Dioecious species and arbuscular mycorrhizal symbioses: The case of Antennaria dioica

Sex-specific interactions with herbivores and pollinators have been observed in female and male plants of dioecious species. However, only a limited number of studies have revised sex-specific patterns in mycorrhizal symbiosis. To test whether female and male plants of Antennaria dioica differ in their relationship with arbuscular mycorrhizal (AM) fungi, we examined the temporal and spatial variation in AM fungi in female, male and non-reproductive A. dioica plants in three natural populations in Finland during flowering and after seed production. Our results are consistent with previous studies both under greenhouse and field conditions with the same species showing differences in AM colonization between the sexes linked with allocation to reproduction. Taken together, the results indicate that there is a sex-specific interaction between A. dioica and AM fungi. Overall, females have a greater investment in AM fungi, likely to enhance their uptake of soil nutrients and support the reproduction by seed.     

Competitive interactions are mediated in a sex‐specific manner by arbuscular mycorrhiza in Antennaria dioica

• Plants usually interact with other plants, and the outcome of such interaction ranges from facilitation to competition depending on the identity of the plants, including their sexual expression. Arbuscular mycorrhizal (AM) fungi have been shown to modify competitive interactions in plants. However, few studies have evaluated how AM fungi influence plant intraspecific and interspecific interactions in dioecious species.
• The competitive abilities of female and male plants of Antennaria dioica were examined in a greenhouse experiment. Females and males were grown in the following competitive settings: (i) without competition, (ii) with intrasexual competition, (iii) with intersexual competition, and (iv) with interspecific competition by Hieracium pilosella – a plant with similar characteristics to A. dioica. Half of the pots were grown with Claroideoglomus claroideum, an AM fungus isolated from the same habitat as the plant material. We evaluated plant survival, growth, flowering phenology, and production of AM fungal structures.
• Plant survival was unaffected by competition or AM fungi. Competition and the presence of AM fungi reduced plant biomass. However, the sexes responded differently to the interaction between fungal and competition treatments. Both intra‐ and interspecific competition results were sex‐specific, and in general, female performance was reduced by AM colonization. Plant competition or sex did not affect the intraradical structures, extraradical hyphae, or spore production of the AM fungus.
• These findings suggest that plant sexual differences affect fundamental processes such as competitive ability and symbiotic relationships with AM fungi.

     

Differences in response to defoliation between males and females of Silene dioica

Summary The response by male and female plants to herbivory was studied by experimental defoliation of the dioecious perennial herb Silene dioica in a green-house. Male and female plants were defoliated prior to and during the early flowering phase at two intensities (50% and 100% of leaf-area removed) in two consecutive years. Defoliation resulted in a decrease in the number of flowers initiated in both sexes, while a larger delay of peak flowering and a higher mortality was observed in males compared to females. In female plants, severe defoliation resulted in a reduction in seed number per capsule and in seed size compared to control. Females showed a negative correlation between the production of flowers in the first and second season in all treatments, while flowering in males the first season was not correlated with flowering in the second season. Females also showed a lower frequency of flowering than males during the two seasons studied. However, during the flowering period, males allocated significantly more biomass to flowers than did females. This outcome supports the idea that females may have a higher total reproductive expenditure than males, but males have a higher reproductive effort during flowering. Male rosette leaves were significantly preferred by the generalist herbivore Arianta arbustorum in experiments. This preference was most pronounced in trials with leaves from fertilized plants compared to nonfertilized plants. A greater storage of resources in aboveground leaves during winter by males compared to females may explain the higher preference for male leaves and the higher male mortality following early defoliation. Furthermore, males are smaller than females and may have a lower ability than females to replace lost resources needed for reproduction when defoliated early in the season.     

Environmental stress and the evolution of dioecy: Wurmbea dioica (Colchicaceae) in Western Australia

