The Causes of self-sterility in natural populations of the relictual angiosperm, Illicium floridanum (Illiciaceae)

BACKGROUND AND AIMS: Illicium floridanum, a species belonging to the basal extant angiosperm taxon Illiciaceae, reportedly exhibits self-incompatibility (SI). To date, the site and timing of SI within the carpel of this species remains unidentified. Thus, the objective of this research was to determine the cellular and temporal aspects of SI in I. floridanum. METHODS: Following controlled application of cross- and self-pollen in natural populations of I. floridanum, embryo sac development and temporal aspects of stigma receptivity, as well as pollen tube growth, fertilization, and embryo and endosperm development, were investigated with the aid of light and fluorescence microscopy. KEY RESULTS: Flowers of I. floridanum exhibited complete dichogamy whereby stigmas only supported cross- and self-pollen tube growth prior to anther dehiscence. In contrast to earlier reports of SI in this species, a prezygotic SI resulting in rejection of self-pollen tube growth at the stigma was absent and there were no significant differences between cross- versus self-pollen germination and pollen tube growth within the style and ovary during the first 5 d after pollination. Structural development of the four-celled embryo sac was not differentially influenced by pollen type as noted to occur in other angiosperms with late-acting ovarian SI. The ovule micropyle and embryo sac were penetrated equally by cross- and self-pollen tubes. In addition, there were no statistically significant differences in cross- versus self-fertilization. A resting zygote and multicellular endosperm at a variety of developmental stages was present by 30 d after application of cross- or self-pollen. CONCLUSIONS: In the clear absence of a prezygotic SI that was previously reported to result in differential self-pollen tube growth at the stigma, self- sterility in I. floridanum is likely due to early-acting inbreeding depression, although late-acting post-zygotic ovarian SI cannot be ruled out.     

Sexual interference within flowers of Chamerion angustifolium

Hermaphroditism is prevalent in plants but may allow interference between male function (pollen removal and dispersal) and female function (pollen receipt and seed production) within a flower. Temporal or spatial segregation of gender within a hermaphroditic flower may evolve to reduce this interference and enhance male and female reproductive success. We tested this hypothesis using Chamerion angustifolium (Onagraceae), in which pollen removal (male) and pollen deposition (female) were measured directly on hermaphroditic and experimentally produced unisexual flowers. During a single flower visit in the field, bees deposited 159±24 (SE) pollen grains on a stigma and removed 1058±198 grains from each flower. Anther removal did not alter deposition rates. In the laboratory, bees removed 2669±273 pollen grains and deposited 209±72.3 cross-pollen and 120±28.4 facilitated self-pollen grains per visit. The presence of anthers significantly reduced cross-pollen deposition on the stigma. In contrast, pollen removal was not affected by presence of the pistil. These results suggest that within-flower interference affects female function and represents a fitness cost that can be reduced through temporal segregation of gender within the flower. Co-ordinating editor: S.-M. Chang     

The cross-fertilization mechanism and reproductive interference of self-pollen of Medicago varia ‘Xinmu No. 1’

Some control tests and a technique employing the fluorescent properties of pollen and pollen tube stained with aniline blue dye were utilized to study the stigma receptivity, the optimum pollination period, the self-pollen clogging effect, pollen germination and the pollen tube growth state in alfalfa. The results can be summarized as follows: (1) The stigma receptivity and flower longevity barely overlap. On the first day after pollination, the pollination efficiency was the highest, with a pod-seting rate per raceme of 65.17%±2.01% (P<0.01). The period of the highest pollination efficiency was the same as the peak of stigma receptivity. When pollination was on the fourth day after flowering, the pod-setting percentage per raceme was still 34.25%±6.73%. (2) Self-pollination led to a lower pollen germination frequency and a lower pollen tube growth rate than cross-pollination. When artificial cross-pollination was applied after self-pollen grains on the stigma was re-moved, the pod-setting percentage per raceme was 72.31%±6.24%, and the number of seed per pod was 3.46±0.25, In contrast, these two values were 51.76%±5.37% (P<0.01) and 2.11%±0.18 respectively (P<0.01) when the self-pollen grains were not removbed in the artificial cross-pollination experiements. Hence, the clog-ging effect of self-pollen to stigma was significant. (3) In the breeding system of alfalfa, there are two mecha-nisms that help the plants to avoid self-fertilization: dichogamy and self-incompatibility. However, due to the structure of flowers, they were not effective enough to improve cross-fertilization rate, even if the stamen matured earlier than the stigma. The typical self-incompatibility system guaranteed the lower self-fertilization rate, but cannot avoid the reproductive interference of self-pollen.     

