Water status and associated processes mark critical stages in pollen development and functioning

Background

The male gametophyte developmental programme can be divided into five phases which differ in relation to the environment and pollen hydration state: (1) pollen develops inside the anther immersed in locular fluid, which conveys substances from the mother plant – the microsporogenesis phase; (2) locular fluid disappears by reabsorption and/or evaporation before the anther opens and the maturing pollen grains undergo dehydration – the dehydration phase; (3) the anther opens and pollen may be dispersed immediately, or be held by, for example, pollenkitt (as occurs in almost all entomophilous species) for later dispersion – the presentation phase; (4) pollen is dispersed by different agents, remaining exposed to the environment for different periods – the dispersal phase; and (5) pollen lands on a stigma and, in the case of a compatible stigma and suitable conditions, undergoes rehydration and starts germination – the pollen–stigma interaction phase.

Scope

This review highlights the issue of pollen water status and indicates the various mechanisms used by pollen grains during their five developmental phases to adjust to changes in water content and maintain internal stability.

Conclusions

Pollen water status is co-ordinated through structural, physiological and molecular mechanisms. The structural components participating in regulation of the pollen water level, during both dehydration and rehydration, include the exine (the outer wall of the pollen grain) and the vacuole. Recent data suggest the involvement of water channels in pollen water transport and the existence of several molecular mechanisms for pollen osmoregulation and to protect cellular components (proteins and membranes) under water stress. It is suggested that pollen grains will use these mechanisms, which have a developmental role, to cope with environmental stress conditions.     

Carpeloidy in flower evolution and diversification: a comparative study in Carica papaya and Arabidopsis thaliana

Background and Aims

Bisexual flowers of Carica papaya range from highly regular flowers to morphs with various fusions of stamens to the ovary. Arabidopsis thaliana sup1 mutants have carpels replaced by chimeric carpel–stamen structures. Comparative analysis of stamen to carpel conversions in the two different plant systems was used to understand the stage and origin of carpeloidy when derived from stamen tissues, and consequently to understand how carpeloidy contributes to innovations in flower evolution.

Methods

Floral development of bisexual flowers of Carica was studied by scanning electron microscopy and was compared with teratological sup mutants of A. thaliana.

Key Results

In Carica development of bisexual flowers was similar to wild (unisexual) forms up to locule initiation. Feminization ranges from fusion of stamen tissue to the gynoecium to complete carpeloidy of antepetalous stamens. In A. thaliana, partial stamen feminization occurs exclusively at the flower apex, with normal stamens forming at the periphery. Such transformations take place relatively late in development, indicating strong developmental plasticity of most stamen tissues. These results are compared with evo-devo theories on flower bisexuality, as derived from unisexual ancestors. The Arabidopsis data highlight possible early evolutionary events in the acquisition of bisexuality by a patchy transformation of stamen parts into female parts linked to a flower axis-position effect. The Carica results highlight tissue-fusion mechanisms in angiosperms leading to carpeloidy once bisexual flowers have evolved.

Conclusions

We show two different developmental routes leading to stamen to carpel conversions by late re-specification. The process may be a fundamental aspect of flower development that is hidden in most instances by developmental homeostasis.     

False paracarpy in Seemannaralia (Araliaceae): from bilocular ovary to unilocular fruit

Background and Aims

Seemannaralia appears to be fundamentally different from all other Araliaceae in the presence of a well-developed symplicate zone in its gynoecium, as well as in the ovule insertion in the symplicate zone (rather than in the cross-zone). The present investigation re-examined the floral structure of Seemannaralia with emphasis on the morphology and evolution of its gynoecium.

Methods

Flowers and fruits of Seemannaralia gerrardii at various developmental stages were examined using light microscopy and scanning electron microscopy.

