ECOLOGICAL ASPECTS OF SEX EXPRESSION IN SUBDIOECIOUS SCHIEDEA GLOBOSA (CARYOPHYLLACEAE)

The importance of ecological factors such as sex lability, spatial segregation, and resource allocation in the evolution of dioecy were examined in Schiedea globosa. S. globosa is a subdioecious species with equal numbers of plants possessing strictly male or female function and a small proportion of hermaphrodites. The propensity for labile sex expression was under both environmental and genetic control; some plants with male function became hermaphroditic (by producing female flowers) under better growing conditions in the field and in the greenhouse. There was some spatial segregation of the sexes. Because of sex lability, more hermaphrodites than males occurred on moister slopes. Although there were not measurable sex-related differences in mortality within or between two flowering seasons, more females than males and hermaphrodites occurred at the bottom of slopes. Males and females produced the same number of ramets and inflorescences, but females had a greater number of flowers per inflorescence. Males and females had the same number of ovules (vestigial in males), but females had larger ovules and longer stigmas. Hermaphrodites and males had the same amount of pollen per flower despite the production of fruit by the hermaphrodites. In hermaphrodites, there was no apparent tradeoff within flowers between pollen production and ovule production. These results indicate that spatial segregation, sex lability, and environmental conditions influence allocation patterns of S. globosa, and in combination with high inbreeding depression and selling rates, may promote the further evolution of dioecy in S. globosa.     

Morphological variation in the flowers of Jacaratia mexicana A. DC. (Caricaceae), a subdioecious tree

The Caricaceae is a small family of tropical trees and herbs in which most species are dioecious. In the present study, we extend our previous work on dioecy in the Caricaceae, characterising the morphological variation in sexual expression in flowers of the dioecious tree Jacaratia mexicana . We found that, in J. mexicana , female plants produce only pistillate flowers, while male plants are sexually variable and can bear three different types of flowers: staminate, pistillate and perfect. To characterise the distinct types of flowers, we measured 26 morphological variables. Our results indicate that: (i) pistillate flowers from male trees carry healthy-looking ovules and are morphologically similar, although smaller than, pistillate flowers on female plants; (ii) staminate flowers have a rudimentary, non-functional pistil and are the only flowers capable of producing nectar; and (iii) perfect flowers produce healthy-looking ovules and pollen, but have smaller ovaries than pistillate flowers and fewer anthers than staminate flowers, and do not produce nectar. The restriction of sexual variation to male trees is consistent with the evolutionary path of dioecy from hermaphrodite ancestors through the initial invasion of male-sterile plants and a subsequent gradual reduction in female fertility in cosexual individuals (gynodioecy pathway), but further work is needed to confirm this hypothesis.     

Functional dioecy in <Emphasis Type="Italic">Morinda parvifolia</Emphasis> (Rubiaceae), a species with stigma-height dimorphism

The evolution of dioecy from heterostyly has been well documented, but detailed studies on this transitional process are rare. Here we report the occurrence of cryptic dioecy in a perennial liana species with stigma-height dimorphism, Morinda parvifolia Bartl. ex DC. (Rubiaceae). Floral morphology, ancillary characters and cross compatibility of long-styled (L-morph) and short-styled (S-morph) were examined. L-morph and S-morph display obvious pistil dimorphisms, with the stigma of S-morph lacking papillae cells. Both floral morphs show similar pollen morphology, although pollen viability is higher in S-morph than in L-morph. S-morph flowers produce viable pollen grains but much reduced stigma and set no fruits, functioning as males; L-morphs, although with viable pollen grains and receptive stigmas, exhibit strong self- and intramorph incompatibility, with self- and intramorph pollen tubes arrested in the stigma lobes and the upper part of style, respectively, resulting in L-morphs functioning only as females. The species thus has physiological androdioecy but functional dioecy. This might be the first case showing the possibility that androdioecy could be a mid-stage in the pathway of dioecy evolving from stigma-height dimorphism.     

