Monosulcate Pollen Research Articles

Variation of microsporogenesis in monocots producing monosulcate pollen grains

Microsporogenesis leading to monosulcate pollen grains has already been described for a wide range of monocot species. However, a detailed study of additional callose deposition after the completion of the cleavage walls has been neglected so far. The study of additional callose deposition in monosulcate pollen grain has gained importance since a correlation between additional callose deposition and aperture location has recently been revealed. Microsporogenesis is described for 30 species belonging to eight families of the monocots: Acoraceae, Amaryllidaceae, Alstroemeriaceae, Asparagaceae, Butomaceae, Commelinaceae, Liliaceae and Xanthorrhoeaceae. Five different microsporogenesis pathways are associated with monosulcate pollen grain. They differ in the type of cytokinesis, tetrad shape, and the presence and shape of additional callose deposition. Four of them present additional callose deposition. In all these different microsporogenesis pathways, aperture location seems to be linked to the last point of callose deposition.

The trichotomosulcate asparagoids: pollen morphology of Hemerocallidaceae in relation to systematics and pollination biology

We examined pollen of 19 genera of Hemerocallidaceae by using scanning electron microscopy (SEM), and one genus (Dianella) by using transmission electron microscopy (TEM). Pollen was generally small in size, with a rounded triangular outline when hydrated, and a characteristic three-armed aperture, a distal trichotomosulcus. The pollen surface was finely sculptured and the exine was thin. Microreticulate pollen is a potential synapomorphy for several species of the ‘crown phormioid’ subclade recognised in molecular analyses. Perforate and fossulate pollen supports a relationship between several species of Dianella. Microrugulate pollen is more frequent in the johnsonioids than in the phormioids. Hemerocallis is distinguished by elongated monosulcate pollen, a relatively thick exine with a pronounced reticulate surface, and large globules of attached pollenkitt. We hypothesise that Hemerocallidaceae are ancestrally buzz-pollinated, and their pollen morphology is an adaptation to this pollination type. A reversal to butterfly or moth pollination occurred in Hemerocallis, with associated changes in pollen morphology.

Angiosperm-like pollen and Afropollis from the Middle Triassic (Anisian) of the Germanic Basin (Northern Switzerland).

Here we report on angiosperm-like pollen and Afropollis from the Anisian (Middle Triassic, 247.2–242.0 Ma) of a mid-latitudinal site in Northern Switzerland. Small monosulcate pollen grains with typical reticulate (semitectate) sculpture, columellate structure of the sexine and thin nexine show close similarities to early angiosperm pollen known from the Early Cretaceous. However, they differ in their extremely thin inner layer (nexine). Six different pollen types (I–VI) are differentiated based on size, reticulation pattern, and exine structure. The described pollen grains show all the essential features of angiosperm pollen. However, considering the lack of a continuous record throughout the lower part of the Mesozoic and the comparison with the oldest Cretaceous finds we suggest an affinity to an angiosperm stem group. Together with the previously published records from the Middle Triassic of the Barents Sea area the angiosperm-like pollen grains reflect a considerable diversity of the parent plants during the Middle Triassic. Sedimentological evidence and associated palynofloras also suggest a remarkable ecological range for these plants. Associated with these grains we found pollen comparable to the genus Afropollis. Representatives of this genus are commonly recorded in Lower Cretaceous sediments of low latitudes, but until now had no record from the lower part of the Mesozoic.

Androecium development in the bromeliad Dyckia pseudococcinea L.B.Sm. (Pitcairnioideae-Bromeliaceae), an endangered species endemic to Brazil: implications for conservation