Stressful ecological conditions have been implicated in the evolution of separate sexes in plants. Gender dimorphic species are often found in drier habitats than their sexually monomorphic relatives, and gynodioecious populations appear closer to a dioecious state as resources, particularly water, become limiting. This pattern could result if dry conditions decrease the relative seed fitness of cosexual plants, allowing female plants to become established in monomorphic populations. We studied geographical variation in gender expression and biomass allocation among 12 monomorphic and dimorphic populations of Wurmbea dioica along a latitudinal precipitation gradient in southwestern Australia to provide insight into mechanisms by which aridity might favor transitions between sexual systems. Plants in monomorphic and dimorphic populations exhibited contrasting gender expression and patterns of biomass allocation in areas with different levels of precipitation. Among dimorphic populations, lower precipitation was associated with a higher frequency of female plants, and reduced allocation to female function by hermaphrodites during flowering. In contrast, stress conditions had no effect on female allocation at flowering in monomorphic populations. Across latitudes, unisexuals and cosexuals exhibited consistent differences in above ground traits, with cosexuals having larger leaves, taller stems and larger flowers. Although all plants were smaller under drier conditions, cosexuals decreased above ground allocation to vegetative and reproductive structures with decreasing latitude. In contrast, unisexuals increased allocation to reproduction in drier areas at the expense of below ground size. Aridity was associated with reduced flower size among all gender classes, but not with changes in flower number. These data do not support the hypothesis that resource limitation of female allocation in cosexual populations favors the establishment of gender dimorphism in W. dioica. Alternative hypotheses, involving higher selfing rates and enhanced survival of unisexuals relative to cosexuals under resource-limited conditions, are discussed as possible explanations for the origin of dioecy in W. dioica. This revised version was published online in August 2006 with corrections to the Cover Date.     

Masting uncoupling: mast seeding does not follow all mast flowering episodes in a dioecious juniper tree

Evolutionary selective forces, like predator satiation and pollination efficiency, are acknowledged to be major causes of masting (the variable, periodic and synchronic production of seeds in a population). However, a number of recent studies indicate that resources might also play an important role on shaping masting patterns. Dioecious masting species offer a privileged framework to study the role of resources on masting variation, since male and female plants often experience different reproductive costs and selective pressures. We followed masting and reproductive investment (RI) of the dioecious tree Juniperus thurifera in two populations along 10 years and studied the different response of males and females to experimentally increased water and nutrient availability in a third population. Juniperus thurifera females invested in reproduction three times more resources than males. Such disparity generated different resource‐use strategies in male and female trees. Tree‐ring growth and water use efficiency (WUE) confirmed that sexes differed in their resource investment temporal pattern, with males using current resources for reproduction and females using resources accumulated during longer periods. Watered and fertilized female trees presented significantly higher flowering reproductive investments than males and experienced an extraordinary mast‐flowering event. However, seeding RI and mast seeding were not affected by the treatment. This suggests that although resource availability affects the reproductive output of this species, there are other major forces regulating masting on J. thurifera. During 10 years, J. thurifera male and female trees presented high and low flowering years more or less synchronously. However, not all mast flowering episodes resulted in mast seeding, leading to masting uncoupling between flowering and seeding. Since flowering costs represent only 1% of females’ total reproductive investments, masting uncoupling could be a beneficial bet‐hedging strategy to maximize reproductive output in spite of unpredictable catastrophic events.     

Land-use change impact on mycorrhizal symbiosis in female and male plants of wild <Emphasis Type="Italic">Carica papaya</Emphasis> (Caricaceae)

It has been acknowledged that land-use change has negative effects on genetic diversity and sex ratio in dioecious species, but less attention has been paid on the influence that land-use change has on the biotic interactions, especially between dioecious species and arbuscular mycorrhizal (AM) fungi. AM mutualism involves reciprocal transfer of carbohydrates and mineral nutrients between the host plant’s roots and these fungi. Here, we report spatial and temporal variation in AM colonization in dioecious wild Carica papaya plants growing in sites with different land use intensity. We tagged, recorded the basal stem circumference and collected roots of reproductive female and male Carica papaya plants in three wild sites during dry and rainy seasons of western Mexico. We also collected soil samples in each site to conduct soil chemical analyses. The sexes of C. papaya did not show significant differences in the frequency (percentage of root colonized by intraradical fungal structures) and abundance (length of intraradical hyphae) of AM fungi but the higher AM colonization was observed during the dry season, and in the site with the lowest disturbance. There was no relationship between soil chemistry and AM colonization. Overall, our findings suggest that land-use intensity has a negative effect on AM colonization and we discuss the consequences of habitat loss for the reproductive female and male plants, the implications of decreasing AM colonization for wild Carica papaya plants an important species that provides a source of genetic variation for the C. papaya varieties.     

