Dioecy,more than monoecy,affects plant spatial genetic structure: the case study of Ficus

In this analysis, we attempt to understand how monoecy and dioecy drive spatial genetic structure (SGS) in plant populations. For this purpose, plants of the genus Ficus were used as a comparative model due to their particular characteristics, including high species diversity, variation in life histories, and sexual systems. One of the main issues we assessed is whether dioecious fig tree populations are more spatially genetically structured than monoecious populations. Using the Sp statistic, which allows for quantitative comparisons among different studies, we compared the extent of SGS between monoecious and dioecious Ficus species. To broaden our conclusions we used published data on an additional 27 monoecious and dioecious plant species. Furthermore, genetic diversity analyses were performed for two monoecious Ficus species using 12 microsatellite markers in order to strengthen our conclusions about SGS. Our results show that dioecy, more than monoecy, significantly contributes to SGS in plant populations. On average, the estimate of Sp was six times higher for dioecious Ficus species than monoecious Ficus species and it was two times higher in dioecious than monoecious plant species. Considering these results, we emphasize that the long‐distance pollen dispersal mechanism in monoecious Ficus species seems to be the dominant factor in determining weak spatial genetic structure, high levels of genetic diversity, and lack of inbreeding. Although Ficus constitute a model species to study SGS, a more general comparison encompassing a wider range of plants is required in order to better understand how sexual systems affect genetic structure.     

Phylogenetic relationships of functionally dioecious FICUS (Moraceae) based on ribosomal DNA sequences and morphology

Figs (Ficus, Moraceae) are either monoecious or gynodioecious depending on the arrangement of unisexual florets within the specialized inflorescence or syconium. The gynodioecious species are functionally dioecious due to the impact of pollinating fig wasps (Hymenoptera: Agaonidae) on the maturation of fig seeds. The evolutionary relationships of functionally dioecious figs (Ficus subg. Ficus) were examined through phylogenetic analyses based on the internal transcribed spacer (ITS) region of nuclear ribosomal DNA and morphology. Forty-six species representing each monoecious subgenus and each section of functionally dioecious subg. Ficus were included in parsimony analyses based on 180 molecular characters and 61 morphological characters that were potentially informative. Separate and combined analyses of molecular and morphological data sets suggested that functionally dioecious figs are not monophyletic and that monoecious subg. Sycomorus is derived within a dioecious clade. The combined analysis indicated one or two origins of functional dioecy in the genus and at least two reversals to monoecy within a functionally dioecious lineage. The exclusion of breeding system and related characters from the analysis also indicated two shifts from monoecy to functional dioecy and two reversals. The associations of pollinating fig wasps were congruent with host fig phylogeny and further supported a revised classification of Ficus.     

The genetics of sex determination in stinging nettle (<Emphasis Type="Italic">Urtica dioica</Emphasis>)

Urtica dioica (“stinging nettle”) includes both dioecious and monoecious forms. In most sexually dimorphic angiosperm species, the genetic mechanisms of sex determination are completely unknown. The few species that include both monoecious and dioecious forms provide an unusual opportunity to examine the genetic mechanisms that underlie the separation of sexual functions, through crossing experiments and analysis of progeny segregation. Our focus is on the genetic mechanisms distinguishing monoecious and dioecious forms of U. dioica. A complicated picture of sex determination in this species has resulted from crosses between dioecious and monoecious subspecies, as well as between dioecious and monoecious forms of the same subspecies. Most significant is evidence for a maternal influence on sex determination and for the possibility of gynodioecy as an intermediate stage in the evolutionary pathway to dioecy. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Hybridization, polyploidy, and evolutionary transitions between monoecy and dioecy in Bryonia (Cucurbitaceae)

