuals (mean 0.78) and in seeds from open-pollinated flowers (mean 0.59). The female bias in seeds was weakly associated with the frequency of male flowering individuals in populations in the direction predicted when certation occurs. Under glasshouse conditions, females outperformed males at several life-history stages, although male seeds were heavier than female seeds. Poor performance of Y1Y2 gametophytes and male sporophytes in R. nivalis may be a consequence of the accumulation of deleterious mutations on Y-sex chromosomes.

Intrapopulation Sex Ratio Variation in the Salt GrassDistichlis spicata

ABSTRACT In many dioecious plant populations, males and females appear to be spatially segregated, a pattern that is difficult to explain given its potentially high costs. However, in asexually propagating species, spatial segregation of the sexes may be indistinguishable from superficially similar patterns generated by random establishment of a few genets followed by extensive clonal spread and by gender-specific differences in rates of clonal spread. In populations where a significant fraction of individuals are not flowering and gender cannot be assigned to this fraction, apparent spatial segregation of the sexes may be due to differential flowering between the sexes. We confirm reports that flowering ramets of the clonal, perennial grass Distichlis spicata are spatially segregated by sex. We extend these studies in two fundamental ways and demonstrate that this species exhibits true spatial segregation of the sexes. First, using RAPD markers, we estimated that at least 50% of ramets in patches with biased sex ratios represent distinct genotypes. Second, we identified a RAPD marker linked to female phenotype (eliminating the possibility that gender is environmentally determined) and used it to show that the majority of patches exhibit significantly biased sex ratios for both ramets and genets, regardless of flowering status.

Evolutionarily Stable Reproductive Strategies in Sexual Organisms: III. The Effects of Lottery Density Dependence and Pollen Limitation

This paper extends our previous work on modelling, within a single framework, the allocation of resources to reproduction vs. survival and the male vs. female components of reproduction in perennial plants. We derive the evolutionarily stable strategy (ESS) results under pollen limitation for both hermaphroditic and dioecious plant populations held stable through density-dependent juvenile recruitment. Pollen limitation affects female reproductive allocation in our model because there is post-flowering provisioning of offspring. We find that pollen limitation is unimportant to the ESS reproductive allocation and sex allocation so long as there are enough seeds to fill the empty sites left by the death of adults. To the extent that the relationships between gamete output and resource investment are linear for both sexes or sex functions, the separate treatment of reproductive and sex allocation in modern life-history and sex-allocation theories is adequate. In such cases, the ESS sex allocation is exactly what is found in traditional sex allocation theory, and the ESS reproductive allocation of hermaphrodites or females in a dioecious species maximizes the amount of resources allocated to reproduction during an average lifespan, an analogue of the usual maximization principle in life-history theory modified to include the possibility of pollen limitation and extended seed maturation. The ESS reproductive allocation of males in a dioecious species maximizes lifetime pollen production, independent of pollen limitation and the female's resource allocation.

Sexual Dimorphism of Dormancy and Survivorship in Buried Seeds of Silene Latifolia

1 To test for differences in survivorship between buried male and female seeds in the dioecious perennial, Silene latifolia (Caryophyllaceae), the behaviour of large numbers of seeds buried under field conditions and unearthed after 5 and 10 months was compared with unburied seeds. 2 After 5 months of burial there was significant mortality of seeds and a large fraction of the remaining seeds displayed induced dormancy, i.e. they did not germinate under glasshouse conditions. The sex ratio of germinating seeds was not significantly different from the sex ratio of seeds germinated without burial (0 months). 3 Seed survivorship was constant from 5 to 10 months. However, by 10 months levels of dormancy had decreased so that most seeds germinated when unearthed and tested in a growth chamber. Furthermore, the proportion of females in germinating seed was significantly higher after 10 months than after 5 months. 4 Sex differences in mortality were therefore only revealed after 10 months, when dormancy had been broken. This result suggests that female seeds were more likely to become dormant during the first 5 months of burial, whereas males suffered greater mortality. Thus, sex differences in both survivorship and dormancy of buried seeds affected the sex ratio of germinating seeds over time. Such sexual dimorphism in seed bank dynamics may have an important effect on sex ratio variation in natural populations of dioecious plants.

Spatial Segregation of the Sexes of Dioecious Plants

Several recent studies have shown that males and females in some populations of dioecious plants are spatially segregated with respect to an environmental gradient. The inference is often made that such spatial segregation of the sexes (SSS) is favored by selection because it reduces competition between individuals of opposite sex (sexual niche partitioning). This paper was written to clarify the evolutionary interpretation of cases of SSS. We describe the possible proximate mechanisms capable of producing SSS and evaluate their plausibility. Then, we identify the selective factors that could favor the evolution of SSS. We argue that SSS can be favored if male fitness and female fitness respond differently across environments (because of differences in reproductive biology), that a reduction in competition between males and females is unlikely to be an evolutionary cause of SSS, and that differential mortality is unlikely to evolve as a proximate mechanism for achieving adaptive SSS. Such a pattern is r...