Movement-assisted dichogamy in Atamisquea emarginata (Capparaceae)

 Various aspects of the reproductive biology of Atamisquea emarginata Miers ex Hook. et Arn. were studied in a population from Tucumán, northern Argentina. Flowering took place in late (austral) spring to early summer. The creamy white, entomophilous flowers were protandrous and lasted 3.5–4.5 days. A pleasant odor along with nectar and pollen attracted 36 mostly diurnal insect species of which 41.7% were Hymenoptera, 38.9% Lepidoptera and 19.4% Diptera. However, 75% from all recorded species were infrequent, and domestic bees made ca. 94% of all visits. One halictid bee, one sphecid wasp, one myzinid wasp, four small- to medium-sized butterflies and a hawkmoth completed the set of relatively frequent insect visitors. Pollen and stigma were presented in the same place within the flower, but interference was prevented by a movement-assisted, complete dichogamy mechanism in which the bending ability of the gynophore played a fundamental role. This form of dichogamy was previously unknown in the Capparaceae. In comparison with the known species of the closely allied genus Capparis L., A. emarginata has diurnal, smaller, longer-lasting flowers, that offer much smaller nectar amounts, and that attract a rather generalized array of non-hovering visitors, while Capparis behaves as a more crepuscular-biased sphingophile or ornithophile or chiropterophile. Low levels of seed set under bagging indicates dependence on insect visits and are partly congruent with an earlier report of self-incompatibility in A. emarginata. Pollen tubes arrived to ovules in 90.6% of the inspected flowers. Fruit set of open-pollinated flowers was 43.6%–53.7%. Ninety-five percent of all ovules were regularly aborted in each fruiting ovary; thus, each fruit contained 1.2 developed seeds. Full-sized individuals were expected to produce ca. 10,700 potential offspring per year. Received August 19, 2002; accepted October 8, 2002 Published online: February 7, 2003     

Floral traits mediate the vulnerability of aloes to pollen theft and inefficient pollination by bees

Background and Aims

Pollen-collecting bees are among the most important pollinators globally, but are also the most common pollen thieves and can significantly reduce plant reproduction. The pollination efficiency of pollen collectors depends on the frequency of their visits to female(-phase) flowers, contact with stigmas and deposition of pollen of sufficient quantity and quality to fertilize ovules. Here we investigate the relative importance of these components, and the hypothesis that floral and inflorescence characteristics mediate the pollination role of pollen collection by bees.

Methods

For ten Aloe species that differ extensively in floral and inflorescence traits, we experimentally excluded potential bird pollinators to quantify the contributions of insect visitors to pollen removal, pollen deposition and seed production. We measured corolla width and depth to determine nectar accessibility, and the phenology of anther dehiscence and stigma receptivity to quantify herkogamy and dichogamy. Further, we compiled all published bird-exclusion studies of aloes, and compared insect pollination success with floral morphology.

Key Results

Species varied from exclusively insect pollinated, to exclusively bird pollinated but subject to extensive pollen theft by insects. Nectar inaccessibility and strong dichogamy inhibited pollination by pollen-collecting bees by discouraging visits to female-phase (i.e. pollenless) flowers. For species with large inflorescences of pollen-rich flowers, pollen collectors successfully deposited pollen, but of such low quality (probably self-pollen) that they made almost no contribution to seed set. Indeed, considering all published bird-exclusion studies (17 species in total), insect pollination efficiency varied significantly with floral shape.