The dioecious, sphingophilous species Citharexylum myrianthum (Verbenaceae): Pollination and visitor diversity

Citharexylum myrianthum is an example of cryptic dioecy in morphologically perfect flowers. Flowers are small, tubular, white-colored, and crepuscular, with a sweet, pleasant scent and occur in raceme-like inflorescences. Functionally, male flowers have a less developed ovary and females have anthers lacking pollen. They are pollinated by five species of Sphingidae. The low fruit set under natural conditions, compared with the very high fruit set after hand-pollination, apart from other possible factors, suggests that pollen flow is limited mainly by pollinators. The overabundance of nectar provided by C. myrianthum flowers attracts a large number of opportunistic nectar foragers in the morning. Hummingbirds, the most frequent visitors, are nectar thieves and display territorialism, being aggressive even to Passeriformes. Despite its visitor diversity C. myrianthum is pollinated by sphingids, and in this sense this species is a specialist rather than a generalist.     

Synchronized dichogamy and dioecy in neotropicalLauraceae

In hermaphrodite neotropicalLauraceae a highly evolved dichogamous system is present which represents a kind of temporal dioecy. This system involves the existence of two flower morphs which are characterized by reciprocal phases of receptivity of the stigmas and pollen release. In some genera (Persea, Cinnamomum), nectar is produced as a reward for the flower visitors, while in other genera (Aniba, Clinostemon, Licaria), nectar is absent and pollen seems to be the only reward. This implies that in this case the flowers in the female stage must be deceptive flowers. In dioecious species of the generaOcotea andNectandra, both the male and female flowers attract the visitors with nectar. The pollen-ovule ratio of theLauraceae is comparatively low. — The type of reproductive system that characterizes theLauraceae, comprising functional dioecy, small, inconspicuously coloured flowers, pollination by small bees, and large, one-seeded fruits dispersed by birds, is quite prominent among trees of various families in the tropical lowland forest. The relationship between the different modes of flowering within theLauraceae and the causes for the correlation of their reproductive traits are discussed.     

Pollination of Oncocyclus irises (Iris: Iridaceae) by night-sheltering male bees

Irises in the section Oncocyclus (Siems.) Baker ( IRIS: Iridaceae) grow throughout the Middle East and have large and dark-coloured flowers but no nectar reward available to flower visitors. Consequently, no reward-collecting pollinators have been observed visiting the flowers during daytime. The only visitors are solitary male bees ( Eucera spp.: Apidae) that enter the flowers at dusk and stay there overnight. Here we describe the mating system of Oncocyclus irises, and the role of night-sheltering male bees in their pollination system. Pollen viability in I. haynei on Mt. Gilboa was very high (>90%) throughout all floral life stages. Stigmas were receptive in buds and in open flowers, but not in older ones. Self-pollination yielded no fruits in three species, confirming complete self-incompatibility in Oncocyclus irises. On average, 1.9 flowers were visited by each male bee before it settled for the night in the last one. Moreover, Iris pollen was present on the dorsal side of 38.8% of males caught sheltering in flower models mounted near an I. atrofusca population, indicating that pollen is transferred between flowers by night-sheltering solitary male bees. We have surveyed 13 flowering populations of six Oncocyclus species for the presence of night-sheltering male bees as well as for fruit set. We found a positive correlation, indicating that sexual reproduction in Oncocyclus irises is dependent on night-sheltering solitary male bees. Based on their complete self-incompatibility, the absence of nectar-collecting visitors during the day, and the transfer of pollen grains by the night-sheltering solitary male bees, we conclude that fertilization of Oncocyclus irises is totally dependent on pollination by night-sheltering solitary male bees.     

Pollination in a cryptically dioecious plant Decaspermum parviflorum (Lam.) A. J. Scott (Myrtaceae) by pollen-collecting bees in Sulawesi, Indonesia

Decaspermum parviflorum is a common shrub or treelet in clearings in eastern Sulawesi, Indonesia. It is widely distributed in SE Asia. It produces numerous white flowers and small blue-black fruits. Individual shrubs were of two types in 1:1 ratio, one bearing staminate flowers, the other bearing flowers with both stamens and carpels. The 'hermaphrodite' flowers were shown to have sterile pollen so the species is functionally dioecious. The flowers and floral sprays of the two sexes differed in several measurements: the male flowers had more anthers and were larger and more numerous, making the corporate visual image of male sprays more than twice as large as that for sprays of female flowers. Flowers, individuals of which last only one day, opened only every second day, when all bushes flower synchronously. When the anthers burst in the morning, pollen and sterile pollen was collected vigorously by a variety of bees, mostly Apis dorsata and Nomia spp. Most of the pollen was collected within 1 h of anthesis. Anthesis in males takes place about 20 min earlier than in females; insect visitation follows this pattern. The minute quantity of nectar was collected by only few insects and mostly after the pollen had gone, if at all. Fruits were eaten by birds. The species displays many features typical of dioecious tropical plants with the unusual feature of pollen being the main food reward for pollinators. It was pollinated effectively in Sulawesi.     