Key Results

Ovaries in the flowers of Seemannaralia are bilocular. Each ovary locule corresponds to a carpel whose ascidiate part is distinctly longer than the plicate part. Each carpel contains one fertile ovule attached to the cross-zone, and one sterile ovule as well. The fruit is unilocular: its central cavity is occupied by a single large seed. In the course of fruit development, the growth of one ovule stops while another ovule develops into the mature seed. When this ovule outgrows the available space in the locule, the septum is ruptured, forming a united cavity of two carpels.

Conclusions

Despite literature data, the synascidiate zone is well developed in the gynoecium of Seemannaralia, and the ovules are attached to the cross-zone. Its preanthetic and anthetic gynoecium has nearly the same structure as gynoecia of most other Araliaceae. The Seemannaralia fruit resembles the paracarpous gynoecium but its ground plan is very different because the central cavity is formed by mechanical rupture of the septum. The term ‘pseudoparacarpy’ (‘false paracarpy’) is proposed to describe this condition, which has not been reported to date for indehiscent fruits in any taxa other than Seemannaralia. In this genus, the pseudoparacarpy has probably resulted from a decrease in seed number in the course of the transition from zoochory to anemochory.     

Arabinogalactan-protein secretion is associated with the acquisition of stigmatic receptivity in the apple flower

Background and Aims

Stigmatic receptivity plays a clear role in pollination dynamics; however, little is known about the factors that confer to a stigma the competence to be receptive for the germination of pollen grains. In this work, a developmental approach is used to evaluate the acquisition of stigmatic receptivity and its relationship with a possible change in arabinogalactan-proteins (AGPs).

Methods

Flowers of the domestic apple, Malus × domestica, were assessed for their capacity to support pollen germination at different developmental stages. Stigmas from these same stages were characterized morphologically and different AGP epitopes detected by immunocytochemistry.

Key Results

Acquisition of stigmatic receptivity and the secretion of classical AGPs from stigmatic cells occurred concurrently and following the same spatial distribution. While in unpollinated stigmas AGPs appeared unaltered, in cross-pollinated stigmas AGPs epitopes vanished as pollen tubes passed by.

Conclusions

The concurrent secretion of AGPs with the acquisition of stigmatic receptivity, together with the differential response in unpollinated and cross-pollinated pistils point out a role of AGPs in supporting pollen tube germination and strongly suggest that secretion of AGPs is associated with the acquisition of stigma receptivity.Key words: AGPs, arabinogalactan proteins, apple, Malus × domestica, pollen, pollen tube, stigma, stigmatic receptivity, flower receptivity     

Successive microsporogenesis affects pollen aperture pattern in the tam mutant of Arabidopsis thaliana

Background and Aims

The tam (tardy asynchronous meiosis) mutant of Arabidopsis thaliana, which exhibits a modified cytokinesis with a switch from simultaneous to successive cytokinesis, was used to perform a direct test of the implication of cytokinesis in aperture-pattern ontogeny of angiosperm pollen grains. The aperture pattern corresponds to the number and arrangement of apertures (areas of the pollen wall permitting pollen tube germination) on the surface of the pollen grain.

Methods

A comparative analysis of meiosis and aperture distribution was performed in two mutant strains of arabidopsis: quartet and quartet-tam.

Key Results

While the number of apertures is not affected in the quartet-tam mutant, the arrangement of the three apertures is modified compared with the quartet, resulting in a different aperture pattern.

Conclusions

These results directly demonstrate the relationship between the type of sporocytic cytokinesis and pollen aperture-pattern ontogeny.     

Profiling and functional classification of esterases in olive (Olea europaea) pollen during germination

Background and Aims

A pollen grain contains a number of esterases, many of which are released upon contact with the stigma surface. However, the identity and function of most of these esterases remain unknown. In this work, esterases from olive pollen during its germination were identifided and functionally characterized.

Methods

The esterolytic capacity of olive (Olea europaea) pollen was examined using in vitro and in-gel enzymatic assays with different enzyme substrates. The functional analysis of pollen esterases was achieved by inhibition assays by using specific inhibitors. The cellular localization of esterase activities was performed using histochemical methods.