Dyckia pseudococcinea L.B.Sm. is endemic to the restingas of Maricá, State of Rio de Janeiro, in southeastern Brazil. However, since this area is under intense ecological stress, D. pseudococcinea is considered an endangered plant species. An important step toward the conservation of this species is establishing the developmental stage most favorable to in vitro propagation. Accordingly, the present study analyzed the androecium to establish the developmental stages of anthers, emphasizing anther wall development, androsporogenesis and androgametogenesis. The relationships between the size of flowers and anthers were also studied. Anther wall development follows the basic-type, while the tapetum, which is originated from the subepidermal layer (inner secondary parietal layer 2 and archesporial initials), follows the secretory-type. Androsporogenesis is successive and originates isobilateral and decussate tetrads. The oblate and monosulcate pollen grains exhibit microreticulate exine and are dispersed at the bicellular stage. At the vacuolated stage, the androspore (microspore) is still proliferating and not yet completely differentiated. Therefore, since the androspore has not yet lost its embryonic capacity, we suggest the preferential use of this developmental stage, which is present in flower buds 6.4–7.1mm in length, in conservation protocols focused on the androgenesis of D. pseudococcinea.

Molecular and Fossil Evidence on the Origin of Angiosperms

Molecular data on relationships within angiosperms confirm the view that their increasing morphological diversity through the Cretaceous reflected their evolutionary radiation. Despite the early appearance of aquatics and groups with simple flowers, the record is consistent with inferences from molecular trees that the first angiosperms were woody plants with pinnately veined leaves, multiparted flowers, uniovulate ascidiate carpels, and columellar monosulcate pollen. Molecular data appear to refute the hypothesis based on morphology that angiosperms and Gnetales are closest living relatives. Morphological analyses of living and fossil seed plants that assume molecular relationships identify glossopterids, Bennettitales, and Caytonia as angiosperm relatives; these results are consistent with proposed homologies between the cupule of glossopterids and Caytonia and the angiosperm bitegmic ovule. Jurassic molecular dates for the angiosperms may be reconciled with the fossil record if the first angiosperms were restricted to wet forest understory habitats and did not radiate until the Cretaceous.

Evolution and phylogenetic significance of pollen in Annonaceae

The remarkable diversity of pollen morphology in Annonaceae has attracted systematic attention since the 1970s. Optimization of pollen characters on a molecular tree confirms that granular monosulcate pollen, as in Anaxagorea and subfamily Ambavioideae, was ancestral in the family. However, granular structure is derived for angiosperms as a whole, and exine characters are highly homoplastic in Annonaceae. Because columellar taxa (Annickia, Bocageeae) diverged earlier than previously thought, columellae may have re-originated in the common ancestor of Malmeoideae and Annonoideae, which make up most of the family, with several later reversals to granular structure. In Malmeoideae, loss of the sulcus and origin of a verrucate tectum occurred in the tribe Miliuseae, followed by an uncertain number of origins of disulculate pollen. Tetrads arose several times in Annonoideae, but reverted to monads in Isolona and probably Uvarieae. Recognition that microspores rotate during development in Annona and Cymbopetalum suggests that the proximal thin area in these taxa is a modified distal sulcus, but rotation does not occur in all tetrad groups; a broader survey is needed to evaluate this phenomenon. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 169, 190–221.

Exine ultrastructure of in situ pollen from the cycadalean cone Androstrobus prisma Thomas et Harris 1960 from the Jurassic of England

Pollen grains extracted from the cycad pollen cone Androstrobus prisma Thomas et Harris 1960 from the Bajocian of Yorkshire were studied by means of LM, SEM and TEM. The species is characterized by rounded-oval, inaperturate pollen with an indistinctly verrucate surface and predominantly homogeneous exine with occasional alveolate areas. Present and earlier published data on the fine morphology of pollen grains of fossil cycads are discussed in the light of the differentiation between Mesozoic non-saccate and presumably monosulcate pollen produced by a number of gymnosperm groups. Although the exine pattern in pollen grains of fossil cycads is usually indistinct in transmitted light, SEM reveals minute, but various sculpturing that can be used for taxonomic purposes, particularly to differentiate between species. Elongated ectexinal alveolae, situated mostly in one row and covered by a thin tectum, is an unequivocal cycadalean character. Sufficiently well preserved fossil cycad pollen clearly shows this type of exine ultrastructure. Poorly preserved cycad pollen grains commonly show an alternation of alveolate and homogeneous regions in the exine with predominance of homogeneous regions. This peculiar mode of preservation can be used as a hint to reveal cycadalean affinity. Inaperturate pollen grains are unknown in bennettites or ginkgophytes. Therefore, if a dispersed, non-saccate and boat-shaped pollen grain is proved by means of electron microscopy to lack an aperture, this would indicate a possible cycadalean affinity.