FUNCTIONAL DIOECY AND ANDROMONOECY IN SOLANUM

Field and laboratory studies of 19 diclinous species endemic to Australia help to clarify the nature and evolution of andromonoecy, androdioecy, and dioecy in the genus Solanum. Ten species are andromonoecious; typically these species bear inflorescences with a single, large basal hermaphroditic flower and 12–60 distal, smaller staminate flowers. We suggest that the andromonoecious condition was derived from hermaphroditic-flowered ancestors in part by hemisterilization of flowers but largely by addition of staminate flowers. The resultant larger inflorescences are hypothesized to serve both to attract and to entrain pollinators, yielding more or higher-quality seed set in hermaphroditic flowers and/or greater dispersion of pollen from staminate flowers. We suggest that andromonoecy may also serve to reduce selling. Nine other species are morphologically androdioecious but functionally dioecious. In these species, staminate flowers, like those of the andromonoecious species, bear anthers with copious tricolporate pollen and a highly reduced gynoecium. The morphologically hermaphroditic flowers are functionally pistillate and borne singly in inflorescences, and they bear anthers with inaperturate pollen. The inaperturate pollen, although viable, never germinates and is hypothesized to be retained in pistillate flowers as a reward to pollinators in the nectarless Solanum flowers. All other species of Solanum studied with pollen dimorphism in which one pollen morph is inaperturate are also best treated as functionally dioecious. We conclude that there is no evidence for androdioecy in Solanum. A review of other families suggests that there is little support for this unusual breeding system in any other angiosperm group either. Preliminary analyses suggest that andromonoecy and dioecy are polyphyletic in Solanum. Furthermore, dioecy is as likely to have arisen from hermaphroditic as from andromonoecious ancestors.     

白背叶(大戟科)性系统和传粉生物学研究

为探讨野桐属(Mallotus)雌雄异株的进化和传粉机制,对白背叶(Mallotus apelta)野生居群的性系统和传粉生物学进行了研究。结果表明,所调查的白背叶居群均由雌性(F)、雄性(M)和少数两性(B)个体组成,平均性比为1：0.66：0.18(F：M：B);3种性别植株的基径大小差异不显著;雌株与两性植株的单花胚珠数、单果结籽数和自然结实率无显著差异;雄株与两性植株的花粉萌发率、花粉组织化学和花粉微形态特征也无显著差异,但雄株的单花花粉量是两性植株的1.26~1.63倍,且差异显著;雌株的异交结实率为96.67%,两性植株的异交结实率为76.00%,两者差异显著,说明居群内雌株的潜在种子生产力明显高于两性植株;野外观察到雄株和两性植株上的雄花具有访花者而雌株没有;雌株经套网处理后结实率超过30%而套袋处理不结实。这些表明白背叶具典型的亚雌雄异株性系统,雌株和雄株的适合度均高于两性植株;雌株以风媒传粉结实,两性植株可能兼有风媒和虫媒传粉特征。     

Sexual dimorphism and fruit production in a dioecious understory tree,Ilex opaca Ait.

