Postgenital Fusion Research Articles

Floral development of Hydrocera and Impatiens reveals evolutionary trends in the most early diverged lineages of the Balsaminaceae

Balsaminaceae consist of two genera, the monospecific Hydrocera and its species-rich sister Impatiens. Although both genera are seemingly rather similar in overall appearance, they differ in ecology, distribution range, habitat preference and morphology. Because morphological support for the current molecular phylogenetic hypothesis of Impatiens is low, a developmental study is necessary in order to obtain better insights into the evolutionary history of the family. Therefore, the floral development of H. triflora and I. omeiana was investigated, representing the most early-diverged lineage of Impatiens, and the observations were compared with the literature. Flowers at all developmental stages were examined using scanning electron microscopy and light microscopy. In Hydrocera, two whorls of five free perianth primordia develop into a less zygomorphic perianth compared with its sister genus. The androecial cap originates from five individual stamen primordia. Post-genital fusion of the upper parts of the filaments result in a filament ring below the anthers. The anthers fuse forming connivent anther-like units. The gynoecium of Hydrocera is pentamerous; it is largely synascidiate in early development. Only then is a symplicate zone formed resulting in style and stigmas. In I. omeiana, the perianth is formed as in Hydrocera. Five individual stamen primordia develop into five stamens, of which the UPPER PART OF THE FILAMENTS CONVERGE WITH EACH OTHER. THE GYNOECIUM OF I. OMEIANA IS TETRAMEROUS; IT APPEARS ANNULAR IN EARLY DEVELOPMENT. Comparison of the present results with developmental data from the literature confirms the perianth morphocline hypothesis in which a congenital fusion of the parts of the perianth results in a shift from pentasepalous to trisepalous flowers. In addition, the development of the androecial cap and the gynoecium follows several distinct ontogenetic sequences within the family.

Floral ontogeny of Schisandra chinensis (Schisandraceae): implications for androecial evolution within Schisandra and Kadsura

The organogenesis of staminate and carpellate flowers of Schisandra chinensis (Schisandraceae) was investigated with scanning electron microscopy, with observations on the development of tepals reported for the first time. The results showed that there is no interval between the initiation of the last tepal and that of the first stamen or carpel, and that the shapes of tepal, stamen, and carpel primordia are similar. The tepals and stamens of staminate flowers are initiated acropetally in a continuous spiral Fibonacci phyllotaxis, with no carpel structures observed; the filaments are not connate. The organogenesis of the carpellate flowers is similar to that of the staminate flowers, but with no evidence of stamen development. The carpels are ascidiate without postgenital fusion. Three androecial characters of Schisandra and Kadsura are discussed in a phylogenetic context. The subglobose or obovoid androecium of Schisandrapropinqua and Schisandra plena may be homologous with that in sections Kadsura and Sarcocarpon. The plesiomorphic form of the androecium within the two genera is likely to be elongate with more than ten free stamens.

Theglossyhead1Allele ofACC1Reveals a Principal Role for Multidomain Acetyl-Coenzyme A Carboxylase in the Biosynthesis of Cuticular Waxes by Arabidopsis

A novel mutant of Arabidopsis (Arabidopsis thaliana), having highly glossy inflorescence stems, postgenital fusion in floral organs, and reduced fertility, was isolated from an ethyl methanesulfonate-mutagenized population and designated glossyhead1 (gsd1). The gsd1 locus was mapped to chromosome 1, and the causal gene was identified as a new allele of Acetyl-Coenzyme A Carboxylase1 (ACC1), a gene encoding the main enzyme in cytosolic malonyl-coenzyme A synthesis. This, to our knowledge, is the first mutant allele of ACC1 that does not cause lethality at the seed or early germination stage, allowing for the first time a detailed analysis of ACC1 function in mature tissues. Broad lipid profiling of mature gsd1 organs revealed a primary role for ACC1 in the biosynthesis of the very-long-chain fatty acids (C(20:0) or longer) associated with cuticular waxes and triacylglycerols. Unexpectedly, transcriptome analysis revealed that gsd1 has limited impact on any lipid metabolic networks but instead has a large effect on environmental stress-responsive pathways, especially senescence and ethylene synthesis determinants, indicating a possible role for the cytosolic malonyl-coenzyme A-derived lipids in stress response signaling.