Ecology of flowers and morphology of pollen grains of selected Narcissus varieties (Narcissus pseudonarcissus x Narcissus poëticus)

The investigations carried out in the years 2004-2005 covered five varieties of Narcissus (<i>Narcissus pseudonarcissus</i> L. x <i>Narcissus poëticus</i> L.): 'Fire Bird', 'Hardy', 'Ivory Yellow', 'Pomona' and 'The Sun'. Longevity of a flower and the flowering period of the studied Narcissus varieties determined, flower elements were compared, as well as the anatomic structure of ovary and nectary and the morphology of pollen grains. Pollen grains of 'Hardy' and 'The Sun' var. were observed in a scanning electron microscope. The flowering period of 'The Sun' plants appeared to be the longest, while the shortest in the case of 'Fire Bird' and 'Ivory Yellow'. Each perianth of the studied taxons was characterized by a different corolla appendage, from slightly corrugated ('Pomona') to greatly curled, with the notches getting inside and giving impression of a few whort in this part of the perianth ('Ivory Yellow', 'Hardy'). Monosulcate pollen grains with regards to 9 size was numbered among medium ('Pomona', 'The Sun') and large ('Fire Bird', 'Hardy', 'Ivory Yellow'). Their shape was assumed as circular elongated (<i>prolato-spheroides</i>) in polar view and flattened (<i>oblatum</i>) and slightly flat (<i>suboboblatum</i>) in equatorial view. The highest pollen vitality was shown by 'Hardy' variety (92%), whereas for 'The Sun' it proved to be the lowest (22%). In the narcissus flowers, septal nectaries are situated in the upper part of the ovary.

The ultrastructure of fossil dispersed monosulcate pollen from the Early Cretaceous of Transbaikalia, Russia

The general morphology, surface sculpturing, and exine ultrastructure have been studied in dispersed monosulcate pollen from the Early Cretaceous of Transbaikalia, Russia. The pollen grains dominate the palynological assemblage extracted from coal deposits of the Khilok Formation in the Buryat Republic, which also contain ginkgoalean leaves of Baierella averianovii as the only constituent of the assemblage of plant megafossils. The relationship between the pollen grains and ginkgoalean leaves from this autochthonous burial is hypothesised on the basis of taphonomical analysis and palaeobiogeographical data. It is shown that the ectexine of the pollen grains includes a thick solid tectum, a thin granular infratectum and a thin foot layer; the endexine is fine-grained, slightly more electron-dense than the ectexine, and is preserved only in places. The distal aperture is formed by a thinning of the exine. No analogous ultrastructure has been described so far in fossil pollen grains of this morphotype studied ultrastructurally from in situ material. For comparison, we also studied the exine ultrastructure of pollen grains Ginkgo biloba. The fossil pollen is not identical to pollen of extant G. biloba, but shows several significant similarities in the exine ultrastructure, which does not contradict the presumable ginkgoalean affinity of the fossil pollen.

Upper Cretaceous sulcate pollen from the Timerdyakh Formation, Vilui Basin (Siberia)

One inaperturate and 16 monosulcate pollen types are described from the latest Campanian to earliest Maastrichtian sediments of the Vilui basin, Siberia, using both light and scanning electron microscopy, and assigned systematically when possible to modern families or subfamilies. Despite their scant occurrence, the pollen grains show considerable diversity. Two new genera have been erected. Lasioideaecidites with two species, Lasioideaecidites hessei and Lasioideaecidites bogneri, represents the earliest record of the subfamily Lasioideae (Araceae). Aristolochiacidites with one species, Aristolochiacidites viluiensis, is assigned to the subfamily Aristolochioideae (Aristolochiaceae) and represents the first fossil pollen record of the family. A new species of Liliacidites, Liliacidites goldblattii, is closely similar to pollen of Isophysioideae (Iridaceae) and/or Doryanthaceae and is the first fossil evidence of the clade that includes these closely related families. A further nine new fossil species are described (Clavatipollenites timerdyakhensis, Liliacidites palaeofritillaria, Retimonocolpites longosucatus, R. microreticulatus, R. microrugulatus, R. lysichitonoides, R. chapmaniae, Monosulcites parvus and Arecipites tyungensis) and assigned to Chloranthaceae, Araceae (Orontioideae, ?Pothoideae), Liliaceae, ?Hypoxidaceae and Arecaceae. The Hydatellaceae (Nymphaeales) may also be present in the palynoflora of the Vilui Basin, represented by pollen similar to M. rivularis Braman from the Santonian to Campanian (Upper Cretaceous) Milk River Formation, Canada.