Key Results

Olive pollen showed high levels of non-specific esterase activity, which remained steady after hydration and germination. Up to 20 esterolytic bands were identified on polyacrylamide gels. All the inhibitors decreased pollen germinability, but only diisopropyl fluorophosphate (DIFP) hampered pollen tube growth. Non-specific esterase activity is localized on the surface of oil bodies (OBs) and small vesicles, in the pollen intine and in the callose layer of the pollen tube wall. Acetylcholinesterase (AChE) activity was mostly observed in the apertures, exine and pollen coat, and attached to the pollen tube wall surface and to small cytoplasmic vesicles.

Conclusions

In this work, for the first time a systematic functional characterization of esterase enzymes in pollen from a plant species with wet stigma has been carried out. Olive pollen esterases belong to four different functional groups: carboxylesterases, acetylesterases, AChEs and lipases. The cellular localization of esterase activity indicates that the intine is a putative storage site for esterolytic enzymes in olive pollen. Based on inhibition assays and cellular localization of enzymatic activities, it can be concluded that these enzymes are likely to be involved in pollen germination, and pollen tube growth and penetration of the stigma.     

Electrophoretic profiling and immunocytochemical detection of pectins and arabinogalactan proteins in olive pollen during germination and pollen tube growth

Background and Aims

Cell wall pectins and arabinogalactan proteins (AGPs) are important for pollen tube growth. The aim of this work was to study the temporal and spatial dynamics of these compounds in olive pollen during germination.

Methods

Immunoblot profiling analyses combined with confocal and transmission electron microscopy immunocytochemical detection techniques were carried out using four anti-pectin (JIM7, JIM5, LM5 and LM6) and two anti-AGP (JIM13 and JIM14) monoclonal antibodies.

Key Results

Pectin and AGP levels increased during olive pollen in vitro germination. (1 → 4)-β-d-Galactans localized in the cytoplasm of the vegetative cell, the pollen wall and the apertural intine. After the pollen tube emerged, galactans localized in the pollen tube wall, particularly at the tip, and formed a collar-like structure around the germinative aperture. (1 → 5)-α-l-Arabinans were mainly present in the pollen tube cell wall, forming characteristic ring-shaped deposits at regular intervals in the sub-apical zone. As expected, the pollen tube wall was rich in highly esterified pectic compounds at the apex, while the cell wall mainly contained de-esterified pectins in the shank. The wall of the generative cell was specifically labelled with arabinans, highly methyl-esterified homogalacturonans and JIM13 epitopes. In addition, the extracellular material that coated the outer exine layer was rich in arabinans, de-esterified pectins and JIM13 epitopes.

Conclusions

Pectins and AGPs are newly synthesized in the pollen tube during pollen germination. The synthesis and secretion of these compounds are temporally and spatially regulated. Galactans might provide mechanical stability to the pollen tube, reinforcing those regions that are particularly sensitive to tension stress (the pollen tube–pollen grain joint site) and mechanical damage (the tip). Arabinans and AGPs might be important in recognition and adhesion phenomena of the pollen tube and the stylar transmitting cells, as well as the egg and sperm cells.     

Flow cytometric analysis of pollen grains collected from individual bees provides information about pollen load composition and foraging behaviour

Background and Aims

Understanding the species composition of pollen on pollinators has applications in agriculture, conservation and evolutionary biology. Current identification methods, including morphological analysis, cannot always discriminate taxa at the species level. Recent advances in flow cytometry techniques for pollen grains allow rapid testing of large numbers of pollen grains for DNA content, potentially providing improved species resolution.

Methods

A test was made as to whether pollen loads from single bees (honey-bees and bumble-bees) could be classified into types based on DNA content, and whether good estimates of proportions of different types could be made. An examination was also made of how readily DNA content can be used to identify specific pollen species.