The Arabidopsis ABCG13 transporter is required for flower cuticle secretion and patterning of the petal epidermis

Previous studies showed that ABCG11 and ABCG12, two ATP-binding-cassette (ABC) transporters, are required for cuticular lipids extracellular secretion. Here, we characterized ABCG13, a third clade member, to widen our limited knowledge regarding assembly of the plant's cuticle. We isolated an abcg13 knockout mutant and used RNAi and artificial microRNA approaches to study the effect of ABCG13 loss-of-function. These plants were subsequently used to conduct a detailed analysis of cuticular lipids composition and cytological observations. ABCG13 loss-of-function resulted in cuticle-related phenotypes that were restricted to flowers, including inter-organ post-genital fusions. Apart from a significant reduction in flower cutin monomers, the macromorphology and micromorphology of abcg13 petal epidermis was strongly affected. We also found that ABCG13 is highly expressed in flowers, predominantly in petals and carpels. The results suggest that ABCG13 is required for the transport of flower cuticular lipids. This work introduces a new component to the recently emerging genetic network that makes the archetypal exterior of Arabidopsis flowers. While the question regarding the substrate specificity of the ABCG12-clade members remains open, these findings will facilitate future investigations regarding the interaction between the half-size ABCG-type transporters that likely take part in cuticle assembly.

Evolutionary History of the Monocot Flower1

Abstract This paper reviews monocot flower structure and gynoecium development and evaluates these data to clarify the evolutionary history of the monocot flower. Despite some congruence between molecular and morphological data regarding the delimitation and phylogenetic relationships of monocots, there is currently no universally accepted view on the morphology of the ancestral monocot flower, reflecting a high degree of parallelism in monocot floral evolution. We focus on two character suites that encompass the key features of monocot flowers: (1) the typical monocot groundplan of trimerous-pentacyclic flowers, and (2) a character suite related to carpel fusion, including postgenital fusion between carpels and the presence of septal nectaries. It is likely that the trimerous-pentacyclic flower represents a major synapomorphy of monocots; this flower groundplan is virtually absent from the closest relatives of monocots. Such close correlation of a particular groundplan with a phylogenetic group is analog...

Pollen tube growth and the pollen-tube pathway of Nymphaea odorata (Nymphaeaceae)

An important aspect of the evolution of carpel closure, or angiospermy, is the relationship between pollen tube growth patterns and internalization of the pollen-tube pathway. True carpel closure, involving postgenital fusion of inner carpel margins, is inferred to have arisen once within the ancient order Nymphaeales, in the common ancestor of Nymphaeaceae. We studied pollen tube development, from pollination to fertilization, in a natural population of Nymphaea odorata, using hand pollinations and timed flower collections. Pollen germinates in stigmatic secretions within 15 min and pollen tubes enter subdermal transmitting tissue within an hour, following wide intercellular spaces towards the zone of postgenital fusion. At the zone of fusion they turn downwards to grow in narrow spaces between interlocked cells and then wander freely to ovules within ovarian secretions. The pollen-tube pathway is 2–6 mm long and upper ovules are first penetrated 2.5 h after pollination. Pollen tubes grow at rates of approximately 1 mm/h whether in stigmatic fluid, transmitting tissues or ovarian secretions. Pollen-tube pathways are structurally diverse across Nymphaeales, yet their pollen tubes have similar morphologies and rapid growth rates. This pattern suggests pollen tube growth innovations preceded and were essential for the evolution of complete carpel closure.

Diversity and evolution of floral structure among early diverging lineages in the Ericales.