Correns's 1903 (Berichte der Deutschen Botanischen Gesellschaft 21: 133-147) crosses between a monoecious and a dioecious species of Bryonia revealed the simple Mendelian inheritance of dioecy and provided the first instance of an XY sex determination system in any organism. Bryonia ranges from the Canary Islands to Central Asia and comprises seven dioecious and three monoecious species; its closest relative, Ecballium elaterium, has dioecious and monoecious populations. We used chloroplast (cp) and nuclear (nr) gene phylogenies to infer sexual system evolution in Bryonia. We also tested for associations between sexual system and ploidy level, based on published and original chromosome counts. Conflicts between cp and nr topologies imply that the dioecious hexaploid B. cretica arose from hybridization(s), probably involving the dioecious diploids B. dioica, B. syriaca, and/or B. multiflora. A tetraploid dioecious endemic on Corsica and Sardinia probably originated from B. dioica via autopolyploidy. While the cp phylogeny resolves few species relationships, the nr tree implies at least two evolutionary changes in sexual system. There is no correlation between sexual system and ploidy level. Molecular clocks suggest that the deepest divergence, between a species on the Canary Islands and the ancestor of all remaining species, occurred ca. 10 million years ago.     

Breeding systems and phylogenetic diversity of seed plants along a large‐scale elevational gradient

Aim The factors affecting the distribution of dioecious species have received surprisingly little attention and, as a consequence, are poorly understood. For example, there is a well‐documented negative association between dioecy and latitude, for which we have no candidate mechanisms. Conversely, it has been suggested that the relative proportion of dioecious species should be positively correlated with changes in elevation. Location Costa Rica, Central America. Methods We made use of data on the distribution of 175 seed plant species from a series of plots along a transect in Costa Rica that ranged from 30 to 2600 m a.s.l. to test the prediction that dioecy is correlated with elevation. Specifically, we examined relationships between the proportion of dioecy, at the species and individual levels, and the elevation, species richness, number of individuals, and phylogenetic diversity (PD) of plots. For comparison, we repeated all analyses with monoecious species, which also have unisexual flowers but do not suffer from reduced mate assurance and the seed shadow effect that may be the outcomes of having spatially separated sexes. Results The relative proportions of dioecious species and individuals displayed a unimodal relationship with elevation, both peaking at 750 m a.s.l. In contrast, the relative proportions of monoecious species and individuals displayed negative associations with elevation. In addition, the proportion of dioecious species was significantly positively associated with species richness and with the PD of plots. The proportion of monoecious species was not associated with species richness and was weakly positively associated with the PD of plots. Main conclusions Our results suggest that the selection pressure of elevation on the pollinators and life‐history strategy of dioecious species is much less than expected, and is overshadowed by the as‐yet unexplained correlation of dioecy with species richness. Additional studies of the ecology of cosexual and unisexual (i.e. dioecious and monoecious) species along large environmental gradients are needed.     

Chloroplast haplotype variation among monoecious and dioecious populations of Sagittaria latifolia (Alismataceae) in eastern North America

Aquatic plants commonly have extensive geographical distributions, implying few restrictions to dispersal. Here we investigate the postglacial history of an aquatic plant with contrasting sexual systems (monoecy and dioecy), which are predicted to affect dispersal ability. We examined the distribution of cpDNA haplotypes using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) among 76 populations (32 monoecious, 38 dioecious, two mixed and four undetermined populations) of Sagittaria latifolia sampled throughout eastern North America. We also use these data to investigate the polarity of the evolutionary transition between monoecy and dioecy. Using PCR-RFLP, we identified eight cpDNA haplotypes. All haplotypes were found in unglaciated areas of the species' range, clustered primarily in the southeastern United States, providing evidence that glacial refugia probably occurred in this area. Genetic diversity (hT) was more than six times greater among monoecious compared to dioecious populations. All seven of the haplotypes for which the sexual system could be determined were represented among monoecious populations. In contrast, only four haplotypes were detected in dioecious populations and 94% of individuals from dioecious populations possessed a single haplotype. Monoecious populations possessing this widespread haplotype were restricted to the southern portion of the range, indicating that dioecy probably originated in this region and then spread northwards. The distribution of cpDNA haplotypes in dioecious populations represents a subset of the variation found in monoecious populations, a pattern expected if dioecy has evolved from monoecy in S. latifolia.     