Key Results

The method allowed DNA contents to be quickly found for between 250 and 9391 pollen grains (750–28 173 nuclei) from individual honey-bees and between 81 and 11 512 pollen grains (243–34 537 nuclei) for bumble-bees. It was possible to identify a minimum number of pollen species on each bee and to assign proportions of each pollen type (based on DNA content) present.

Conclusions

The information provided by this technique is promising but is affected by the complexity of the pollination environment (i.e. number of flowering species present and extent of overlap in DNA content). Nevertheless, it provides a new tool for examining pollinator behaviour and between-species or cytotype pollen transfer, particularly when used in combination with other morphological, chemical or genetic techniques.     

The unusual gynoecium structure and extragynoecial pollen-tube pathway in <Emphasis Type="Italic">Phytolacca</Emphasis> (Phytolaccaceae)

The fusion of carpels into a unified compound gynoecium is considered a dominant feature of angiosperm evolution and it also occurs by postgenital fusion during the gynoecium development in some apocarpous species. However, we found the reverse process, the separation of carpels from combined carpel primordia, during the development of the gynoecium in Phytolacca. Semithin sectioning and scanning electron microscopy were utilised to observe the structure and development of the gynoecia in Phytolacca acinosa and Phytolacca americana, fluorescence microscopy was utilised to observe the pollen tube growth in the gynoecia of the two species, and the topology method was applied to analyze the relationship between the gynoecium structure and pollen tube pathway. Although the gynoecia of P. acinosa and P. americana are both syncarpous, the degree of carpel fusion in the mature gynoecia of the two syncarpous species is different as a result of variant developmental processes. However, change in the degree of carpel fusion during the development of gynoecia in Phytolacca does not affect pollen tube growth because of the existence of the extragynoecial pollen-tube pathway. Thus, the change in the degree of carpel fusion in Phytolacca is primarily the result of diversification of developmental processes related to selection pressure.     

Effects of pollination timing on seed paternity and seed mass in Silene latifolia (Caryophyllaceae)

Background and Aims

Competition among genetically different pollen donors within one recipient flower may play an important role in plant populations, increasing offspring genetic diversity and vigour. However, under field conditions stochastic pollen arrival times may result in disproportionate fertilization success of the first-arriving pollen, even to the detriment of the recipient plant''s and offspring fitness. It is therefore critical to evaluate the relative importance of arrival times of pollen from different donors in determining siring success.

Methods

Hand pollinations and genetic markers were used to investigate experimentally the effect of pollination timing on seed paternity, seed mass and stigmatic wilting in the the dioecious plant Silene latifolia. In this species, high prevalence of multiply-sired fruits in natural populations suggests that competition among different donors may often take place (at fertilization or during seed development); however, the role of variation due to pollen arrival times is not known.

Key Results

First-arriving pollen sired significantly more seeds than later-arriving pollen. This advantage was expressed already before the first pollen tubes could reach the ovary. Simultaneously with pollen tube growth, the stigmatic papillae wilted visibly. Individual seeds were heavier in fruits where one donor sired most seeds than in fruits where both donors had more even paternity shares.

Conclusions

In field populations of S. latifolia, fruits are often multiply-sired. Because later-arriving pollen had decreased chances of fertilizing the ovules, this implies that open-pollinated flowers often benefit from pollen carry-over or pollinator visits within short time intervals, which may contribute to increase offspring genetic diversity and fitness.Key words: Reproduction, reproductive success, pollen, siring success, microsatellite DNA, paternity, pollen tube growth, seed mass, Silene alba, stigma wilting     

Pollen-pistil interactions and early fruiting in parthenocarpic citrus

Background and Aims

An intense pollen–pistil interaction precedes fertilization. This interaction is of particular relevance in agronomically important species where seeds or fruits are the edible part. Over time some agronomically species have been selected for the ability to produce fruit without seeds. While this phenomenon is critical for commercial production in some species, very little is known about the events behind the production of seedless fruit. In this work, the relationship between pollen–pistil interaction and the onset of fruiting was investigated in citrus mandarin.