This is a combination of review and original data on floral structure and diversity in the two earliest diverging lineages of the Ericales, i.e. the balsaminoids, comprising Balsaminaceae, Marcgraviaceae and Tetrameristaceae, and the polemonioids, comprising Fouquieriaceae and Polemoniaceae. Each clade is strongly supported in molecular studies, while structural synapomorphies have largely been lacking. For the balsaminoid families, we compare floral morphology, anatomy and histology among selected taxa and find that the entire clade is strongly supported by the shared presence of nectariferous tissue in the floral periphery, thread-like structures on anthers, truncate stigmas, secretion in the ovary, as well as mucilage cells, raphides and tannins in floral tissues. A possible sister group relationship between Balsaminaceae and Tetrameristaceae is supported by the shared presence of post-genital fusion of filaments and ovary and a star-shaped stylar canal. For polemonioids, we document unexpected diversity of floral features in Polemoniaceae, partly providing structural links to Fouquieriaceae. Features include cochlear and quincuncial corolla aestivation, connective protrusions, ventrifixed anthers and nectariferous tissue in the base of the ovary. In addition, we outline future directions for research on floral structure in the Ericales and briefly discuss the general importance of structural studies for our understanding of plant phylogeny and evolution.

Comparative floral structure and systematics in the Asian palm genus Rhapis (Arecaceae, Coryphoideae)

A comparative study of the floral structure in the species of the genus Rhapis (Arecaceae, Coryphoideae, Rhapidinae) is presented. Flowers are mainly unisexual, with three sepals, three petals, 6 stamens or staminodes and three carpels or carpellodes. Some evidences of basal congenital and apical postgenital fusion of the carpels, first time reported in the genus, were observed in the gynoecium. Ovules are basally attached and crassinucellate; they appear to be slightly anatropous. The morphology of the filaments suggests a division of the species into two groups: Rhapis excelsa and R. subtilis exhibit thick and keeled filaments, whereas R. gracilis, R. humilis, R. laosensis, R. micrantha and R. multifida have slender, non-keeled filaments. Relationships of Rhapis with the rest of the genera of Rhapidinae are inferred on the light of floral structure.

Floral evolution in the monocot family Nartheciaceae (Dioscoreales): evidence from anatomy and development inMetanarthecium luteo-virideMaxim.

The placement of Nartheciaceae within Dioscoreales is an unexpected result of molecular phylogenetics. Nartheciaceae generally differs from the rest of Dioscoreales in having less specialized flowers. Studies of this family are important to elucidate the evolutionary history of the order. Using scanning electron microscopy and light microscopy, we describe the details of the flower structure, initiation, and development in Metanarthecium, which is unique amongst Nartheciaceae in possessing both an almost superior ovary and septal nectaries. This is the first member of Dioscoreales for which all stages of organogenesis have been studied. Within Nartheciaceae, the presence/absence of septal nectaries and the position of the ovary are labile. The presence of post-genital fusion in the gynoecium correlates with the presence of septal nectaries. Septal nectary morphology is complicated in Metanarthecium, which raises the question of whether its floral structure (including superior ovary) is plesiomorphic within Dioscoreales. The septal nectaries of Metanarthecium show homoplastic similarity with those of Allium (Asparagales). The presence/absence of a compitum is probably variable at the infraspecific level in Metanarthecium as a result of alternative possibilities of post-genital fusion between ventral carpel margins.

The ArabidopsisDESPERADO/AtWBC11Transporter Is Required for Cutin and Wax Secretion

The cuticle fulfills multiple roles in the plant life cycle, including protection from environmental stresses and the regulation of organ fusion. It is largely composed of cutin, which consists of C(16-18) fatty acids. While cutin composition and biosynthesis have been studied, the export of cutin monomers out of the epidermis has remained elusive. Here, we show that DESPERADO (AtWBC11) (abbreviated DSO), encoding a plasma membrane-localized ATP-binding cassette transporter, is required for cutin transport to the extracellular matrix. The dso mutant exhibits an array of surface defects suggesting an abnormally functioning cuticle. This was accompanied by dramatic alterations in the levels of cutin monomers. Moreover, electron microscopy revealed unusual lipidic cytoplasmatic inclusions in epidermal cells, disappearance of the cuticle in postgenital fusion areas, and altered morphology of trichomes and pavement cells. We also found that DSO is induced by salt, abscisic acid, and wounding stresses and its loss of function results in plants that are highly susceptible to salt and display reduced root branching. Thus, DSO is not only essential for developmental plasticity but also plays a vital role in stress responses.