Life-history differentiation and the maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae)

The existence of monoecious and dioecious populations within plant species is rare. This limits opportunities to investigate the ecological mechanisms responsible for the evolution and maintenance of these contrasting sexual systems. In Sagittaria latifolia, an aquatic flowering plant, monoecious and dioecious populations exist in close geographic proximity but occupy distinct wetland habitats differing in the relative importance of disturbance and competition, respectively. Life-history theory predicts contrasting evolutionary responses to these environmental conditions. We propose that the maintenance of monoecy and dioecy in S. latifolia is governed by ecological selection of divergent life-history strategies in contrasting habitats. Here we evaluate this hypothesis by comparing components of growth and reproduction between monoecious and dioecious populations under four conditions: natural populations, a uniform glasshouse environment, a common garden in which monoecious and dioecious populations and their F1 progeny were compared, and a transplant experiment using shaded and unshaded plots in a freshwater marsh. Plants from dioecious populations were larger in size and produced heavier corms in comparison with monoecious populations. Monoecious populations flowered earlier and produced more flowers, clonal ramets, and corms than dioecious populations. The life-history differences between the sexual systems were shown to have a quantitative genetic basis, with F1 progeny generally exhibiting intermediate trait values. Survival was highest for each sexual system in field plots that most closely resembled the habitats in which monoecious (unshaded) and dioecious (shaded) populations grow. These results demonstrate that monoecious and dioecious populations exhibit contrasting patterns of investment in traits involved with growth and reproduction. Selection for divergent life histories between monoecious and dioecious populations of S. latifolia appears to be the principal mechanism maintaining the integrity of the two sexual systems in areas of geographic overlap.     

The evolution of dioecy, heterodichogamy, and labile sex expression in Acer

The northern hemisphere tree genus Acer comprises 124 species, most of them monoecious, but 13 dioecious. The monoecious species flower dichogamously, duodichogamously (male, female, male), or in some species heterodichogamously (two morphs that each produce male and female flowers but at reciprocal times). Dioecious species cannot engage in these temporal strategies. Using a phylogeny for 66 species and subspecies obtained from 6600 nucleotides of chloroplast introns, spacers, and a protein-coding gene, we address the hypothesis (Pannell and Verdú, Evolution 60: 660-673. 2006) that dioecy evolved from heterodichogamy. This hypothesis was based on phylogenetic analyses (Gleiser and Verdú, New Phytol. 165: 633-640. 2005) that included 29-39 species of Acer coded for five sexual strategies (duodichogamous monoecy, heterodichogamous androdioecy, heterodichogamous trioecy, dichogamous subdioecy, and dioecy) treated as ordered states or as a single continuous variable. When reviewing the basis for these scorings, we found errors that together with the small taxon sample, cast doubt on the earlier inferences. Based on published studies, we coded 56 species of Acer for four sexual strategies, dioecy, monoecy with dichogamous or duodichogamous flowering, monoecy with heterodichogamous flowering, or labile sex expression, in which individuals reverse their sex allocation depending on environment-phenotype interactions. Using Bayesian character mapping, we infer an average of 15 transformations, a third of them involving changes from monoecy-cum-duodichogamy to dioecy; less frequent were changes from this strategy to heterodichogamy; dioecy rarely reverts to other sexual systems. Contra the earlier inferences, we found no switches between heterodichogamy and dioecy. Unexpectedly, most of the species with labile sex expression are grouped together, suggesting that phenotypic plasticity in Acer may be a heritable sexual strategy. Because of the complex flowering phenologies, however, a concern remains that monoecy in Acer might not always be distinguishable from labile sex expression, which needs to be addressed by long-term monitoring of monoecious trees. The 13 dioecious species occur in phylogenetically disparate clades that date back to the Late Eocene and Oligocene, judging from a fossil-calibrated relaxed molecular clock.     