Gender variation and the evolution of dioecy in Wurmbea dioica (Liliaceae)

A major obstacle for empirical tests of hypotheses concerning the evolution of dioecy in flowering plants is the limited number of species that possess both cosexual and dioecious populations. Wurmbea dioica (Liliaceae) is a diminutive, fly-pollinated geophyte native to temperate Australia. Marked geographical variation of floral traits is evident, particularly with respect to sex expression. A survey of phenotypic gender in 45 populations from Western Australia (WA), South Australia (SA), Victoria (Vic) and the Australian Capital Territory (ACT) revealed two contrasting patterns. Populations in SA, Vic, and ACT were uniformly dimorphic for gender, containing female and male plants, whereas populations in WA were either monomorphic or dimorphic. In most dimorphic populations varying numbers of male plants produced hermaphrodite flowers (male inconstancy). There was a significant negative relationship between female frequency and the proportion of inconstant male plants. Depending on region and population, male plants produced more flowers of larger size than females. In WA monomorphic populations often occurred on rich, moist soils at high density, whereas dimorphic populations were more commonly found at lower density on shallow soils in drier areas. In an area of sympatry, plants with contrasting sexual systems flowered at different times and were ecologically differentiated. The patterns of gender variation in W. dioica indicate that dioecy has evolved via the gynodioecious pathway. The spread of females in monomorphic populations may be favoured where ecological conditions result in increased selfing and inbreeding depression in hermaphrodites.     

Complex sex determination in the stinging nettle Urtica dioica

The dioecious species Urtica dioica harbours wide variation in sex ratio of seeds. We conducted a series of crosses to analyse the genetic basis of sex determination in this species. Dutch populations of U. dioica contain low proportions of monoecious individuals beside male and female plants. Self-pollination of monoecious plants always yielded female, male and monoecious plants, generally in a ratio of one female to three male/monoecious individuals. This motivated us to write down a simple model in which gender is determined by one major sex-determination locus with four alleles. In the model males and monoecious plants have distinct genotypes but are both heterozygous at the sex-determination locus. We first made crosses among progeny obtained after self-pollination of monoecious plants. These crosses showed that the monoecious trait generally showed Mendelian inheritance and was passed on to the next generation via both pollen and seeds. Further crosses between monoecious plants and plants from dioecious system indicated that alleles from the dioecious system are often dominant. However, many exceptions to our genetic model are observed which suggest that dominance is incomplete and/or that more genes are involved in sex determination. We discuss to what extent sex determination genes explain the strongly biased seed sex ratios and argue that additional genes, for instance genes for female choice, must also be involved.     

Differential competitive ability between sexes in the dioecious Antennaria dioica (Asteraceae)

Background and Aims

Differences in competitive ability between the sexes of dioecious plants are expected as a result of allocation trade-offs associated with sex-differential reproductive costs. However, the available data on competitive ability in dioecious plants are scarce and contradictory. In this study sexual competition was evaluated using the dioecious plant Antennaria dioica in a common garden transplantation experiment.

Methods

Male and female plants were grown for 3 years either in isolation, or in competition with a plant of the same sex or the opposite sex. Flowering phenology, sexual and asexual reproduction, plant growth, nutrient content and arbuscular mycorrhizal colonization in the roots were assessed.

Key Results

Our results showed little evidence of sexual differences in competitive ability. Both sexes suffered similarly from competition, and competitive effects were manifested in some traits related to fitness but not in others. Survival was unaffected by competition, but competing plants reduced their vegetative growth and reproductive investment compared with non-competing plants. In addition, differences in sexual competitive ability were observed in relation to flowering frequency, an important life history trait not reported in previous studies.

Conclusions

The findings indicate that female and male A. dioica plants possess similar intersexual competitive abilities which may be related to the similar costs of reproduction between sexes in this species. Nevertheless, intrasexual competition is higher in females, giving support for asymmetric niche segregation between the sexes.     

Sex ratio, reproductive investment and flowering phenology in dioecious Rhamnus alaternus (Rhamnaceae)

Guitián, J. 1995. Sex ratio, reproductive investment and flowering phenology in dioecious Rhamnus alaternus (Rhamnaceae). - Nord. J. Bot. 15: 139–143. Copenhagen. ISSN 0107–055X.
During 1992 and 1993 I investigated the reproductive biology of the dioecious Mediterranean shrub Rhamnus alaternus in a population in the northwest Iberian Peninsula. Reproductive investment was estimated as the mean total dry weight of reproductive organs per branch tip. I also estimated number of flowers produced per plant and population sex ratio, and investigated the spatial distribution of the sexes and flowering phenology. The sex ratio was 1:1, and the spatial distribution of the sexes was random. Male plants produced 2.6 times more flowers than female plants, but the overall reproductive investment by females was 5- to 9-fold higher. In both years of study, male plants commenced flowering first. The male and female flowering peaks coincided closely (in late March) in both years. The results of this study suggest that male and female R. alaternus differ most notably in the amount of resources allocated to reproduction.     