Summary This study suggested that sexual selection is potentially an important factor in the maintenance of dioecy in the American holly, Ilex opaca (Aquifoliaceae). Sexual dimorphisms in flower production and phenology were highly significant in this understory tree. On average, individual males produced 7.4 times as many flowers as did female trees. Staminate flowers lasted only a single day, whereas pistillate flowers lasted 3–4 days, during which they showed no significant decline in their ability to produce fruit after pollination. Individual male trees opened their flower buds asynchronously during the season, maximizing the number of days they were in flower. Individual females opened their buds more synchronously, maximizing their floral display at one point in time. Females produced fruits in numbers that were somewhat less than proportional to their flower production. Fruit development was initiated from only 38.9% and 69.5% of pistillate flowers in 1987 and 1988, respectively. By the time of ripening, an average female had lost 62.3%, 24.3%, and 11.1% of its initial fruit crop in 1986, 1987, and 1988, respectively. The proportion of fruit lost in 1986 was independent of the number of fruit that initially began development. In 1988, artifically supplementing pollen to a large number of flowers failed to increase either fruit or seed production relative to control branches with unsupplemented flowers. This suggested that resource levels were likely more important than pollen availability in limiting female reproductive success. These observations on I. opaca were consistent with the expectations for a population in which male reproductive success continues to benefit from continued pollinator service and female reproductive success does not.     

Dioecy and wind pollination in Pernettya rigida (Ericaceae) of the Juan Fernández Islands

Pernetttya rigida is endemic to the Juan Fernández archipelago. Although all flowers are complete, with seemingly fertile stamens and pistils, differences in fruit production and detailed field, anatomical, and morphological studies indicate they are functionally unisexual, and the species is consequently dioecious. A comparison of 15 features demonstrated sigdicant differences between the sexes. The populational sex ratio is 1:1. Nectaries located between the filaments produce small amounts of floral nectar with similar sugar composition in both sexes. There are =33,357 (or = 133,429) pollen tetrads (or pollen grains)/male flower and =109 ovules/female flower. No tetrads in either hand self-pollinated or open pollinated male flowers showed any pollen germination. Tetrads on open- and hand-pollinated female flowers germinate. Female flowers do not show autogamy or apomixis. During more than 80 hours of field observation, we recorded only seven floral visitors (representing three insect species). In spite of this, openpollinated female flowers have abundant fruit and seed-set. Thus, we conclude that pollen is transferred abiotically and the ever-present wind over the exposed ridges of the islands is the likely dispersal agent. A number of anemophilous features, such as dry pollen and exposed habitat, support this conclusion. Thus, dioecy and anemophily have evolved independently, in situ , in this species in this remote locality. Preservation of habitat and elimination of competitive invasives are the primary conservation challenges.     

Dioecy inHonkenya peploides var.major (Caryophyllaceae)

Sexual difference and pollination system inHonkenya peploides var.major were investigated. The present analyses indicate that sex expression of this plant is functionally dioecious, rather than androdioecious, as described previously. Its flowers are largely pollinated by small bees, hover-flies, flies, and ants. Female flowers were morphologically characterized by conspicuously longer styles and smaller petals, and had no pollen grains, while male flowers had shorter styles, larger petals, and longer filaments, and produced viable pollen grains. Although the male flowers possess many ovules in an ovary, they set fruits at a very low frequency in natural conditions and did not produce fruits by pollination experiments. We suggest that the dioecy of this species has been derived from hermaphrodite through gynodioecy, and not directly from hermaphrodite.     

Foraging by Male and Female Solitary Bees with Implications for Pollination

Both male and female solitary bees visit flowers for rewards. Sex related differences in foraging efficiency may also affect their probability to act as pollinators. In some major genera of solitary bees, males can be identified from a distance enabling a comparative foraging-behavior study. We have simultaneously examined nectar foraging of males and females of three bee species on five plant species in northern Israel. Males and females harvested equal nectar amounts but males spent less time in each flower increasing their foraging efficiency at this scale. The overall average visit frequencies of females and males was 27.2 and 21.6 visits per flower per minute respectively. Females flew shorter distances increasing their visit frequency, relative foraging efficiency and their probability to pollinate. The proportion of conspecific pollen was higher on females, indicating higher floral constancy and pollination probability. The longer flights of males increase their probability to cross-pollinate. Our results indicate that female solitary bees are more efficient foragers; females seem also to be more efficient pollinators but males contribute more to long-distance pollen flow.     