Sex determination and the evolution of dioecy from monoecy in Sagittaria latifolia (Alismataceae)

The role of mutations of small versus large effect in adaptive evolution is of considerable interest to evolutionary biologists. The major evolutionary pathways for the origin of dioecy in plants (the gynodioecy and monoecy-paradioecy pathways) are often distinguished by the number of mutations involved and the magnitude of their effects. Here, we investigate the genetic and environmental determinants of sex in Sagittaria latifolia, a species with both monoecious and dioecious populations, and evaluate evidence for the evolution of dioecy via gynodioecy or monoecy-paradioecy. We crossed plants of the two sexual systems to generate F1, F2 and backcross progeny, and grew clones from dioecious populations in low-and high-fertilizer conditions to examine sex inconstancy in females and males. Several lines of evidence implicate two-locus control of the sex phenotypes. In dioecious populations sex is determined by Mendelian segregation of alleles, with males heterozygous at both the male- and female-sterility loci. In monoecious populations, plants are homozygous for alleles dominant to male sterility in females and recessive to female sterility in males. Experimental manipulation of resources revealed sex inconstancy in males but not females. These results are consistent with predictions for the evolution of dioecy via gynodioecy, rather than the expected monoecy-paradioecy pathway, given the ancestral monoecious condition.     

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.     

The evolution and maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae)

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.     

海南尖峰岭热带山地雨林60 ha大样地木本植物性别系统数量特征

热带山地雨林中植物不同性别系统的数量和空间分布特征如何? 是否受自然环境条件影响? 这些问题的回答有助于更深入理解群落物种多样性是如何形成的。本文以海南尖峰岭热带山地雨林60 ha大样地中胸径大于1 cm的木本植物为研究对象, 描述了样地内木本植物性别系统的数量特征, 分析了雌雄异株植物空间分布与地形因子的相关关系。结果表明: 大样地内289种木本种子植物中有两性花植物176种, 单性花植物113种; 在单性花植物中, 51种为雌雄同株, 62种为雌雄异株。单性花植物植株数占所有植株总数的36.2%; 雌雄异株植物植株数占所有植株总数的21.5%, 占单性花植物植株数的59.5%。在20 m × 20 m样方的空间尺度, 雌雄异株植物呈现聚集分布的物种共有31种。以20 m × 20 m样方为分析单元, 雌雄异株植物种类的性别比例与海拔、凹凸度和坡度呈弱正相关; 个体相对多度仅和海拔呈弱正相关, 与坡度和凹凸度无相关; 而3个性别系统多样性指数与海拔、凹凸度和坡度均无显著关联。可见雌雄异株植物在海南热带山地雨林中占据了较大的比例, 但大部分种类种群较小, 其数量分布特征与地形因素紧密相关, 雌雄异株植物的存在对群落物种多样性的形成有较大贡献。     

Sex in advertising: dioecy alters the net benefits of attractiveness in Sagittaria latifolia (Alismataceae)

The flowers and inflorescences of animal-pollinated dioecious plants are generally small and inconspicuous in comparison with outcrossing cosexual species. The net benefits of an attractive floral display may be different for dioecious compared to cosexual populations because dioecious species experience a more severe reduction in pollen delivery when pollinators forage longer on fewer individuals. Here, we develop a model that predicts the decrease in pollen delivery in dioecious relative to cosexual populations from female-female, female-male and male-male visit sequences as the number of individuals visited varies. To evaluate the predictions of our model we conducted a common garden experiment with dioecious and monoecious (cosexual) arrays of the insect-pollinated herb Sagittaria latifolia. We find that, although increasing the advertisements of floral rewards (i.e. increasing floral display) attracts more pollinators to individuals, the probability that these pollinators subsequently deliver pollen to neighbouring plants depends on sexual system. Because the number of individual plants visited per foraging trip did not increase significantly with floral display, the relative pollination success of dioecious versus monoecious populations decreases with increased floral display. We propose that this could explain why dioecy is strongly correlated with reduced floral display among angiosperm species.     