Sex in Ephedra trifurca (Ephedraceae) with Relation to Chihuahuan Desert Habitats

Spatial patterns of Ephedra trifurca populations were examined in seven vegetative zones along an elevational gradient in the Chihuahuan Desert near Las Cruces, New Mexico. Ephedra density and mean biomass differed significantly among zones, with variations that were not a consistent function of soil moisture. Higher proportions of female plants did not occur in areas of higher soil moisture availability, as has been previously reported for another Ephedra species in desert habitats. INTRODUCTION Plants of the genus Ephedra are dioecious gymnosperm shrubs which occur in arid and semiarid areas of the western United States. Freeman et al. (1976) reported that in several populations of dioecious plants, including Ephedra viridis Coville, in Utah, males and females were segregated spatially, and that the sex ratios were significantly correlated to conditions of the immediate environment, with higher proportions of female plants associated with more mesic sites. Spatial distributions of males and females determined by an environmental gradient would dictate the presence of areas along the gradient where male survivorship is higher and other areas where female survival is favored (Freeman et al., 1976; Freeman et al., 1980; Meagher, 1980), or that sex is not genetically fixed and expression is subject to microsite environmental influence (Freeman et al., 1976). We tested for this pattern in a population of Ephedra trifurca Torr. occurring along an elevational gradient at a Chihuahuan Desert site, where the soil moisture availability (as a function of soil moisture and morphology) varied markedly over the gradient. We also examined variation in density and sex-specific biomass associated with gradient features under the assumption that these variables generally indicate site suitability for the species. METHODS The study was conducted at the New Mexico State University College Ranch, 40 km NNE of Las Cruces, Dona Ana Co., New Mexico. The study site is located on the slopes of a small watershed, extending along an elevational gradient from a rocky mountain slope (1417 m) to an ephemeral playa lake (1307 m). Soil and vegetation studies on the site (Wierenga et al., 1987) have identified seven zones, from high to low elevation: (1) rockland (boulder-covered mountain slopes); (2) upper piedmont [grassland dominated by black g'ama (Bouteloua eriopoda)]; (3) lower piedmont [grassland featuring black grama, dropseed (Sporobolus spp.), and fluffgrass (Erioneuron pulchellum)]; (4) bajada [shrubland dominated by creosote bush (Larrea tridentata)]; (5) basin slope (area featuring a diversity of small perennial and annual herbaceous plants and scattered shrubs); (6) playa fringe [band of large mesquite (Prosopis glandulosa)]; and (7) playa (ephemeral lake bed dominated by Panicum obtusum). Soils in the area present soil gradation typical of elevational gradients in arid regions, with coarse gravel and sand at higher elevation and clay in the playa, and are underlaid by a calcium carbonate deposition layer (caliche) at 60-100 cm (Wierenga et al., 1987). The

POPULATION BIOLOGY OF CHAMAELIRIUM LUTEUM, A DIOECIOUS LILY. I. SPATIAL DISTRIBUTIONS OF MALES AND FEMALES.

The present series of papers considers the population biology of the dioecious herb Chamaelirium luteum (L.) Gray (Liliaceae) with special reference to the impact of sexual dimorphism on spatial distribution, resource allocation, life history, and sex ratios. This species is a perennial forest floor herb found typically on slopes of mesic hardwood forests. Its range extends from southwestern Massachusetts, as far west as Illinois and throughout the southeastern United States into northern Florida (Fernald, 1950; Radford et al., 1968). The plant consists of a basal rosette which does not undergo vegetative reproduction. It flowers from late April through May. The infloresence is produced from the center of the rosette and is an upright spike with leaves along 2/3 to 3/4 of its length and flowers on the remaining apical portion. There is a marked morphological sexual dimorphism: females have tall persistent inflorescences with many leaves whereas males have shorter inflorescences with fewer leaves. The male inflorescence withers away in late spring shortly after flowering, but the female inflorescence persists until the fall when the fruits mature. In many animal groups sexual dimorphism has been demonstrated in feeding behavior (Morse, 1968; Jackson, 1970), habitat choice (Feduccia and Slaughter, 1974), and, in some extreme cases, in geographical range (Bartholomew, 1970). Sexual dimorphism has also been documented in many dioecious plant species (see Lloyd and Webb, 1977, for recent review); and, since plants are nonmotile, it seems likely that ecological consequences of sexual dimorphism should be reflected in the relative distribution patterns of male and female plants. Studies into distribution of the sexes in dioecious plant species have either looked at nearest neighbor effects (Bawa and Opler, 1977; Melampy and Howe, 1977), or have examined the distribution of males and females along environmental gradients (Davey and Gibson, 1917; Harris, 1968; Dzhaparidge, 1969; Rathore, 1969; Freeman et al., 1976; Brockman and Bocquet, 1978; Opler and Bawa, 1978; Grant and Mitton, 1979). None of these studies used mapping techniques to follow the histories of individual plants in order to analyze in detail the influence of sexual dimorphism on the ecology and evolution of dioecious plant populations. In this paper, I address the problem of spatial patterns in the distribution of males and females in four populations of the markedly sexually dimorphic C. luteum with regard to the following questions. What is the scale of pattern in populations of this species? What factors might bring about differences in the distributions of the two sexes? Are the spatial distributions of males and females influenced by differences in microhabitats? What are the evolutionary consequences of differences in the distribution of males and females in a plant population?