A NEW EXPLANATION FOR THE EVOLUTION OF POLLINIA AND LOSS OF CARPEL FUSION IN ASCLEPIAS AND THE APOCYNACEAE S.L.1

The demonstration of postzygotic self-incompatibility in Asclepias L. prompted us to reconsider explanations for several unusual features of the Apocynaceae. With such late-acting self-incompatibility, mixed loads of self- and cross-pollen can be extremely wasteful by causing abortion of whole fruits and consequent wastage of both cross-pollen and cross-fertilized ovules. We contend that the evolution of pollinia and loss of carpel fusion in certain Apocynaceae represent adaptations to prevent or compensate for these negative effects of mixed pollen loads. Trends in the agglutination of pollen can be seen in the tetrads of Apocynum L., which are transported on sticky band-like translators; the masses of tetrads of Periplocoideae, which are deposited onto the sticky scoop portion of a spoon-like translator; the pollinia of Secamonoideae, which have a clip-like translator but no outer covering; and the pollinia of Asclepiadoideae, which are completely enclosed by a waxy outer covering. In many genera of the Rauvolfioideae, the ovary is syncarpous, but in most Apocynoideae and all Periplocoideae and Secamonoideae, the gynoecium consists of two carpels that are free in the ovule-bearing region but fused in the upper region to produce a compitum, which enables pollen placed on one stigmatic area to supply pollen tubes to both ovaries. In at least some Asclepiadoideae, however, a compitum is lacking, although postgenital fusion between the carpel apices still takes place. Parallel trends in pollen delivery and receipt also occur in taxa from other angiosperm families in which late-acting self-incompatibility systems have been implicated.

Floral development in three species of Acacia (Leguminosae, Mimosoideae)

The complete sequence of floral development in three species of Acacia was analysed. These species were sampled from each of the three Acacia subgenera. The species were Acacia berlandieri Benth. (subg. Aculeiferum), A. pennatula (Schltdl. & Cham.) Benth. (subg. Acacia) and A. saligna (Labill.) H.L.Wendl. (subg. Phyllodineae). The aim of the study was to determine whether the different subgenera share developmental pathways during flower formation. This study showed that development in the genus Acacia is heterogeneous. Each species studied showed different inception patterns of the calyx and androecium, whereas the inception patterns of the corolla and gynoecium were similar. These differences of inception in the calyx are not necessarily constant within each subgenus. Nevertheless, each subgenus was differentiated on the basis of inception patterns of the androecium, and other features such as the presence or absence of congenital or postgenital fusion in the calyx and corolla, and the time of differentiation of calyx and corolla tubes and the style.

Cloning and characterization of the WAX2 gene of Arabidopsis involved in cuticle membrane and wax production.

Insertional mutagenesis of Arabidopsis ecotype C24 was used to identify a novel mutant, designated wax2, that had alterations in both cuticle membrane and cuticular waxes. Arabidopsis mutants with altered cuticle membrane have not been reported previously. Compared with the wild type, the cuticle membrane of wax2 stems weighed 20.2% less, and when viewed using electron microscopy, it was 36.4% thicker, less opaque, and structurally disorganized. The total wax amount on wax2 leaves and stems was reduced by >78% and showed proportional deficiencies in the aldehydes, alkanes, secondary alcohols, and ketones, with increased acids, primary alcohols, and esters. Besides altered cuticle membranes, wax2 displayed postgenital fusion between aerial organs (especially in flower buds), reduced fertility under low humidity, increased epidermal permeability, and a reduction in stomatal index on adaxial and abaxial leaf surfaces. Thus, wax2 reveals a potential role for the cuticle as a suppressor of postgenital fusion and epidermal diffusion and as a mediator of both fertility and the development of epidermal architecture (via effects on stomatal index). The cloned WAX2 gene (verified by three independent allelic insertion mutants with identical phenotypes) codes for a predicted 632-amino acid integral membrane protein with a molecular mass of 72.3 kD and a theoretical pI of 8.78. WAX2 has six transmembrane domains, a His-rich diiron binding region at the N-terminal region, and a large soluble C-terminal domain. The N-terminal portion of WAX2 is homologous with members of the sterol desaturase family, whereas the C terminus of WAX2 is most similar to members of the short-chain dehydrogenase/reductase family. WAX2 has 32% identity to CER1, a protein required for wax production but not for cuticle membrane production. Based on these analyses, we predict that WAX2 has a metabolic function associated with both cuticle membrane and wax synthesis. These studies provide new insight into the genetics and biochemistry of plant cuticle production and elucidate new associations between the cuticle and diverse aspects of plant development.