The mating system ofElatostema (Urticaceae) in relation to morphology: A comparative study

To investigate why some species are dioecious and others monoecious, a data set of 14 morphological characters in 106 species ofElatostema was regressed onto mating system using partial multiple regression (controlling for subgeneric classification to avoid spurious associations). Morphology and mating system are significantly related (p = 0.05), and dioecious species tend to have large stipules, woody habit, and large leaves. The correlation of dioecy with large stipules is reported for the first time, but this character, like woodiness, is an indicator of plant size, suggesting a functional relationship of large plants (of the under-canopy) with dioecy.     

Phylogeny of the Cucurbitales based on DNA sequences of nine loci from three genomes: implications for morphological and sexual system evolution

The Cucurbitales are a clade of rosids with a worldwide distribution and a striking heterogeneity in species diversity among its seven family members: the Anisophylleaceae (29-40 species), Begoniaceae (1400 spp.), Coriariaceae (15 spp.), Corynocarpaceae (6 spp.), Cucurbitaceae (800 spp.), Datiscaceae (2 spp.), and Tetramelaceae (2 spp.). Most Cucurbitales have unisexual flowers, and species are monoecious, dioecious, andromonoecious, or androdioecious. To resolve interfamilial relationships within the order and to polarize morphological character evolution, especially of flower sexual systems, we sequenced nine plastids (atpB, matK, ndhF, rbcL, the trnL-F region, and the rpl20-rps12 spacer), nuclear (18S and 26S rDNA), and mitochondrial (nad1 b/c intron) genes (together approximately 12,000 bp) of 26 representatives of the seven families plus eight outgroup taxa from six other orders of the Eurosids I. Cucurbitales are strongly supported as monophyletic and are closest to Fagales, albeit with moderate support; both together are sister to Rosales. The deepest split in the Cucurbitales is that between the Anisophylleaceae and the remaining families; next is a clade of Corynocarpaceae and Coriariaceae, followed by Cucurbitaceae, which are sister to a clade of Begoniaceae, Datiscaceae, and Tetramelaceae. Based on this topology, stipulate leaves, inferior ovaries, parietal placentation, and one-seeded fruits are inferred as ancestral in Cucurbitales; exstipulate leaves, superior ovaries, apical placentation, and many-seeded fruits evolved within the order. Bisexual flowers are reconstructed as ancestral, but dioecy appears to have evolved already in the common ancestor of Begoniaceae, Cucurbitaceae, Datiscaceae, and Tetramelaceae, and then to have been lost repeatedly in Begoniaceae and Cucurbitaceae. Both instances of androdioecy (Datisca glomerata and Schizopepon bryoniifolius) evolved from dioecious ancestors, corroborating recent hypotheses about androdioecy often evolving from dioecy.     

The role of hybridization in the evolution of sexual system diversity in a clonal,aquatic plant

The stable coexistence within populations of females, males, and hermaphrodites (subdioecy) is enigmatic because theoretical models indicate that maintenance of this sexual system involves highly restricted conditions. Subdioecy is more commonly interpreted as a transitory stage along the gynodioecious pathway from hermaphroditism to dioecy. The widespread, North American, aquatic plant Sagittaria latifolia is largely composed of monoecious or dioecious populations; however, subdioecious populations with high frequencies of hermaphrodites (mean frequency = 0.50) characterize the northern range boundary of dioecy in eastern North America. We investigated two hypotheses for the origin of subdioecy in this region. Using polymorphic microsatellite loci, we evaluated whether subdioecy arises through selection on standing genetic variation for male sex inconstancy in dioecious populations, or results from hybridization between monoecious and dioecious populations. We found evidence for both pathways to subdioecy, although hybridization was the more common mechanism, with genetic evidence of admixture in nine of 14 subdioecious populations examined. Hybridization has also played a role in the origin of androdioecious populations in S. latifolia, a mechanism not often considered in the evolution of this rare sexual system. Our study demonstrates how hybridization has the potential to play a role in the diversification of plant sexual systems.     

