Floral constraint resulting from intersexual mimicry in a gynodioecious fig tree

Fig trees (Ficus: Moraceae) are pollinated by female fig wasps (Agaonidae) whose larvae develop inside galled flowers of unusual inflorescences (figs). Most fig trees also support communities of non‐pollinating fig wasps. Figs of different species display great size variation and contain tens to tens of thousands of flowers. Around one‐half the species of fig trees have the gynodioecious breeding system, where female trees have figs that produce seeds and male trees have figs that support development of pollinators. Mutual mimicry between receptive male and female figs ensures that pollinators enter female figs, even though the insects will die without reproducing, but the need to give no sex‐specific cues to the pollinators may constrain differences in size between receptive male and female figs. We compared relationships between inflorescence size and some measures of reproductive success in male and female figs of Ficus montana grown under controlled conditions in the presence of the pollinator Kradibia tentacularis and its main parasitoid Sycoscapter sp. indesc. Female figs that contained more flowers produced more seeds, but male figs did not increase the production of female pollinator K. tentacularis fig wasps in proportion of the flower number. Although more flowers were galled by the pollinators in male figs containing more female flowers, the high larval mortality caused by parasitism and nutritional limitation prevented the increase in the production of adult female offspring. Selection may favor the increase in flower numbers within figs in female plants of F. montana, but contrarily constrain this attribute in male plants.     

Expression of AODEF,a B-functional MADS-box gene,in stamens and inner tepals of the dioecious species Asparagus officinalis L.

Garden asparagus (Asparagus officinalis L.) is a dioecious species with male and female flowers on separate unisexual individuals. Since B- and C-functional MADS-box genes specify male and female reproductive organs, it is important to characterize these genes to clarify the mechanism of sex determination in monoecious and dioecious species. In this study, we isolated and characterized AODEF gene, a B-functional gene in the development of male and female flowers of A. officinalis. Southern hybridization identified a single copy of AODEF gene in asparagus genome. Northern blot analysis showed that this gene was specifically expressed in flower buds and not in vegetative tissues. In situ hybridization showed that during early hermaphrodite stages, AODEFgene was expressed in the inner tepal and stamen whorls (whorls 2 and 3, respectively), but not in the outer tepals (whorl 1), in both male and female flowers. In late unisexual developmental stages, the expression of AODEF gene was still detected in the inner tepals and stamens of male flowers, but the expression was reduced in whorls 2 and 3 of female flowers. Our results suggest that AODEF gene is probably not involved in tepal development in asparagus and that the expression of AODEF gene is probably controlled directly or indirectly by sex determination gene in the late developmental stages.     

Characterization of unisexual flower development in the endangered mahogany tree Swietenia macrophylla King. (Meliaceae)

The selection of candidate plus trees of desirable phenotypes from tropical forest trees and the rapid devastation of the natural environments in which these trees are found have created the need for a more detailed knowledge of the floral and reproductive biology of tropical tree species. In this article, the organogenic processes related to unisexual flower development in tropical mahogany, Swietenia macrophylla , are described. Mahogany inflorescences at different developmental stages were evaluated using scanning electron microscopy or optical microscopy of histological sections. The unisexual flowers of S. macrophylla are usually formed in a thyrse, in which the positions of the female and male flowers are not random. Differences between male and female flowers arise late during development. Both female and male flowers can only be structurally distinguished after stage 9, where ovule primordia development is arrested in male flowers and microspore development is aborted in female flower anthers. After this stage, male and female flowers can be distinguished by the naked eye as a result of differences in the dimensions of the gynoecium. The floral characteristics of S. macrophylla (distribution of male and female flowers within the inflorescence, and the relative number of male to female flowers) have practical implications for conservation strategies of this endangered species.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 529–535.     

天仙果的繁殖生物学研究

研究了天仙果（Ficus erecta Thunb.var.beecheyana(Hook.et Arn.)King）的繁殖生物学特性。春季雌株不形成雌花序,雄瘿株的花序中雄花也不完全发育,只有瘿花发育良好,这些瘿花为培育传粉者（Blastophaga sp.)提供了必需的繁殖及发育场所;夏秋季是天仙果自身的繁殖时期,雌花充雄花花药发育良好。雌花期与雄花期每年在3－4月份及6－8月份相遇。通常进入花序传粉或产卵的小蜂只有1－2只。对天仙果隐头花序的配置及花的解剖研究还表明,其繁殖习性是榕树－榕小蜂共生体系繁殖对策多样性的体现。     

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.     