Conclusions

Species-specific floral and inflorescence characteristics, especially nectar accessibility and dichogamy, control the efficiency of pollen-collecting bees as pollinators of aloes.     

Reproductive barriers in Annona cherimola (Mill.) outside of its native area

Cherimoya (Annona cherimola) is a subtropical tree crop of Andean origin whose fruit set results extremely low in farming areas outside of its natural occurrence. The lack of efficient pollinators and dichogamy are often argued to be the main constraints resulting in this low reproductive success. Herein, we describe the reproductive barriers exhibited by this crop and whether wind and insects play a role in cherimoya pollination in Spain, the main region of cultivation. A.?cherimola exhibits marked protogynous dichogamy with large differences in the duration of female (around 28?h) and male (<8?h) phases. Stigma receptivity and pollen release do not fully coincide with the morphological changes of the petals defining the female and male phases. Synchronization of sexual phases among different flowers from different trees of the same genotype was high during the whole blooming season. Effective herkogamy of approach type also limits pollen deposition within the same flower. Wind does not play any role in cherimoya pollination. Insect visitors to cherimoya flowers in Spain were found to be inefficient in transferring pollen grains. Cherimoya flowers do not reject self-pollen to achieve fertilization. A.?cherimola shows preferential allogamy based on efficient dichogamy reinforced by elevated synchrony among flowers in their sexual phases. Herkogamy hampers autogamy, although pollen deposition by gravity in cherimoya pendulous flowers explains the reduced reproductive success observed in isolated flowers.     

A model of the evolution of dichogamy incorporating sex-ratio selection, anther-stigma interference, and inbreeding depression

Historically, explanations for the evolution of floral traits that reduce self-fertilization have tended to focus on selection to avoid inbreeding depression. However, there is growing support for the hypothesis that such traits also play a role in promoting efficient pollen dispersal by reducing anther-stigma interference. The relative importance of these two selective pressures is currently a popular topic of investigation. To date, there has been no theoretical exploration of the relative contributions of selection to avoid the genetic costs of self-fertilization and selection to promote efficient pollen dispersal on the evolution of floral traits. We developed a population genetic model to examine the influence of these factors on the evolution of dichogamy: the temporal separation of anther maturation and stigma receptivity. Our analysis indicates that anther-stigma interference can favor dichogamy even in the absence of in-breeding depression. Although anther-stigma interference and inbreeding depression are the key forces driving the initial evolution of dichogamy, selection to match the timing of pollen dispersal to the availability of ovules at the population level becomes a more potent force opposing the further evolution of dichogamy as the extent of temporal separation increases. This result may help to explain otherwise puzzling phenomena such as why dichogamy is rarely complete in nature and why dichogamy tends to be associated with asynchronous flower presentation.     

Permian ginkgophyte fossils from the Dolomites resemble extant <Emphasis Type="Italic">O-ha-tsuki</Emphasis> aberrant leaf-like fructifications of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> L

Background  

Structural elucidation and analysis of fructifications of plants is fundamental for understanding their evolution. In case of Ginkgo biloba, attention was drawn by Fujii in 1896 to aberrant fructifications of Ginkgo biloba whose seeds are attached to leaves, called O-ha-tsuki in Japan. This well-known phenomenon was now interpreted by Fujii as being homologous to ancestral sporophylls. The common fructification of Ginkgo biloba consists of 1-2 (rarely more) ovules on a dichotomously divided stalk, the ovules on top of short stalklets, with collars supporting the ovules. There is essentially no disagreement that either the whole stalk with its stalklets, collars and ovules is homologous to a sporophyll, or, alternatively, just one stalklet, collar and ovule each correspond to a sporophyll. For the transition of an ancestral sporophyll resembling extant O-ha-tsuki aberrant leaves into the common fructification with stalklet/collar/ovule, evolutionary reduction of the leaf lamina of such ancestral sporophylls has to be assumed. Furthermore, such ancestral sporophylls would be expected in the fossil record of ginkgophytes.     