Pollen Evolution in the Early‐Divergent Monocot Order Alismatales

Alismatales are a key clade in monocot evolution, as they are sister to all other monocots, excluding Acorales. However, phylogenetic relationships within the order are poorly resolved, prompting the need for a reevaluation of pollen and tapetum characters, which have proved useful in other monocot groups, such as lilioids. Development of monosulcate pollen in Aponogeton (Aponogetonaceae) and Butomus (Butomaceae) and of pantoporate pollen in Echinodorus (Alismataceae) is examined here. Phylogenetic mapping of pollen and tapetum characters, based on both original observations and an extensive literature search, demonstrates that Alismatales are characterized by a potential synapomorphy: a plasmodial tapetum. Microsporogenesis is successive, a character shared with Acorus and probably other potential outgroups. Linked character syndromes, involving thin‐walled or exineless, inaperturate (omniaperturate) pollen with a smooth or reticulate surface, have evolved iteratively within the order, probably related to the aquatic or semiaquatic habit of many alismatids. Extremely long (up to 1500 μm) filamentous pollen, associated with hydrophilous pollination, occurs in three closely related families (Cymodoceaceae, Posidoniaceae, and Zosteraceae). Pantoporate spiny pollen is a potential synapomorphy for Alismataceae (including Limnocharitaceae). Alismatales share some characters (plasmodial tapetum and reniform pollen) with Pandanales, although further work is required to investigate whether this apparent homoplasy is of systematic significance. Tofieldiaceae possess two interesting apomorphies (simultaneous microsporogenesis and a secretory tapetum) that are also found in Petrosaviales and some Dioscoreales.

Pennsylvanian and Cisuralian palynofloras from the Los Sauces area, La Rioja Province, Argentina: Chronological and paleoecological significance

Abstract Three outcrops of the Libertad and Sauces Formations from the Los Sauces area La Rioja Province, western Argentina, yielded the nine palynoassemblages studied here. Two assemblage zones are defined on the basis of the stratigraphic distribution and ranges of seventy five species of palynomorphs (42 species of spores, 32 pollen taxa and one fungus). Only thirteen species are common to both assemblages and ten species are first records for the Paganzo Basin. Assemblage 1 from the Libertad Formation is dominated by trilete spores of Cristatisporites (lycophyte) and Punctatisporites (pteridophyte). Monosaccate pollen (Coniferales/Cordaitales) is frequently present. Pteridosperms, mostly represented by Cyclogranisporites, are especially abundant in one level together with scarce striate bisaccate pollen grains. Assemblage 2 of the Sauces Formation is dominated by trilete spores related to the Pteridophyta (e.g., Horriditriletes, Converrucosisporites, Granulatisporites) and Sphenophyta. Monosaccate (Cordaitales/Coniferales) and taeniate and non-taeniate bisaccate pollen grains (Pteridospermales/Coniferales), are equally subordinated. Monosulcate pollen (Cycadophyta) and fungi (Portalites gondwanensis) are rare. Assemblage 1 is mainly Moscovian; assemblage 2 Asselian–Sakmarian. This interpretation is based on correlation of assemblage 1 to the DMb (Mid Pennsylvanian) and assemblage 2 to the FS (Early Cisuralian) Biozones of the Paganzo Basin (Argentina). The taxonomic composition of the Ahrensisporites cristatus–Crucisaccites monoletus (Mid–Late Pennsylvanian) and the Protohaploxypinus goraiensis Subzone (Asselian–Sakmarian) of the Vittatina costabilis (Early Cisuralian) Biozones of the Parana Basin (Brazil) support this correlation. The continental freshwater depositional setting of this part of the Paganzo Basin is supported by the dominance of terrestrial palynomorphs and phytoclasts, the presence of coal and carbonaceous shales, and the occurrence of plant megafossils and plant debris in the sediments. The floristic changes between assemblages 1 and 2 are consistent with the interpretation of climatic amelioration associated with paleogeographic changes in Gondwana occurring during the Pennsylvanian and Cisuralian.