Developmental analyses reveal early arrests of the spore-bearing parts of reproductive organs in unisexual flowers of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

To understand the regulatory mechanisms governing unisexual flower development in cucumber, we conducted a systematic morphogenetic analysis of male and female flower development, examined the dynamic changes in expression of the C-class floral organ identity gene CUM1, and assessed the extent of DNA damage in inappropriate carpels of male flowers. Accordingly, based on the occurrence of distinct morphological events, we divided the floral development into 12 stages ranging from floral meristem initiation to anthesis. As a result of our investigation we found that the arrest of stamen development in female flowers, which occurs just after the differentiation between the anther and filament, is mainly restricted to the primordial anther, and that it is coincident with down-regulation of CUM1 gene expression. In contrast, the arrest of carpel development in the male flowers occurs prior to the differentiation between the stigma and ovary, given that no indication of ovary differentiation was observed even though CUM1 gene expression remained detectable throughout the development of the stigma-like structures. Although the male and female reproductive organs have distinctive characteristics in terms of organ differentiation, there are two common features regarding organ arrest. The first is that the arrest of the inappropriate organ does not affect the entirety of the organ uniformly but occurs only in portions of the organs. The second feature is that all the arrested portions in both reproductive organs are spore-bearing parts.Abbreviations SEM Scanning electron microscopy - TEM Transmission electron microscopy - TUNEL TdT-mediated dUTP nick-end labeling     

Phenological responses to extreme droughts in a Mediterranean forest

Mediterranean regions are projected to experience more frequent, prolonged and severe drought as a consequence of climate change. We used a retractable rainfall shelter, to investigate the impact of extreme droughts on the development of Quercus ilex leaves, flowers and fruit. In 2008, 97% of rainfall was excluded from a forest plot during the autumn, representing 50% of the 1127 mm of rain that fell during the year. In 2009, 87% of rainfall was excluded during the spring, representing 58% of the 749 mm that fell during the year. The rainfall shelter did not impact neither incident radiation nor air temperature. Autumn rainfall exclusion did not significantly affect leaf, flowers or fruit development. Spring rainfall exclusion resulted in larger and more sustained depression of leaf water potential during the key phases of foliar and floral development. Consequently, only half of the sampled trees (6) reached the shoot lengthening stage which leads to functionally mature leaves (phenophase 4), with one abandoning leaf development at budburst (phenophase 3) and the other two at the bud swelling stage (phenophase 2). All trees of the control plot passed phenophase 4, with most reaching complete leaf development. The impact of extreme droughts on flower development differed between the sexes. The spring exclusion had no effect on male flower, but only one of six trees completed female fruit maturation, compared with four in the control plot. The difference between the male and female drought impacts is likely attributable to the occurrence of male floral development before the period of lowest leaf water potential, and to the lower resource allocation requirements of male flowers. The information provided by our experimental approach may constitute a crucial step to evaluate the impact of increasing drought due to climate change on the most dominant Mediterranean tree species and to help drawing a full picture of the ecological consequences of the decline in water resource on forest dynamics under changing conditions.     

Development-related changes of protein ubiquitination in pollen from male and female kiwifruit (Actinidia deliciosa)

Kiwifruit (Actinidia deliciosa) is a dioecious vine whose staminate and pistillate flowers nonetheless develop non-functional reproductive structures of the ompposite sex. Ubiquitin is a small, highly conserved protein found in all eucaryotes: a covalent ATP-dependent attachment of ubiquitin marks proteins for degradation. In the present paper, we used immunoblotting to investigate the presence of free ubiquitin and ubiquitin conjugates during pollen development in male (androfertile) and in female (androsterile) genotypes of kiwifruit. In the male, several high molecular mass protein conjugates were present throughout development. On the contrary, such a pattern characterized only early stages of pollen from the female genotype, where conjugates progressively disamppeared, until they were detectable only in trace amounts at anthesis. The highest content of conjugates in the male genotype was observed when microspores were ampproaching the first mitosis. Free ubiquitin increased continuously during development of the male microgametophyte so that mature pollen contained considerable amounts of the ubiquitin monomer at the time of its release from the anther. By contrast, only low levels were detectable in the degenerating microspores in the pistillate flowers. In vitro experiments using labeled ubiquitin indicated that early-uninucleate microspores of the female genotype had a much higher conjugation rate than those of the male genotype at the same stage. However, after feeding α-lactalbumin as exogenous substrate, the rate of ubiquitin conjugation strongly increased and was quite similar in both sexes. Nuclear features of pollen development in both genotypes are also described. The nucleus progressively degenerated in the microspores of the pistillate flowers starting from the early-uninucleate stage, in parallel with the progressive decrease in ubiquitin content and activity. At anthesis, the microspores in the pistillate flowers either had no nucleus or showed only traces of chromatin. Thus, the ubiquitin system seems to play an important role in protein turnover occurring during the normal developmental pathway of the kiwifruit microgametophyte, while it was mainly involved in regressive events related to microspore degeneration in the female genotype.     