Experimental and Analytical Studies of Pteridophytes: XL. Factors in the Formation and Distribution of Sori in Leptosporanglate Ferns

In further studies of soral morphogenesis in common Britishferns, consideration is given to environmental and other factorswhich may be causally involved. Accounts are given of the normalcourse of sporophyll development in Dryopteris austriaca andof the effects of more or less extensive defoliation on theinception of sori in several ferns. Some anomalous developmentsin sporophylls of Blechnum spicant are described and illustrated.Various general ideas regarding factors involved in the onsetof the reproductive phase are discussed. It is concluded that,on the evidence which is admittedly very incomplete, the onsetof the reproductive phase in the fern sporophyte is determinedby the interaction of a number of factors, rather than by thespecial action of some particular factor. Because the nature,functional sorus is a complex and highly organized structure,its further investigation is considered to be a task of veryconsiderable magnitude.     

美国凌霄柱头行为--一种促进传粉的机制

引起美国(Campsis radicans(L.)Seem.ex Bureau.)柱头开合行为的原因迄今未知.在本研究中,机械刺激、自花授粉、异花授粉以及不同授粉数量分别给予美国凌霄的柱头,以期找到影响其柱头行为的原因.研究结果表明,单纯的机械剌激可以使柱头闭合,但不能使之水永久闭合;只有柱头表明落置了足够量的花粉(>350)时柱头才会最终闭合.美国凌霄的柱头对换自花或者花粉没有差异,两种花粉授于柱头后花粉管都能成功萌发并生长.研究结果表明:美国凌霄的柱头行为不是一种异交机制,而是一种促进传粉进而增强繁殖成功的机制.     

美国凌霄柱头行为——一种促进传粉的机制(英文)

引起美国凌霄(campsis radicans(L.)Seem.ex Bureau.)柱头开合行为的原因迄今未知。在本研究中,机械刺激、自花授粉、异花授粉以及不同授粉数量的授粉分别给予美国凌霄的柱头,以期找到影响其柱头行为的原因。研究结果表明,单纯的机械刺激可以使柱头闭合,但不能使之永久闭合;只有柱头表面落置了足够量的花粉(>350)时柱头才会最终闭合。美国凌霄的柱头行为对自花或者异花花粉没有差异,两种花粉授于柱头后花粉管都能成功地萌发并生长。研究结果说明:美国凌霄的柱头行为不是一种异交机制,而是一种促进传粉进而增强繁殖成功的机制。     

雄蕊合生植物半边莲的花部综合征与繁育系统

 为了解花内雄蕊合生结构的繁殖适应意义, 初步研究了雄蕊合生植物半边莲(Lobelia chinensis)的花部综合征、传粉特性和繁育系统。半边莲花大且鲜艳, 花瓣中下部弯折并合生成背部有裂缝的不封闭的花冠筒。5雄蕊的花药紧密合生成花药筒, 花丝中上半部也合生在一起, 只有花丝基部分离插生于花冠筒上。柱头被包裹在花药筒内。半边莲单花寿命可达5 d左右。雄性先熟, 柱头在伸出花药筒之后才具活性。花的主要访问者为蚂蚁、食蚜蝇和苍蝇类等小型昆虫。半边莲单花花粉数约为(5 200±130)粒、胚珠数约为(55±6)颗, 花粉胚珠比为94.54, 应属于兼性自交繁育系统, 但异交指数和其它特征都显示其以异交为主, 部分自交亲和。套袋和人工授粉实验发现, 半边莲不存在无融合生殖与自发自交, 但自交亲和性高。雄蕊合生(尤其是花药的合生)能把花药中的花粉聚拢在一起在传粉者的一次访问中就被同时带出, 与同样具有较低花粉胚珠比的花粉聚联(Pollen aggregation)传粉过程近似。半边莲的雄蕊合生结构(花药合生成筒、花丝上部也合生)可能与一些特定的花部特征, 如花被合生成未完全封闭的筒、雌雄异熟以及低花粉胚珠比等联系在一起, 形成了适应小型传粉者的“花部综合征”     