Intersexual differences in phenology and damage by herbivores and pathogens in dioecious Rubus chamaemorus L.

Summary Shoot development and damage by herbivores and pathogens to male, female and non-floral ramets of the dioecious, perennial herb Rubus chamaemorus were studied in the field during three consecutive years. Leaves on male ramets were usually consumed more by herbivores and attacked more by fungi than were leaves on female ramets. Male ramets unfolded their leaves later than did female ramets. In 1983, when the level of herbivory was comparatively high, ramets that were fully developed in early June were more damaged by herbivores than were ramets that unfolded their leaves later. Non-floral ramets usually showed intermediate levels of damage compared to male and female flowering ramets.Defoliation caused a greater increase in ramet mortality among females than among males in a field experiment. It is suggested that the different reproductive roles of males and females differently constrain the evolution of defense against herbivores and pathogens in dioecious plants. This may result in intersexual differences both in palatability to herbivores and susceptibility to pathogens.     

Impact of defoliation intensities on plant biomass,nutrient uptake and arbuscular mycorrhizal symbiosis in Lotus tenuis growing in a saline‐sodic soil

The impact of different defoliation intensities on the ability of Lotus tenuis plants to regrowth, mobilise nutrients and to associate with native AM fungi and Rhizobium in a saline‐sodic soil was investigated. After 70 days, plants were subjected to 0, 25, 50, 75 and 100% defoliation and shoot regrowth was assessed at the end of subsequent 35 days. Compared to non‐defoliated plants, low or moderate defoliation up to 75% did not affect shoot regrowth. However, 100% treatment affected shoot regrowth and the clipped plants were not able to compensate the growth attained by non‐defoliated plants. Root growth was more affected by defoliation than shoot growth. P and N concentrations in shoots and roots increased with increasing defoliation while Na⁺ concentration in shoots of non‐defoliated and moderately defoliated plants was similar. Non‐defoliated and moderately defoliated plants prevented increases of Na⁺ concentration in shoots through both reducing Na⁺ uptake and Na⁺ transport to shoots by accumulating Na⁺ in roots. At high defoliation, the salinity tolerance mechanism is altered and Na⁺ concentration in shoots was higher than in roots. Reduction in the photosynthetic capacity induced by defoliation neither changed the root length colonised by AM fungi nor arbuscular colonisation but decreased the vesicular colonisation. Spore density did not change, but hyphal density and Rhizobium nodules increased with defoliation. The strategy of the AM symbiont consists in investing most of the C resources to preferentially retain arbuscular colonisation as well as inoculum density in the soil.     

Reversal of height dimorphism promotes pollen and seed dispersal in a wind-pollinated dioecious plant

Variation in the timing of reproductive functions in dioecious organisms may result in adaptive changes in the direction of sexual dimorphism during the breeding season. For plants in which both pollen and seeds are wind-dispersed, it may be advantageous for male plants to be taller when pollen is dispersed and female plants to be taller when seeds are dispersed. We examined the dynamics of height dimorphism in Rumex hastatulus, an annual, wind-pollinated, dioecious plant from the southern USA. A field survey of seven populations indicated that females were significantly taller than males at seed maturity. However, a glasshouse experiment revealed a more complex pattern of height growth during the life cycle. No dimorphism was evident prior to reproduction for six of seven populations, but at flowering, males were significantly taller than females in all populations. This pattern was reversed at reproductive maturity, consistent with field observations. Males flowered later than females and the degree of height dimorphism was greater in populations with a later onset of male flowering. We discuss the potential adaptive significance of temporal changes in height dimorphism for pollen and seed dispersal, and how this may be optimized for the contrasting reproductive functions of the sexes.     