Reproductive Biology of Erythrina falcata (Fabaceae: Papilionoideae)1

Reproductive phenology, floral biology, degree of self‐incompatibility, and floral visitors of Erythrina falcata were studied in an Argentinean population. Flowering occurs during the dry season from late August to late October. Flower lifetime is 5–6 d. Phylogenetic studies indicate that E. falcata, together with E. fusca and E. crista‐galli, are included in a basal clade within Erythrina. Its phylogenetic position, floral morphology, and nectar characteristics suggest a hummingbird–passerine mixed pollination system. The flowers are nontubular, and the vexillum (the upper petal of the corolla) covers the other remaining floral parts until displaced by a visiting passerine (Icterus cayanensis) or a hummingbird (Amazilia chionogaster). Both birds act as pollen vectors. Bees were observed as occasional pollinators. Nectar production begins at anther dehiscence and coincides with maximum stigmatic receptivity. The base of the keel forms a secondary nectar reservoir. Controlled pollinations showed that this species is self‐incompatible, although a few fruits develop from selfing. Pollen:ovule ratio (43,200:7) is as expected for a xenogamous plant. Only 1 percent of the flowers set seeds under natural conditions. Possible explanations for the low reproductive success are discussed.     

Acer pseudoplatanus (Sapindaceae): Heterodichogamy and thrips pollination

One of the pathways to dioecy is via heterodichogamy, a system including protandrous (flowering male first) and protogynous (female first) plants. Using a research crane the reproductive ecology of the heterodichogamous Acer pseudoplatanus was studied in 74 mature trees over 2 years. The synchronized flowering phenology of the trees resulted in reciprocal pollination between the two morphs. Protandrous trees were more numerous (3:1), had more female flowers (2–3:1), had much less pollen on their stigmas (1:15) and had a much lower seed to fruit ratio (1:3–4). The pollinators were probably breeding thrips. The heterodichogamy of A. pseudoplatanus is confirmed and underlined as a functioning ecological system. Depending on the way pollination efficiency changes in time, either of the morphs can be interpreted as “more female” or “more male”. The evolution of heterodichogamy towards dioecy thus depends on more components of the reproductive ecology than have been assumed.     

Intersexual mimicry and flowering phenology facilitate pollination in a dioecious habitat specialist species, <Emphasis Type="Italic">Myristica fatua</Emphasis> (Myristicaceae)

Myristica fatua is a dioecious specialist species restricted to the endangered, freshwater Myristica swamp forests in the Western Ghats, India. Earlier studies have alluded to pollination by deception in members of the Myristica genus, and thus we examined the pollination ecology comprising floral biology, flower production, flower visitors, and reproductive success in M. fatua and inferred the potential strategies that could permit such deception in this habitat specialist tree. Male flowers provide pollen rewards for an extended period of time while female flowers are rewardless and both sexes are visited by generalist insects, mainly by honeybees and stingless bees. Bee visits were significantly more frequent and longer on male than on female flowers as bees collected pollen from male flowers. We found that flower production patterns create a preponderance of males compared to females in the swamp populations. Using a model of honeybee color vision, we found the distance between the color loci of male and female flowers and based on minimum visual angle subtended by these flowers, we suggest that the two floral sexes cannot be discriminated by bees. Bees are likely deceived by the perceptual similarity of rewardless female flowers to pollen-offering male flowers and pollination is the consequence of foraging errors made by pollinators that encounter largely male–rarely female flower mosaics as they forage among clump-distributed M. fatua trees in the swamp habitat.     

Effects of some natural and synthetic growth regulators on sex-expression of Myrica esculenta Buch-Ham. ex D. Don

Gibberellic acid (GA,), chlorflurenol and ethrel were applied at different concentrations to either male or female trees of Myrica esculenta. GA₃ induced intersexual flowers both on the male and female trees whereas chlorflurenol and ethrel induced similar (intersexual) flowers but only on the male plants. When GA₃ was applied in combination with chlorflurenol, fewer intersexual inflorescences appeared on the male plants along with a significant decline in the number of flowers per inflorescence. In combination, chlorflurenol and ethrel induced copious female and intersexual inflorescences per male plant, especially when two successive treatments of 100 mg/I of chlorflurenol and 1920 mg/l of ethrel were applied. The induced female inflorescences later bore fruits.     