The effectiveness of some mechanisms of reproductive isolation in Arum maculatum and A. italicum (Araceae)

Mechanisms of reproductive isolation were studied in two species of Arum , A. italicum and A. maculatum , growing in England and the south of France. The study focused on three potential mechanisms for reproductive isolation: the effectiveness of dichogamy as a barrier to autogamy; the ability of self and outcrossed pollen to germinate on stigmas at different stages of anthesis and to effect pollination; and postzygotic barriers to selfing. Dichogamy was found to provide a very effective barrier to within-inflorescence selfing in these species, as no seeds were produced by spontaneous self-pollination (i.e. autogamy) in any population of either species. However, the study found that geitonogamy (cross-pollination between inflorescences of a same individual or clone) was possible, as genotypes frequently produced several inflorescences and stigmas were found to be receptive to pollen from before anthesis until their contraction at the end of the female phase of flowering. Hand pollination with self pollen from clone inflorescences produced as numerous and heavy seeds as outcross pollination. In addition, the germination and growth of geitonogamously produced seed was similar to that of outcrossed seed, suggesting that this potential postzygotic barrier is absent or weak. These findings suggest that geitonogamous seed production may be unrestricted by significant pre- or postzygotic barriers. The possible advantages of geitonogamy in Arum are discussed.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 323–328.     

核桃揪雌雄异熟性的初步研究

对核桃揪开花特性进行了观察。结果表明：核桃揪多数植株为雌雄异熟,有的植株上雌花先开（雌先型）,有的植株上雄花先开（雄先型）。每一植株上雌雄异熟的次序是相当稳定的。并且多数能为雌先型植株提供授粉机会的雄先型群体,其雌花期与雌先型群体的雄花期也能较好的吻合。但是少数花期不同的同一类型（即：雌先型与雌先型或雄先型与雄先型）植株之间也有授粉机会。在坐果率、产量、核果的整齐度、放叶早晚、落叶期、形态差异的调查分析中未发现异熟性与这些性状上有必然的联系。     

Photoperiodic long-day control of sporophyll and hair formation in the brown alga Undaria pinnatifida

Two photoperiodic responses, the development of sporophylls and hairs, havebeen quantified in sporophytes of the brown alga Undaria pinnatifida. In a finalexperiment, the algae were cultivated in outdoor, 2000-L seawater tanks in agreenhouse for up to 12 weeks, and daylength was regulated by automatic blindsmounted on top of the tanks. Vegetative young sporophytes were treated undershort-day (SD; 8 h light per day) or long-day conditions (LD; 16 h light perday), at 12 h light per day or in a night-break regime (NB; 8 h light per day,7.5 h dark, 1 h light, 7.5 h dark). The earliest sporophyll development wasobserved 6, 7 or 9 weeks under LD, NB or SD conditions, respectively. After 12 weeksthe sporophylls were significantly longer and wider under LD or NB conditions than inthe SD regime, and only half of the experimental algae had formed sporophyllsunder SD conditions, but all algae under LD or NB conditions. In a foregoing 7-weekculture experiment performed in 300-lL indoor tanks, enhanced sporophyll formationhad also been observed under LD and not under SD conditions (NB omitted). In bothexperiments, blade elongation rates remained high until the end of theexperiments in SD, but declined during sporophyll initiation in LD, NB or at 12 hlight per day. Another difference caused by photoperiod was observed in regard to thedevelopment of surface hair spots which occurred in both experiments on the bladesin LD, NB or at 12 h light per day with identical densities, but were completelylacking under SD conditions. It is concluded that U. pinnatifida is afacultatative long-day plant in regard to reproduction forming vigorously sporophyllsin long days, and an obligate long-day plant in regard to hair formation.     