Aperture variation in the pollen of Nelumbo (Nelumbonaceae)

The evolutionary and developmental origin of tricolpate pollen is of great interest because pollen of this kind defines a major clade of angiosperms (eudicots), a clade that is also well supported by molecular data. We examined evidence that tricolpate and monosulcate pollen types are produced alongside each other in the anthers of Nelumbo flowers, as has previously been reported. Observations of pollen in situ within individual anthers revealed mainly tricolpate pollen produced in tetrahedral tetrads, but also a small percentage of clearly aberrant pollen grains that have a great variety of aperture configurations. Previously published evidence for tetragonal tetrads is not supported, and previously reported monosulcate grains are part of a continuum of variation among the aberrant grains in aperture number, position and form. Other eudicots show similar variability in their pollen apertures. The variation in the pollen of Nelumbo is not exceptional, and may not be more significant than variation seen in the other taxa with regard to the origin of the tricolpate and tricolpate‐derived pollen characteristic of eudicots. Nevertheless further studies of aberrant pollen in Nelumbo and other eudicots, together with comparisons of pollen development in “normal” eudicots and closely related species that show radical, and developmentally fixed, reorganization of apertures and pollen polarity, may be helpful in understanding the processes that controlled the transition from the monosulcate to the tricolpate condition.

The Unique Pollen Morphology of Duparquetia (Leguminosae: Caesalpinioideae): Developmental Evidence of Aperture Orientation Using Confocal Microscopy

The phylogenetic affinities of the aberrant monotypic genus Duparquetia (subfamily Caesalpinioideae) are at present unresolved. Preliminary results from molecular analyses suggest a basal, isolated position among legumes. A study of Duparquetia pollen was carried out to provide further morphological characters to contribute to multi-data set analyses. Understanding the development of Duparquetia pollen was necessary to clarify the orientation of the apertures. Pollen grains and developing microspores were examined using light microscopy, confocal microscopy and scanning electron microscopy. Evidence for the orientation of the apertures was provided by the examination of microspores within developing tetrads, using (a) confocal microscopy to locate the position of the ectoapertures, and (b) light microscopy and Alcian blue stain to locate the position of the endoapertures. Confocal microscopy has been used for the first time to examine developing microspores in order to obtain information on ectoapertures that was unavailable using other techniques. Pollen in Duparquetia develops in tetrahedral tetrads as in other eudicots, with the apertures arranged in a modified pattern following Fischer's rule. Pollen grains are asymmetrical and have one equatorial-encircling ectoaperture with two equatorial endoapertures, a unique feature in Leguminosae, and in eudicots. The pollen morphology of Duparquetia is so unusual that it provides little information to help determine its closest relatives. However, it does fit with a pattern of greater pollen morphological diversity in the first-branching caesalpinioid legume groups than in the more derived clades. The latitudinal ectoaperture of Duparquetia is unique within the Fabales and eudicot clades, resembling more closely the monosulcate pollen found in monocots and basal angiosperms; however, developmental patterns are recognizably similar to those of all other legume pollen types.

Variations in the microsporogenesis of monosulcate palm pollen

Pollen morphology has been extensively studied in the Arecaceae, and pollen aperture organization is usually distal monosulcate, as in many monocot families. Much is known about the influence of microsporogenesis on aperture configuration, but the key processes during microsporogenesis responsible for aperture type, number and arrangement are still poorly understood. In order to clarify the developmental sequence underlying aperture type and organization in palm monosulcate pollen, a study of the characteristics of male postmeiotic development was carried out in representative species of four genera of subfamily Coryphoideae, and four genera of subfamily Arecoideae. We found evidence for the occurrence of successive cytokinesis in addition to simultaneous cytokinesis in three Coryphoideae species. Tetrad shape was highly diverse within all species. Our results reveal an unexpected diversity in microsporogenesis from which it may be possible to gain further insight into pollen evolution within the family. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society, 2006, 151, 93–102.