Female flowers and inflorescences of Didymelaceae

In molecular analyses Didymelaceae together with Buxaceae form a fairly well-supported clade among families near the base of eudicots. Only little is known, however, about the flowers and inflorescences of Didymelaceae. In this study, the structure of the female flowers and inflorescences of Didymeles integrifolia was studied. Flowers are unicarpellate and orientation of the carpel is slightly deflected abaxially as in Proteaceae. Otherwise, Didymelaceae share many features of the gynoecium with Buxaceae and some other basal eudicots: the carpels are ascidiate in the lower half; anthetic carpels are completely closed by postgenital fusion; stigma is double-crested and widely decurrent; stigmatic papillae are unicellular and pear-shaped; the pollen tube transmitting tract is extensive and prominently differentiated; fruits are fleshy drupes with persistent stigma and style. However, the exceedingly elongate base of the integuments of Didymelaceae is an unusual feature among basal eudicots and even angiosperms.

The Flowers in Extant Basal Angiosperms and Inferences on Ancestral Flowers

This is a combination of a review and original data on floral structure, development, and biology of representatives of all families of the ANITA grade and, in addition, Chloranthaceae and Ceratophyllaceae. Since the ANITA grade has been identified as the basalmost grade of extant angiosperms based on molecular studies by a number of authors, it has become possible to search for potential plesiomorphies among flowers of extant basal angiosperms. They may include the following traits: flowers small, pollination by small insects (dipters, thrips, moths); flowers with moderate or low number of floral organs, in spiral (or whorled) arrangement, with a tendency to form organ series in Fibonacci numbers (3, 5, 8); flowers bisexual (but easily becoming unisexual because of low level of synorganization between organs), protogynous; tepals (in spiral flowers) with gradual transitions between bractlike, sepal‐like, and petal‐like forms; stamens with short filaments, anthers with a connective tip, with more or less bulging disporangiate thecae; thecae opening by a longitudinal slit and not by valves. Carpels free, styleless, extremely ascidiate, with one or only few anatropous ovules, sealed by secretion and not by postgenital fusion; stigmas wet, with multicellular protrusions. Among members of the ANITA grade, there is a trend to form extragynoecial compita. In those taxa with the relatively most complicated gynoecium architecture (including an extragynoecial compitum), there is a concomitant trend to have less strongly ascidate to almost plicate carpels (Nymphaeaceae, Schisandraceae, Illiciaceae).

Origins of flower morphology.

Flowers evolved in many steps, probably starting long before flowering plants (angiophytes) originated. Certain parts of flowers are conservative and have not changed much during evolution; others are evolutionarily highly plastic. Here conservative features are discussed and an attempt is made to trace them back through their evolutionary history. Microsporangia and ovules (which develop into seeds) are preangiophyte floral elements. Angiospermy, combined with postgenital fusion, was the most prominent key innovation in angiophytes. Angiospermy and thecal organization of stamens originated earlier than all clades of extant angiosperms (the crown group of angiophytes). Differentiation of a perianth into calyx and corolla and syncarpy appeared after the first branching of the basalmost clades of extant angiosperms. Sympetaly and floral tubes as well as tenuinucellar, unitegmic ovules originated as major innovations in the clade that led to asterids. An obvious trend in flower evolution is increased synorganisation of parts, which led to new structures. Fixation of floral organ number and position was a precondition for synorganization. Concomitantly, plasticity changed from number and position of organs to shape of the new structures. Character distribution mapped onto cladograms indicates that key innovations do not appear suddenly, but start with trials and only later become deeply rooted genetically in the organization. This is implied from the common occurrence of reversals in the early history of an innovation. J. Exp. Zool. (Mol. Dev. Evol.) 291:105-115, 2001.