A new lycopsid megaspore cone from the Upper Devonian of Chaohu, China

A new small lycopsid cone, Minostrobus chaohuensis gen. et sp. nov. , is described from the upper part of Wutung Formation (Famennian) of Chaohu, Anhui, China. Six sporophylls in each whorl are tightly arranged on the cone axis. Each sporophyll consists of a short pedicel and a narrow lamina with smooth margins. One spherical to spherical-ellipsoidal sporangium is attached directly to the adaxial surface of each sporophyll. Each sporangium contains four megaspores. A solid exarch protostele occurs at the centre of the cone axis, and a mesarch protostele in the base of the sporophyll. M. chaohuensis is interpreted as a small, herbaceous, heterosporous lycopsid.     

Experimental analysis of protogyny in Aquilegia canadensis (Ranunculaceae)

Dichogamy is very common in flowering plants and is widely thought to reduce pollen-pistil interference, especially self-pollination. Yet, the functional significance of dichogamy has rarely been studied. We investigated the nature and functioning of dichogamy in eastern Ontario populations of Aquilegia canadensis, a highly selfing columbine previously described as protogynous. We then manipulated flowers to determine whether increased protogyny would reduce self-fertilization. Contrary to previous reports, A. canadensis is not dichogamous. Controlled pollinations in a greenhouse showed that pollen tubes generally begin to develop after anther dehiscence. Although stigmas can collect pollen early in floral development, naturally pollinated flowers collected from four populations had few pollen grains on stigmas and almost no pollen tubes in styles until after anther dehiscence. Limited pollen deposition before anther dehiscence was also associated with low nectar availability and limited sepal expansion. Because inbreeding depression is strong in this species, selection may favor increased protogyny if it reduces selfing. We tested this hypothesis by comparing the level of selfing in flowers rendered protogynous by the removal of the first 19 (of 39) anthers to develop, with nonprotogynous control flowers. Contrary to expectations, protogyny did not reduce selfing. Our results emphasize the importance of detailed field observations and manipulative experiments in understanding the nature and functional significance of dichogamy.     

Self-interference is reduced in a secondary pollen presentation species, Duperrea pavettifolia (Rubiaceae)

Occurrence of secondary pollen presentation (SPP) is evidenced for Duperrea pavettifolia. For this, self-pollen was presented on the stigma and reduced fertilization was observed in this case. The receptive area located in the furrow of the stigma becomes receptive from the third day of elongation growth. Fruit set upon hand pollination with pollen from other individuals was significantly higher than the results of selfed, bagged, emasculated and control treatments. However there was no difference in pollen tube growth rate between selfed and crossed pollen on a receptive stigma. Hawkmoths and butterflies were effective pollinators for D. pavettifolia, but the visiting frequencies were very low. Stingless bees removed pollen from the unreceptive stigma which had no contribution to reproductive success. High level of outcrossing in D. pavettifolia was demonstrated by molecular analyses using the simple sequence repeats (SSR) method. Although the wild populations of D. pavettifolia are small and with fragmented distribution, the genetic diversity of seedlings was high, with fluctuations among years. Our results indicated that protandry and the visitation of stingless bees reduced the amount of self-pollen on the still unreceptive stigma and self-incompatibility prevented fertilization by un-removed self-pollen.     

PETRIFIED PLANTS FROM THE UPPER MISSISSIPPIAN OF NORTH AMERICA. II. LEPIDOSTROBUS FAYETTEVILLENSE SP. N.

Structurally preserved arborescent lycopsid fructifications are described from the Fayetteville Shale (Upper Mississippian) of northwestern Arkansas. Specimens of Lepidostrobus fayettevillense sp. n. range from complete cones 22.5 cm long and approximately 1.0 cm in diameter to smaller water-worn fragments. The cones consist of a central axis bearing closely spaced, spirally arranged sporophylls which extend from the cone axis at right angles. Each sporophyll consists of a pedicel which is turned up at its end to form a distal lamina. Sporangia are large and attached to the adaxial surface of each sporophyll. The vascular cylinder consists of a centrally located exarch protostele at least 1.1 mm in diameter. The new species is compared with morphologically similar lycopsid fructifications of equivalent age.