Physiological performance and xylem water isotopic composition underlie gender‐specific responses in the dioecious shrub Corema album

Gender‐specific requirements of reproduction in dioecious species can lead to different physiological responses in male and female plants, made in relation to environmental constraints, and influencing growth, survival and population structure. Gender‐related physiological differences and seasonal responses, indicating the existence of compensatory mechanisms of reproduction, were examined during a drought year in the dioecious shrub species Corema album. To integrate aboveground and belowground physiological responses, chlorophyll fluorescence, leaf gas exchange, water potential and xylem water isotopic composition were monitored throughout the diurnal cycle and annual phenological sequence of the species. Sampling was carried out in Doñana Natural Park (SW Spain) in Mediterranean‐type climate conditions. The gender which bore greater reproductive effort showed higher physiological stress. Intersexual differences in leaf water potential were interpreted as arising from each gender's maximum reproductive allocation; lower values were found during flowering in males and during fruit production in females. Cold temperatures during winter fostered photoinhibitory responses that were most evident in male individuals, as a response to their relatively higher investment in reproduction during flowering. Net assimilation rate was not influenced by reproductive status; however, females tended to show higher values of this parameter at midday. The integrated analysis of photosynthetic variables and water relations indicated a gender effect in the physiological response at midday. The oxygen isotopic composition of xylem water showed a lack of dependence on the water table during the drought period, and indicated intersexual differences in water catchment. Females reached deeper soil layers, suggesting mechanisms compensating for their higher reproductive effort, and giving new evidence of physiological gender dimorphism in the belowground responses of a woody species.     

Mycorrhiza modulates aboveground tri‐trophic interactions to the fitness benefit of its host plant

1. Arbuscular mycorrhiza (AM), the association of AM fungi and plant roots, may alter morphological and physiological attributes of aboveground plant parts and thereby influence plant‐associated organisms such as herbivores and their natural enemies, predators and parasitoids. 2. The interactions between AM and the players of aboveground tri‐trophic systems have mainly been considered in isolation from each other. The effects of AM on aboveground herbivore–carnivore population dynamics and the consequences to plant fitness are unknown. 3. We explored AM‐induced compensatory mechanisms for AM‐promoted proliferation of the herbivorous spider mite, Tetranychus urticae Koch, on whole bean plants, Phaseolus vulgaris L. Vegetative and reproductive plant growth, AM fungal colonisation levels, and mite densities were assessed on spider mite‐infested plants colonised or not by the AM fungus Glomus mosseae Nicol. & Gerd, and harbouring the natural enemy of the spider mites, the predatory mite Phytoseiulus persimilis Anthias‐Henriot or not. 4. AM symbiosis modulated the aboveground tri‐trophic system to the fitness benefit of the plant. AM‐increased plant productivity outweighed the fitness decrease due to AM‐promoted herbivory: at similar vegetative growth, mycorrhizal plants produced more seeds than non‐mycorrhizal plants. 5. AM‐increased spider mite population levels were compensated for by enhanced population growth of the predators and increased plant tolerance to herbivory. 6. AM‐enhanced predator performance looped back to the AM fungus and stabilised its root colonisation levels, providing the first experimental evidence of a mutually beneficial interaction between AM and an aboveground third trophic level natural enemy.     

Sex-specific physiology and source-sink relations in the dioecious plant Silene latifolia

Differences in reproductive demands between the sexes of dioecious plants could cause divergence in physiology between the sexes. We found that the reproductive effort of female Silene latifolia plants increased to more than twice that of male plants or female plants that were prevented from setting fruit by lack of pollination after 4 weeks of flowering. Whole-plant source/sink ratios of pollinated females were significantly lower than those of males or unpollinated females because of investment in fruit. We hypothesized that these differences in source/sink ratio between the sexes and within females, depending on pollination, would lead to differences in leaf photosynthetic rates. Within females, we found that photosynthetic capacity was consistent with measurement of whole-plant source/sink ratio. Females that were setting fruit had 30% higher light-saturated photosynthetic rates by 28 days after flowering than females that were not setting fruit. Males, however, had consistently higher photosynthetic rates than females from 10 days after flowering onwards. Males also had approximately twice the dark respiration rates of fruiting females. We found that female reproductive structures are longer-lived and contribute more carbon to their own support than male reproductive structures. Despite the higher rates of leaf dark respiration and lower calyx photosynthetic rates, males fix more carbon than do females. We conclude that females have a sink-regulated mechanism of photosynthesis that allows them to respond to variations in fruit set. This mechanism is not, however, sufficient to explain why male S. latifolia plants have higher rates of photosynthesis, higher source/sink ratios, and lower reproductive allocation, but fail to grow larger than female plants.     