Methods

Pistils were sequentially examined in hand-pollinated flowers paying attention to pollen-tube behaviour, and to cytochemical changes along the pollen-tube pathway. To evaluate which of these changes were induced by pollination/fertilization and which were developmentally regulated, pollinated and unpollinated pistils were compared. Also the onset of fruiting was timed and changes in the ovary examined.

Key Results

Conspicuous changes occurred in the pistil along the pollen-tube pathway, which took place in a basipetal way encompassing the timing of pollen-tube growth. However, these changes appear to be developmentally regulated as they happened in the same way and at the same time in unpollinated flowers. Moreover, the onset of fruiting occurred prior to fertilization and the very same changes could be observed in unpollinated flowers.

Conclusions

Pollen–pistil interaction in citrus showed similarities with unrelated species and families belonging to other taxa. The uncoupling of the reproductive and fruiting processes accounts for the parthenocarpic ability of unpollinated flowers to produce fruit in citrus. However, the maintenance of a functional reproductive process reflects the potential to produce seeded fruits, providing a basis for the understanding of the production of seeded or unseeded fruits and further understanding of the process of parthenocarpy in other species.     

The cell wall pectic polymer rhamnogalacturonan-II is required for proper pollen tube elongation: implications of a putative sialyltransferase-like protein

Background and Aims

Rhamnogalacturonan-II (RG-II) is one of the pectin motifs found in the cell wall of all land plants. It contains sugars such as 2-keto-3-deoxy-d-lyxo-heptulosaric acid (Dha) and 2-keto-3-deoxy-d-manno-octulosonic acid (Kdo), and within the wall RG-II is mostly found as a dimer via a borate diester cross-link. To date, little is known regarding the biosynthesis of this motif. Here, after a brief review of our current knowledge on RG-II structure, biosynthesis and function in plants, this study explores the implications of the presence of a Golgi-localized sialyltransferase-like 2 (SIA2) protein that is possibly involved in the transfer of Dha or Kdo in the RG-II of Arabidopsis thaliana pollen tubes, a fast-growing cell type used as a model for the study of cell elongation.

Methods

Two heterozygous mutant lines of arabidopsis (sia2-1+/– and qrt1 × sia2-2+/–) were investigated. sia2-2+/– was in a quartet1 background and the inserted T-DNA contained the reporter gene β-glucuronidase (GUS) under the pollen-specific promoter LAT52. Pollen germination and pollen tube phenotype and growth were analysed both in vitro and in vivo by microscopy.

Key Results

Self-pollination of heterozygous lines produced no homozygous plants in the progeny, which may suggest that the mutation could be lethal. Heterozygous mutants displayed a much lower germination rate overall and exhibited a substantial delay in germination (20 h of delay to reach 30 % of pollen grain germination compared with the wild type). In both lines, mutant pollen grains that were able to produce a tube had tubes that were either bursting, abnormal (swollen or dichotomous branching tip) or much shorter compared with wild-type pollen tubes. In vivo, mutant pollen tubes were restricted to the style, whereas the wild-type pollen tubes were detected at the base of the ovary.

Conclusions

This study highlights that the mutation in arabidopsis SIA2 encoding a sialyltransferase-like protein that may transfer Dha or Kdo on the RG-II motif has a dramatic effect on the stability of the pollen tube cell wall.     

Presence of a Polymicrobial Endometrial Biofilm in Patients with Bacterial Vaginosis

Objective

To assess whether the bacterial vaginosis biofilm extends into the upper female genital tract.