Small but attractive: female‐biased nectar production and floral visitors in a dimorphic shrub

• In sexually dimorphic species, hermaphrodite flowers in gynodioecious species or male flowers in dioecious species are often larger and produce more nectar than their conspecific female flowers. As a consequence, hermaphrodite or male flowers frequently receive more pollinator visits.
• Sex ratio, flower size, floral display, nectar production and floral visits were evaluated in two natural populations of Fuchsia thymifolia, a morphologically gynodioecious but functionally subdioecious insect‐pollinated shrub.
• Sex ratio did not differ from the expected 1:1 in the two studied populations. As expected, hermaphrodite flowers were larger than female flowers, but in contrast to the general pattern, hermaphrodite flowers did not produce nectar or produced much less than female flowers. Flower visitors were flies (68%) and bumblebees (24%), both of which showed a preference for female flowers. No sex difference was detected in either flower longevity or floral display across the flowering season.
• Higher nectar production by females may attract more pollinators, and may be a strategy to enhance female reproductive success in this species. Finally, floral dimorphism and insect preferences did not seem to hamper the maintenance of sub‐dioecy or prevent the evolution of dioecy in F. thymifolia.

     

Reproductive biology and relative male and female fitness in a trioecious cactus,Pachycereus pringlei (Cactaceae)

We describe the breeding system of an autotetraploid trioecious cactus, Pachycereus pringlei, provide estimates of the fitnesses of males and females relative to that of hermaphrodites, and discuss the role played by pollinators in the maintenance of three sexual morphs. Relatively high frequencies of females (45%) and males (26%) exist in coastal desert populations around Bahia Kino, Sonora, Mexico. They differ from hermaphrodites in flower size (females only), initiation of the flowering season, number of flowers produced per night and per season, sucrose content of nectar, and, in females, number of fruits produced per season under open pollination and in response to hand-pollination. Major similarities between the sex classes include overall plant size, nectar volume per flower, percent fruit set in open-pollinated flowers of females and hermaphrodites, seed mass and number of seeds per fruit, and pollen mass per flower in males and hermaphrodites. Hermaphrodites are self-compatible, and the selfing rate is high (65%). Levels of inbreeding depression in selfed fruits and seeds appear to be low. Fruit set is strongly pollinator-dependent in females but much less so in hermaphrodites. Relative fitness of males and females, as measured by annual production of pollen or seeds, is at least 1.5 times higher than that of the corresponding sex function in hermaphrodites. Given the high selfing rate and apparent lack of inbreeding depression, these fitness differences are insufficient to explain the occurrence of trioecy in this species.     

The reproductive biology of a distylous tree, Sarcotheca celebica (Oxalidaceae) in Sulawesi, Indonesia

Sarcotheca celebica is a tree endemic lo Sulawesi (and Kabaena Island). Kike many species in the Oxalidaceae it is heterostylous (here distylous). The small red flowers, which last less than one day, are produced in loose inflorescences which bloom over a long flowering season (several months). There are two whorls of stamens in each plant, which overlap in length, perhaps indicating a tristylous origin. Long–styled and short–styled plants differed in the measurements of stamens, styles and pollen grains. Only a small amount of pollen was produced by long–styled plants. Effective insect pollinators were mainly large Hymenoptcra, especially Xylocopa species, which visited many flowers and different trees in rapid succession. A wide diversity of insects visited flowers of both morphs, and numbers visiting each morph were approximately equal. The only successful experimental pollinations were from pollen of short–styled plants onto stigmas of long–styled plants. Automatic self–pollination did not occur. In nature not all flowers produced fruit, some fruits were set with no seeds and a low number of seeds was set in all fruits. The differences in seed set per fruit between short–styled, long–styled and experimental crosses indicate that pollinator visits were insufficient for maximum seed set in some fruits, but selective abortion of seeds must also be occurring. We suggest that S. celebica may represent an intermediate stage in the evolution of dioecy from distyly, with the short–styled flowers making their major genetic contribution through pollen and long–styled flowers through ovules.     

Gender specialization in <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae), a distylous,hummingbird-pollinated treelet

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.     