Floral Structure and Development ofApostasiaandNeuwiedia(Apostasioideae) and their Relationships to Other Orchidaceae

Floral development, structure, and pollination biology of species of Apostasia and Neuwiedia constituting the basalmost orchid subfamily Apostasioideae were studied. The perianth organs arise from a ring primordium. The two adaxial sepals appear first, followed by the petal primordia. This contrasts with monandrous orchids, in which the median petal primordium is the first organ of the inner perianth whorl to develop. Stamens are only formed on the abaxial side of the flower. However, in Neuwiedia veratrifolia and Apostasia nuda, vestiges of stamen primordia are present on the adaxial side of the flower. In Apostasia only, the lateral stamens are fully developed, whereas the median organ is a staminode; in A. nuda the staminode is suppressed. At anthesis the anthers are free in Neuwiedia, but they are postgenitally connected in Apostasia. In both genera, stamens and style are congenitally fused at the base. The inferior ovary is at first unilocular and then becomes trilocular by postgenital fusion of the ovary septa and the development of a short synascidiate zone. The three carpels of both genera are equal from the beginning, but in A. nuda only, the median carpel dominates during early development. The stigma is convex and papillate. Conspicuous raphide idioblast accumulations occur in staminode and style and in the tips of anthers and filaments. In contrast to all other orchids, both genera offer pollen as a reward. Neuwiedia has lip flowers, which are pollinated by Trigona bees; Apostasia has “Solanum‐type” flowers with recurved perianth and united anthers. Our study supports the view that Apostasia and Neuwiedia are closely related and that Apostasioideae are at the base of the orchid clade, sharing characters with orchids and lower Asparagales.

Gynoecium diversity and systematics in basal monocots

Gynoecium and ovule structure was comparatively studied in representatives of the basal monocots, including Acorales (Acoraceae), Alismatales (Araceae, Alismataceae, Aponogetonaceae, Butomaceae, Hydrocharitaceae, JuncÍaginaceae, Limnocharitaceae, Potamogetonaceae, Scheuchzeriaceae, Tofieldiaceae), Dioscoreales (Dioscoreaceae, Taccaceae), and Triuridaceae as a family of uncertain position in monocots. In all taxa studied the carpels or gynoecia are closed at anthesis. This closure is attained in different ways: (1) by secretion without postgenital fusion (Araceae, Hydrocharitaceae); (2) by partly postgenitally fused periphery but with a completely unfused canal (Alismataceae, Aponogetonaceae, Butomaceae, Limnocharitaceae, Scheuchzeriaceae, Dioscoreaceae, Taccaceae); (3) by completely postgenitally fused periphery but with an unfused canal in the centre (Acoraceae, Tofieldiaceae); (4) by complete postgenital fusion and without an (unfused) canal (Juncaginaceae, Potamogetonaceae). In many Alismatales (but without Araceae) carpels have two lateral lobes. The stigmatic surface is restricted to the uppermost part of the ventral slit (if the carpel is plicate); it is never distinctly double-crested (Butomaceae?). Stigmas are commonly unicellular-papillate and secretory in most taxa. The locules are filled with a (often) mucilaginous secretion in a number of taxa. Superficial (probably intrusive) ethereal oil cells were found in the carpel wall of Acorus gramineus (as in Piperales!). Idioblasts in carpels are otherwise rare. A number of basal monocots has orthotropous ovules, which is perhaps the plesiomorphic condition in the group. The presence of almost tenuinucellar (pseudocrassinucellar) ovules is relatively common (Acoraceae, many Araceae, some Alismatales s.s.), whereas completely tenuinucellar ovules are rare and do not characterize larger groups. However, crassinucellar ovules occur in the largest number of families among the study group (basal Araceae, many Alismatales s.s.) The outer integument is always annular in orthotropous ovules. The inner integument is often lobed and it mostly forms the micropyle, whereas the outer integument is always unlobed. Gynoecium structure supports the isolated position of Acoraceae as sister to all other monocots. However, in an overall view, if compared with all other families, Acoraceae clearly shows the greatest similarities with Araceae.