Study Design

Endometrial samples obtained during curettage and fallopian tube samples obtained during salpingectomy were collected. Endometrial and fallopian tube samples were analyzed for the presence of bacteria with fluorescence-in-situ-hybridisation (FISH) analysis with probes targeting bacterial vaginosis-associated and other bacteria.

Results

A structured polymicrobial Gardnerella vaginalis biofilm could be detected in part of the endometrial and fallopian tube specimens. Women with bacterial vaginosis had a 50.0% (95% CI 24.0–76.0) risk of presenting with an endometrial Gardnerella vaginalis biofilm. Pregnancy (AOR  = 41.5, 95% CI 5.0–341.9, p<0.001) and the presence of bacterial vaginosis (AOR  = 23.2, 95% CI 2.6–205.9, p<0.001) were highly predictive of the presence of uterine or fallopian bacterial colonisation when compared to non-pregnant women without bacterial vaginosis.

Conclusion

Bacterial vaginosis is frequently associated with the presence of a structured polymicrobial Gardnerella vaginalis biofilm attached to the endometrium. This may have major implications for our understanding of the pathogenesis of adverse pregnancy outcome in association with bacterial vaginosis.     

Association between floral traits and rewards in Erysimum mediohispanicum (Brassicaceae)

Background and Aims

Floral rewards may be associated with certain morphological floral traits and thus act as underlying factors promoting selection on these traits. This study investigates whether some traits that are under pollinator-mediated selection (flower number, stalk height, corolla diameter, corolla tube length and corolla tube width) in the Mediterranean herb E. mediohispanicum (Brassicaceae) are associated with rewards (pollen and nectar).

Methods

During 2005 the phenotypic traits and the visitation rate of the main pollinator functional groups were quantified in 720 plants belonging to eight populations in south-east Spain, and during 2006 the same phenotypic traits and the reward production were quantified in 400 additional plants from the same populations.

Key Results

A significant correlation was found between nectar production rate and corolla tube length, and between pollen production and corolla diameter. Visitation rates of large bees and butterflies were significantly higher in plants exhibiting larger flowers with longer corolla tubes.

Conclusions

The association between reward production and floral traits may be a factor underlying the pattern of visitation rate displayed by some pollinators.Key words: Erysimum, floral traits, nectar, pollen, pollinator visitation rate, reward     

Floral display size,conspecific density and florivory affect fruit set in natural populations of Phlox hirsuta,an endangered species

Background and Aims

Natural variation in fruit and seed set may be explained by factors that affect the composition of pollen grains on stigmas. Self-incompatible species require compatible outcross pollen grains to produce seeds. The siring success of outcross pollen grains, however, can be hindered if self (or other incompatible) pollen grains co-occur on stigmas. This study identifies factors that determine fruit set in Phlox hirsuta, a self-sterile endangered species that is prone to self-pollination, and its associated fitness costs.

Methods

Multiple linear regressions were used to identify factors that explain variation in percentage fruit set within three of the five known populations of this endangered species. Florivorous beetle density, petal colour, floral display size, local conspecific density and pre-dispersal seed predation were quantified and their effects on the ability of flowers to produce fruits were assessed.

Key Results

In all three populations, percentage fruit set decreased as florivorous beetle density increased and as floral display size increased. The effect of floral display size on fruit set, however, often depended on the density of nearby conspecific plants. High local conspecific densities offset – even reversed – the negative effects of floral display size on percentage fruit set. Seed predation by mammals decreased fruit set in one population.

Conclusions

The results indicate that seed production in P. hirsuta can be maximized by selectively augmenting populations in areas containing isolated large plants, by reducing the population sizes of florivorous beetles and by excluding mammals that consume unripe fruits.     

Lifeact-mEGFP Reveals a Dynamic Apical F-Actin Network in Tip Growing Plant Cells

Background

Actin is essential for tip growth in plants. However, imaging actin in live plant cells has heretofore presented challenges. In previous studies, fluorescent probes derived from actin-binding proteins often alter growth, cause actin bundling and fail to resolve actin microfilaments.