西方蜜蜂和兰州熊蜂在设施桃园的访花偏好性比较

【目的】为了比较西方蜜蜂 Apis mellifera 和兰州熊蜂 Bombus lantschouensis 在设施桃园内对不同时期桃花的访花偏好性、以及这种偏好性与花粉活力和采集花粉花蜜之间的关系。【方法】记录2种蜂在温室桃园内访问早期花、中期花和晚期花的比例,测定桃花不同时期的花粉活力以及2种蜂携带的花粉活力,观察2种蜂采集花蜜和采集花粉的成功率,统计2种蜂访花过程中桃花所处的枝条数及植株数。【结果】桃花不同时期的花粉活力差异显著,早期花花药未开裂,花粉未释放,中期花花粉活力（58.3%）显著高于晚期花花粉活力（34.2%）（P<0.01）;西方蜜蜂更加偏好访问中期花,对中期花的访问率高达75.3%,显著高于兰州熊蜂对中期花的访问率（49.2%）（P<0.01）;西方蜜蜂携带的花粉活力（92.1%）显著高于兰州熊蜂携带的花粉活力（72.9%）,但是西方蜜蜂采集花粉和采集花蜜的成功率均低于兰州熊蜂（P<0.01）;在访问一定数量的桃花过程中,兰州熊蜂在设施桃园内访问的枝条数和植株数较多,分布范围较广（P<0.01）。【结论】和兰州熊蜂相比,西方蜜蜂对活力花粉的辨别能力更强,更加偏好访问花粉活力较高的花朵,这种偏好性导致其采集花粉花蜜的成功率降低。     

Can nectar properties explain sex‐specific flower preferences in the Adonis Blue butterfly Lysandra bellargus?

1. Field observations in the Swiss Jura mountains showed that males and females of the bivoltine Adonis Blue butterfly Lysandra bellargus Rott. differed significantly in their flower visitation patterns. 2. In both generations, females visited a broader range of available nectar plants than did males. The specific flower visitation patterns of males and females were not affected by the general availability and abundance of potential nectar plant species during both flight periods, indicating high selectivity for nectar plants by both males and females. 3. In addition, the sexes differed in their nectar foraging behaviours: distances between successively visited flowers were significantly longer in males than in females, indicating that male and female butterflies have different foraging strategies. 4. Investigations of nectar characteristics showed that the sexes preferred flowers with different nectar compositions. Males of both generations preferred flowers with high proportions of sucrose and high amounts of total sugar, whereas females preferred flowers with high portions of glucose in their nectar, and, in the spring generation, flowers rich in amino acids. 5. Flowers visited exclusively by males or females in spring differed significantly in their amino acid composition. 6. This clear‐cut pattern did not hold for the autumn generation, most probably due to the limited availability of flowers. 7. The observed nectar foraging patterns underline the importance of adult feeding for longevity and reproduction in butterflies. The findings are particularly relevant for conservation, because L. bellargus is an increasingly threatened species in many European countries.     

Gynodioecy to dioecy: are we there yet?

Background

The ‘gynodioecy–dioecy pathway’ is considered to be one of the most important evolutionary routes from hermaphroditism to separate sexes (dioecy). Despite a large accumulation of evidence for female seed fertility advantages in gynodioecious species (females and hermaphrodites coexist) in support of the first step in the gynodioecy–dioecy pathway, we still have very little evidence for the second step, i.e. the transition from gynodioecy to dioecy.

Scope

We review the literature to evaluate whether basic predictions by theory are supported. To establish whether females'' seed fertility advantage and frequencies are sufficient to favour the invasion of males, we review these for species along the gynodioecy–dioecy pathway published in the last 5 years. We then review the empirical evidence for predictions deriving from the second step, i.e. hermaphrodites'' male fertility increases with female frequency, selection favours greater male fertility in hermaphrodites in gynodioecious species, and, where males and hermaphrodites coexist with females (subdioecy), males have greater male fertility than hermaphrodites. We review how genetic control and certain ecological features (pollen limitation, selfing, plasticity in sex expression and antagonists) influence the trajectory of a population along the gynodioecy–dioecy pathway.

Conclusions

Females tend to have greater seed fertility advantages over hermaphrodites where the two coexist, and this advantage is positively correlated with female frequency across species, as predicted by theory. A limited number of studies in subdioecious species have demonstrated that males have an advantage over hermaphrodites, as also predicted by theory. However, less evidence exists for phenotypic selection to increase male traits of hermaphrodites or for increasing male function of hermaphrodites in populations with high female frequency. A few key case studies underline the importance of examining multiple components of male fertility and the roles of pollen limitation, selfing and plasticity, when evaluating advantages. We conclude that we do not yet have a full understanding of the transition from gynodioecy to dioecy.