Gynoecium diversity and systematics in basal monocots

Abstract Gynoecium and ovule structure was comparatively studied in representatives of the basal monocots, including Acorales (Acoraceae), Alismatales (Araceae, Alismataceae, Aponogetonaceae, Butomaceae, Hydrocharitaceae, JuncIaginaceae, Limnocharitaceae, Potamogetonaceae, Scheuchzeriaceae, Tofieldiaceae), Dioscoreales (Dioscoreaceae, Taccaceae), and Triuridaceae as a family of uncertain position in monocots. In all taxa studied the carpels or gynoecia are closed at anthesis. This closure is attained in different ways: (1) by secretion without postgenital fusion (Araceae, Hydrocharitaceae); (2) by partly postgenitally fused periphery but with a completely unfused canal (Alismataceae, Aponogetonaceae, Butomaceae, Limnocharitaceae, Scheuchzeriaceae, Dioscoreaceae, Taccaceae); (3) by completely postgenitally fused periphery but with an unfused canal in the centre (Acoraceae, Tofieldiaceae); (4) by complete postgenital fusion and without an (unfused) canal (Juncaginaceae, Potamogetonaceae). In many Alismatales (but without Araceae) carpels have two lateral lobes. The stigmatic surface is restricted to the uppermost part of the ventral slit (if the carpel is plicate); it is never distinctly double-crested (Butomaceae?). Stigmas are commonly unicellular-papillate and secretory in most taxa. The locules are filled with a (often) mucilaginous secretion in a number of taxa. Superficial (probably intrusive) ethereal oil cells were found in the carpel wall of Acorus gramineus (as in Piperales!). Idioblasts in carpels are otherwise rare. A number of basal monocots has orthotropous ovules, which is perhaps the plesiomorphic condition in the group. The presence of almost tenuinucellar (pseudocrassinucellar) ovules is relatively common (Acoraceae, many Araceae, some Alismatales s.s. ), whereas completely tenuinucellar ovules are rare and do not characterize larger groups. However, crassinucellar ovules occur in the largest number of families among the study group (basal Araceae, many Alismatales s.s. ) The outer integument is always annular in orthotropous ovules. The inner integument is often lobed and it mostly forms the micropyle, whereas the outer integument is always unlobed. Gynoecium structure supports the isolated position of Acoraceae as sister to all other monocots. However, in an overall view, if compared with all other families, Acoraceae clearly shows the greatest similarities with Araceae.

Gynoecium Structure and Evolution in Basal Angiosperms

A comparative study on gynoecium structure in basal angiosperms revealed new features relevant to systematics. In addition, based on the new phylogenetic framework on basal angiosperms gained by molecular systematics, an evolutionary evaluation of gynoecial character states has become possible. Angiospermy patterns are diverse and are here tentatively grouped into four types according to the mode of carpel closure: (1) by secretion, without postgenital fusion; (2) with a continuous secretory canal but partial postgenital fusion at the periphery; (3) with an incomplete secretory canal and complete postgenital fusion at the periphery; (4) by complete postgenital fusion. In most basal angiosperms the carpels are free, and the inner space of each carpel is occluded from the outside by secretion and not by postgenital fusion (angiospermy type 1). Correlated with this feature is a pronounced ascidiate shape of the carpels. The carpels tend to be shortly stipitate. In relatively many basal clades with pluricarpellate gynoecia an extragynoecial compitum is present. Stigmas are secretory and tend to have pluricellular papillae. Ethereal oil cells at the surface of the carpels (probably intrusive) are present in Illiciales (including Trimeniaceae), Chloranthaceae, Piperales s.l. (including Aristolochiales), and Acoraceae. Ovule structure is diverse. Ovules are predominantly anatropous. Although Amborellaceae, the phylogenetically earliest branching extant angiosperm, has orthotropous ovules, it is argued that the anatropous condition is basal in angiosperms. Crassinucellar ovules are predominant in basal angiosperms. However, there are apomorphic tendencies toward ovule reduction: weakly crassinucellar (Nymphaeales, some Alismatales), pseudocrassinucellar (Acorales, some Alismatales, some Ranunculales), and tenuinucellar (some Piperales s.l., including Aristolochiales, Rafflesiales, some Alismatales). Lobation of the inner integument and semiannular shape of the outer integument are variously present or absent. They appear to be due to architectural constraints in ovule development and not to constitute archaic features. In contrast to anatropous ovules, orthotropous ovules often have an outer integument that is thinner than the inner one. This may be due to a function of the outer integument in ovule curvature as indicated by molecular developmental genetics. On the other hand, the presence of an outer integument that is thinner than the inner one characterizes the majority of the Piperales s.l. (including Aristolochiales), irrespective of presence or absence of ovule curvature.