Methodology/Principal Findings

In this report we use Lifeact-mEGFP, an actin probe that does not affect the dynamics of actin, to visualize actin in the moss Physcomitrella patens and pollen tubes from Lilium formosanum and Nicotiana tobaccum. Lifeact-mEGFP robustly labels actin microfilaments, particularly in the apex, in both moss protonemata and pollen tubes. Lifeact-mEGFP also labels filamentous actin structures in other moss cell types, including cells of the gametophore.

Conclusions/Significance

Lifeact-mEGFP, when expressed at optimal levels does not alter moss protonemal or pollen tube growth. We suggest that Lifeact-mEGFP represents an exciting new versatile probe for further studies of actin''s role in tip growing plant cells.     

Breeding systems,hybridization and continuing evolution in Avon Gorge Sorbus

Background and Aims

Interspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). The Avon Gorge, Bristol, UK, is a world ‘hotspot’ for Sorbus diversity and home to diploid sexual species and polyploid apomictic species. This research investigated how mating system variation, hybridization and polyploidy interact to generate this biological diversity.

Methods

Mating systems of diploid, triploid and tetraploid Sorbus taxa were analysed using pollen tube growth and seed set assays from controlled pollinations, and parent–offspring genotyping of progeny from open and manual pollinations.

Key Results

Diploid Sorbus are outcrossing and self-incompatible (SI). Triploid taxa are pseudogamous apomicts and genetically invariable, but because they also display self-incompatibility, apomictic seed set requires pollen from other Sorbus taxa – a phenomenon which offers direct opportunities for hybridization. In contrast tetraploid taxa are pseudogamous but self-compatible, so do not have the same obligate requirement for intertaxon pollination.

Conclusions

The mating inter-relationships among Avon Gorge Sorbus taxa are complex and are the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and we suggest that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations. We propose that Avon Gorge Sorbus represents a model system for studying the establishment and persistence of SI apomicts in natural populations.     

Floral structure of Emmotum (Icacinaceae sensu stricto or Emmotaceae), a phylogenetically isolated genus of lamiids with a unique pseudotrimerous gynoecium,bitegmic ovules and monosporangiate thecae

Background and Aims

Icacinaceae sensu stricto consist of a group of early branching lineages of lamiids whose relationships are not yet resolved and whose detailed floral morphology is poorly known. The most bizarre flowers occur in Emmotum: the gynoecium has three locules on one side and none on the other. It has been interpreted as consisting of three fertile and two sterile carpels or of one fertile carpel with two longitudinal septa and two sterile carpels. This study focused primarily on the outer and inner morphology of the gynoecium to resolve its disputed structure, and ovule structure was also studied. In addition, the perianth and androecium were investigated.

Methods

Flowers and floral buds of two Emmotum species, E. harleyi and E. nitens, were collected and fixed in the field, and then studied by scanning electron microscopy. Microtome section series were used to reconstruct their morphology.

Key Results

The gynoecium in Emmotum was confirmed as pentamerous, consisting of three fertile and two sterile carpels. Each of the three locules behaves as the single locule in other Icacinaceae, with the placenta of the two ovules being identical, which shows that three fertile carpels are present. In addition, it was found that the ovules are bitegmic, which is almost unique in lamiids, and that the stamens have monosporangiate thecae, which also occurs in the closely related family Oncothecaceae, but is not known from any other Icacinaceae sensu lato so far.

Conclusions

The flowers of Emmotum have unique characters at different evolutionary levels: the pseudotrimerous gynoecium at angiosperm level, the bitegmic ovules at lamiid level and the monosporangiate thecae at family or family group level. However, in general, the floral morphology of Emmotum fits well in Icacinaceae. More comparative research on flower structure is necessary in Icacinaceae and other early branching lineages of lamiids to better understand the initial evolution of this large lineage of asterids.