Clonal genetic structure and diversity in populations of an aquatic plant with combined vs. separate sexes

Clonality is often implicated in models of the evolution of dioecy, but few studies have explicitly compared clonal structure between plant sexual systems, or between the sexes in dioecious populations. Here, we exploit the occurrence of monoecy and dioecy in clonal Sagittaria latifola (Alismataceae) to evaluate two main hypotheses: (i) clone sizes are smaller in monoecious than dioecious populations, because of constraints imposed on clone size by costs associated with geitonogamy; (ii) in dioecious populations, male clones are larger and flower more often than female clones because of sex‐differential reproductive costs. Differences in clone size and flowering could result in discordance between ramet‐ and genet‐based sex ratios. We used spatially explicit sampling to address these hypotheses in 10 monoecious and 11 dioecious populations of S. latifolia at the northern range limit in Eastern North America. In contrast to our predictions, monoecious clones were significantly larger than dioecious clones, probably due to their higher rates of vegetative growth and corm production, and in dioecious populations, there was no difference in clone size between females and males; ramet‐ and genet‐based sex ratios were therefore highly correlated. Genotypic diversity declined with latitude for both sexual systems, but monoecious populations exhibited lower genotypic richness. Differences in life history between the sexual systems of S. latifolia appear to be the most important determinants of clonal structure and diversity.     

Variable mycorrhizal benefits on the reproductive output of Geranium sylvaticum,with special emphasis on the intermediate phenotype

In several gynodioecious species, intermediate sex between female and hermaphrodite has been reported, but few studies have investigated fitness parameters of this intermediate phenotype. Here, we examined the interactions between plant sex and arbuscular mycorrhizal (AM) fungal species affecting the reproductive output of Geranium sylvaticum, a sexually polymorphic plant species with frequent intermediate sexes between females and hermaphrodites, using a common garden experiment. Flowering phenology, AM colonisation levels and several plant vegetative and reproductive parameters, including seed and pollen production, were measured. Differences among sexes were detected in flowering, fruit set, pollen production and floral size. The two AM species used in the present work had different effects on plant fitness parameters. One AM species increased female fitness through increasing seed number and seed mass, while the other species reduced seed mass in all sexes investigated. AM fungi did not affect intermediate and hermaphrodite pollen content in anthers. The three sexes in G. sylvaticum did not differ in their reproductive output in terms of total seed production, but hermaphrodites had potentially larger fathering ability than intermediates due to higher anther number. The ultimate female function – seed production – did not differ among the sexes, but one of the AM fungi used potentially decreased host plant fitness. In addition, in the intermediate sex, mycorrhizal symbiosis functioned similarly in females as in hermaphrodites.     

Sex ratio and sexual dimorphism in the dioecious Borderea pyrenaica (Dioscoreaceae)

Sex ratio and sexual dimorphism of Borderea pyrenaica, a long-lived dioecious geophyte endemic to the Pyrenees (north-east Iberian Peninsula), were examined in three alpine populations. In this species, age can be estimated and the sex of nonreproductive adult plants identified. Male plants attain sexual maturity earlier, flower more frequently and grow faster than female plants, whereas females allocate a higher biomass to reproduction than males. These results support the hypothesis that female plants incur a higher cost of sexual reproduction and that this higher cost is measurable as reduced vegetative growth and lower flowering frequency. Variation of sex ratio among young, intermediate and old adults within populations suggests, however, that this higher female reproductive investment does not result in sexual differences in mortality. The overall male-biased sex ratio in B. pyrenaica is mainly a consequence of the tendency of males to reproduce at an earlier age and more frequently than females.     

The true sex ratio in European Pseudocalliergon trifarium (Bryophyta: Amblystegiaceae) revealed by a novel molecular approach

Dioecious plants, including many bryophytes, rarely exhibit discernible sexual dimorphism before sexual maturity. Because many species and populations of dioecious bryophytes do not express their sex, it remains mostly unresolved whether expressing individuals reflect the ratios of genetically male and female plants. The present study assesses the population sex ratio of the wetland moss Pseudocalliergon trifarium in central and northern Europe. For the first time in a bryophyte, we estimate the sex ratio in a population by assessing directly both expressing and non‐expressing plants. Expressed gender ratio was assessed from herbarium specimens. Single shoots from non‐expressing specimens were sexed using a recently developed molecular sex marker. On the basis of the female and male frequencies in these two data sets and the overall proportion of expressing specimens, we estimate the European population sex ratio to be 1.93 : 1 (female/male). Expressed, non‐expressed, and population sex ratios are not significantly different from each other, suggesting that gender differences in rates of sex expression cannot account for the female bias. Earlier studies of P. trifarium failed to reveal gender‐specific growth rates or pre‐zygotic reproductive costs. Gender differences at the spore to protonemal stage, in mortality, or niche preferences could potentially explain the uneven sex ratio. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 132–140.