Functional implications of the specialized staminal appendages in alpine ginger (Roscoea spp.: Zingiberaceae)

Floral organs are widely believed to enhance the pollination and reproductive success of angiosperms. However, the functional implication of some floral structures is still unknown. In this study, we explored the functional role of staminal appendages on male and female reproductive success of Himalayan Roscoea spp. and tested if their function differed between species with biotic pollination and autonomous selfing. Phenotypic manipulation is a powerful approach to test the functional effect of a particular trait on plant fitness. We compared various proxies of pollination success between intact flowers and flowers with manually excised staminal appendages. We found that the rate of visitation did not differ between intact and manipulated flowers. Our results revealed that in outcrossing Roscoea spp., the staminal appendages act as triggering devices to facilitate pollen release and deposition and also to manipulate the foraging position of pollinators to ensure both male and female reproductive success. In contrast, in autonomously selfing Roscoea spp., the removal of staminal appendages did not affect any aspect of pollination processes. Our results suggest that the staminal appendages are an integral component of outcrossing in Roscoea spp. and are maintained by selection pressure through both male and female reproductive success. This study provides important insights on how variation in breeding systems can provoke changes in the structure and function of floral organs among congeners.     

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.     

THE STABILITY OF THE SYMBIOSIS BETWEEN DIOECIOUS FIGS AND THEIR POLLINATORS: A STUDY OF FICUS CARICA L. AND BLASTOPHAGA PSENES L.

Each Ficus species depends on a specific mutualistic wasp for pollination. The wasp breeds on the fig, each larva destroying a female flower. It is, however, not known why the wasps have not evolved the ability to use all female flowers. In “dioecious” figs, the wasp can only breed in the female flowers of the “male” trees, so that pollination of a female tree is always lethal. The wasps should therefore be selected to avoid female trees. Field data is presented showing that the fruiting phenology of the dioecious fig Ficus carica is such that this selection does not occur: syconia are not receptive at the same time on “male” and female trees. Most wasps are forced to emerge from the syconia of “male” trees at a time when they will not be able to reproduce, whether they avoid female trees or not. This aspect of the life cycle of the wasp, although noticed, has been obscured in most previous studies. It is shown that the fruiting phenology of Ficus carica, which stabilizes the symbiosis, is the result of short-term selective pressures on the male function of the trees. Such selective pressures suggest a possible pathway from monoecy to dioecy in Ficus under seasonal climates.     

Making the most of your pollinators: An epiphytic fig tree encourages its pollinators to roam between figs

Ficus species are characterized by their unusual enclosed inflorescences (figs) and their relationship with obligate pollinator fig wasps (Agaonidae). Fig trees have a variety of growth forms, but true epiphytes are rare, and one example is Ficus deltoidea of Southeast Asia. Presumably as an adaptation to epiphytism, inflorescence design in this species is exceptional, with very few flowers in female (seed‐producing) figs and unusually large seeds. Figs on male (pollinator offspring‐generating) trees have many more flowers. Many fig wasps pollinate one fig each, but because of the low number of flowers per fig, efficient utilization by F. deltoidea''s pollinators depends on pollinators entering several female figs. We hypothesized that it is in the interest of the plants to allow pollinators to re‐emerge from figs on both male and female trees and that selection favors pollinator roaming because it increases their own reproductive success. Our manipulations of Blastophaga sp. pollinators in a Malaysian oil palm plantation confirmed that individual pollinators do routinely enter several figs of both sexes. Entering additional figs generated more seeds per pollinator on female trees and more pollinator offspring on male trees. Offspring sex ratios in subsequently entered figs were often less female‐biased than in the first figs they entered, which reduced their immediate value to male trees because only female offspring carry their pollen. Small numbers of large seeds in female figs of epiphytic F. deltoidea may reflect constraints on overall female fig size, because pollinator exploitation depends on mutual mimicry between male and female figs.     