ECOLOGICAL CORRELATES OF REPRODUCTIVE TRAITS OF MEXICAN RAIN FOREST TREES

Sexual systems of 139 tree species from a tropical rain forest at Los Tuxtlas, Mexico were investigated to: 1) estimate the relative proportions of hermaphroditic, monoecious, and dioecious species; 2) describe flowers, fruits, and seeds in terms of size and weight; 3) describe flowering and fruiting phenology; and 4) correlate sexuality to pollination and dispersal syndromes, and the successional status occupied in the forest. Hermaphroditism occurred in 63% of the species, monoecism in 9%, and dioecy in 27%. Nondioecious species had larger flowers, but dioecious species had more seeds per fruit. The dioecious condition was associated with small flowers pollinated with unspecialized insects and fleshy fruits dispersed by animals at both species and generic levels. Reproductive traits were more correlated among nondioecious species than dioecious species. Pioneer species had more seeds per fruit, and longer flowering and fruiting periods, but persistent species produced heavier seeds and fruits. Flower and fruit morphological traits, sexual systems, and tree guilds are related in a comprehensive way, and a flow model based on data from this study is presented.     

The incidence and pattern of copollinator diversification in dioecious and monoecious figs

Differences in breeding system are associated with correlated ecological and morphological changes in plants. In Ficus, dioecy and monoecy are strongly associated with different suites of traits (tree height, population density, fruiting frequency, pollinator dispersal ecology). Although approximately 30% of fig species are pollinated by multiple species of fig‐pollinating wasps, it has been suggested that copollinators are rare in dioecious figs. Here, we test whether there is a connection between the fig breeding system and copollinator incidence and diversification by conducting a meta‐analysis of molecular data from pollinators of 119 fig species that includes new data from 15 Asian fig species. We find that the incidence of copollinators is not significantly different between monoecious and dioecious Ficus. Surprisingly, while all copollinators in dioecious figs are sister taxa, only 32.1% in monoecious figs are sister taxa. We present hypotheses to explain those patterns and discuss their consequences on the evolution of this mutualism.     

Sexual systems in gymnosperms: A review

The aim of this study was to update figures for the presence of dioecy among the gymnosperms and investigate its correlation with climate, growth form, pollination and seed dispersal syndromes, and risk of extinction. Dioecy was found in almost 65% of contemporary gymnosperm species, a higher percentage than previous estimates. It dominates in 8 of the 12 families. As in angiosperms, dioecious gymnosperms are particularly common in climbers and are more commonly found in tropical climates. Analysis of the degree of threat using IUCN red list categories showed that the proportion of threatened species is higher in dioecious than in monoecious species only in temperate climate. The high sensitivity of dioecious species to environmental changes associated with human activity in temperate climate may explain this phenomenon. The monophyly of extant gymnosperms and the relatively small number of species (about 1000) create the possibility of treating them as a model group in investigating the evolution of sexual systems.     

Flower structure and sex expression of leaky dioecy in Eurya obtusifolia

The genus Eurya is described as strictly dioecious, but rare leaky plants have been found in some species, causing much confusion about the sex expression of the genus. Through field investigations from 2009 to 2011, morphologically hermaphroditic flowers in Eurya obtusifolia were discovered. In order to understand the sex expression of these plants, their stamens, pistils and ovules were carefully observed. Staining methods were used to assess the functional gender. The flowers of the leaky plants can be classified into six types: pistillate, staminate, cryptic pistillate, cryptic staminate, hermaphrodite and infertile. Leaky plants usually exhibit a combination of different kinds of flowers, making them either gynoecious, androecious, gynomonoecious or monoecious. Some individuals have infertile flowers. Bagging experiments verified the bisexual function of E. obtusifolia plants, some of which possess the ability to self‐pollinate. The flower morphology of leaky plants varied more than that of dioecious ones, and the number of ovules were significantly negatively correlated with the number of stamens. These plants show lower fitness than normal dioecious plants. This may owe to allocation tradeoffs or sexual genome conflicts. As leaky dioecy is rarely reported in this genus, E. obtusifolia is an important species to study in order to better understand the ecological adaptations and evolutionary pathways that lead to dioecy in Eurya. Our findings provide some evidence that dioecy of Eurya evolved from hermaphroditism, but further studies are still needed.