The arrest of development of abortive reproductive organs in the unisexual flower of<Emphasis Type="Italic"> Vitis vinifera</Emphasis> ssp.<Emphasis Type="Italic"> silvestris</Emphasis>

During the first stages of development, flowers of most dioecious species are hermaphroditic, with their transition to unisexual flowers being the result of the developmental arrest of one set of reproductive organs. In this work, we describe the development of male and female flowers of the dioecious wild grape species Vitis vinifera ssp. silvestris through scanning electron microscopy analysis and cytological observations, focusing our attention on the transition from bisexual to unisexual development. We divide floral development of the wild grape into eight stages. Differences between male and female flowers appear first at stage 6, when the style and stigma start to differentiate in female but not in male flowers. Cytological analysis of the slowly growing abortive pistil of male flowers shows that megagametophyte formation is, surprisingly, not inhibited. Instead of pistil abortion in the male flower, sexual determination is accomplished through programmed death of external nucellus cells and some layers of integumentary cells. Sterility of male structures in female flowers follows a different pattern, with microspore abnormalities evident from the time of their release from the tetrad. Sterile microspores and pollen grains in female flowers display an abnormal round shape, lacking colpi and possessing uniformly thickened cell walls that impede germination.     

The reproductive biology of two poorly known relatives of the fig (Ficus) and insights into the evolution of the fig syconium

We conducted the first detailed investigation of the floral architecture and reproductive biology of two species from the genus Dorstenia, which are poorly known relatives of Ficus (Moraceae). Our aims were to extend and refine knowledge of the understudied genus Dorstenia and to explore possible insights into the evolution of the fig syconium. We characterised four key stages of floral development using light microscopy, scanning electron microscopy and histological staining. Reproductive biology was found to be complex and species‐specific. Both study species are monoecious and produce an inflorescence of minute male and female flowers. Protogyny, associated with a spatial separation of male and female flowers and asynchronous stamen development, was species‐specific, as was seed set. Our results reveal novel insights into the complex reproductive biology of an under‐studied genus in the family Moraceae. We propose that exploring the reproductive biology of Dorstenia and other poorly known Ficus relatives will provide insights into the evolution of the fig syconium – the unique reproductive structure of this economically and ecologically important genus.     

Adaptive function of duodichogamy: Why do chestnut trees have two pollen emission phases?

Premise

Intersexual mating facilitation in flowering plants has been largely underexplored. Duodichogamy is a rare flowering system in which individual plants flower in the sequence male-female-male. We studied the adaptive advantages of this flowering system using chestnuts (Castanea spp., Fagaceae) as models. These insect-pollinated trees produce many unisexual male catkins responsible for a first staminate phase and a few bisexual catkins responsible for a second staminate phase. We hypothesized that duodichogamy increases female mating success by facilitating pollen deposition on stigmas of the rewardless female flowers through their proximity with attractive male flowers responsible for the minor staminate phase.

Methods

We monitored insect visits to 11 chestnut trees during the entire flowering period and explored reproductive traits of all known duodichogamous species using published evidence.

Results

In chestnuts, insects visited trees more frequently during the first staminate phase but visited female flowers more frequently during the second staminate phase. All 21 animal-pollinated duodichogamous species identified are mass-flowering woody plants at high risk of self-pollination. In 20 of 21 cases, gynoecia (female flower parts) are located close to androecia (male flower parts), typically those responsible for the second minor staminate phase, whereas androecia are often distant from gynoecia.

Conclusions

Our results suggest that duodichogamy increases female mating success by facilitating pollen deposition on stigmas by means of the attractiveness of the associated male flowers while effectively limiting self-pollination.

     

Florivores on the dioecious shrub Eurya japonica and the preferences and performances of two polyphagous geometrid moths on male and female plants

Flowers of dioecious plants have sexually dimorphic traits that may affect florivore performances, and florivores may have preferences to plant sex that are correlated with their performance on different plant sexes. We investigated the florivory on a polygamodioecious evergreen shrub Eurya japonica in Japan to reveal florivores and their feeding patterns involved in sexually biased florivory on E. japonica flowers. Flowers of E. japonica were infested by lepidopteran and dipteran larvae and hemipteran insects. Lepidopteran larvae were chewers, dipteran larvae were gall makers and hemipteran insects were suckers. Chewed flowers were most frequent among infested flowers. Of florivores, lepidopteran larvae, mostly of Geometridae, were the important florivore that damaged flowers by chewing. Florivores infested male flower buds more often than female flower buds, but only a geometrid larvae Chloroclystis excise, which exclusively uses flower buds, showed the biased infection on male flowers. Rearing experiments for two other geometrid moths which use both leaves and flowers showed that the preference and performance of Ourapteryx nivea that fed mainly leaves did not differ between the plant sexes, whereas the development of Alcis angulifera larvae which fed both leaves and flowers was slower when they fed female than male leaves and flower buds. In addition, A. angulifera larvae fed fewer flower buds on female than on male plants. These results show that the male‐biased florivory on E. japonica trees is attributed mainly to the specialist florivore and also feeding preference for male flowers in an opportunistic florivore that feed both leaves and flowers.