The Operculum in Pollen of Monocotyledons

Within monocotyledons, monosulcate pollen is the predominant type and probably represents the plesiomorphic condition, but considerable variation occurs in sulcus morphology. An operculum is an exine thickening that covers most of an aperture. Monocot opercula are usually associated with sulci, although they can occur in ulcerate apertures, as in Poaceae. There are several other aperture types closely related to the monosulcate-operculate type, and confusion occurs in the palynological literature between monosulcate-operculate, pontoperculate, disulculate, disulcate, and zona-aperturate (zonasulculate or zonasulcate) pollen. Transmission electron microscopy (TEM) was used to determine the distribution of the thick apertural intine and to accurately identify these aperture types. Operculate pollen most frequently was present in Asparagales (particularly Agavaceae, Doryanthaceae, lridaceae, and Tecophilaeaceae), Liliales (particularly Liliaceae, Melanthiaceae, and Uvulariaceae), and relatively infrequently among commelinid monocots, except for some Arecaceae, Dasypogonaceae, and Poales. Thus, we conclude that opercula have probably evolved several times independently within monocots, particularly in taxa from dry or seasonally dry habitats, and that this adaptation may be related to their function in protecting the aperture. Two transformation series of related aperture types are proposed, one of which involves monosulcate-operculate pollen, although further testing will be required.

Aperture Pattern and Microsporogenesis in Asparagales

The aperture pattern of pollen grains is a character defined as the number, shape, and position of apertures. Although this character is highly variable in angiosperms, two states are particularly widespread. Pollen grains with one polar aperture occur frequently in basal angiosperms and monocots while tricolpate pollen is a synapomorphy of the eudicots. Many morphological characters are the result of a compromise between selective forces (acting on morphology) and developmental constraints (limiting the range of possible morphologies). To investigate what are the respective roles of development and selection in the determination of aperture pattern in angiosperms, we have chosen to study the characteristics of cell division during male meiosis, since it has been shown that aperture pattern is determined during microsporogenesis. The present study focuses on Asparagales. From a selection of species belonging to the major families of Asparagales, we described the type of cytokinesis, the way callose is deposited, the shape of the tetrad, as well as the shape and position of apertures within the tetrad. We show that although pollen morphology is quite uniform in Asparagales (most species produce monosulcate pollen), the characteristics of cell division during male meiosis vary among families. A highly conserved developmental sequence is observed in higher Asparagales whereas lower Asparagales, and particularly Iridaceae, display different ways of achieving cell division.

A new Late Cretaceous ginkgoalean reproductive structure Nehvizdyella gen. nov. from the Czech Republic and its whole‐plant reconstruction

During the Mesozoic Era, gingkoaleans comprised a diverse and widespread group. Here we describe ginkgoalean fossils in their facies context from the Late Cretaceous (Cenomanian) Peruc-Korycany Formation of the Czech Republic and present a reconstruction of tree architecture and ecology. Newly described in this study is the ovuliferous reproductive structure, Nehvizdyella bipartita gen. et sp. nov. (Ginkgoales). This ovuliferous organ consists of a bifurcating axis, terminated by large cupule-like structures, probably homologous to the collar of the recent Ginkgo. Each cupule encloses an orthotropous ovule. In specimens with the early developmental stages preserved, the entire ovule and young seed, with the exception of the micropylar area, is embedded in the cupule. Mature seeds consist of sclerotesta and sarcotesta. Monosulcate pollen grains of Cycadopites-type are found adhering to the seeds. Although similar to Ginkgo in terms of its large size and reduced number of seeds, N. bipartita differs from the extant genus in having ovules completely enclosed in a cupule-like structure. The co-occurrence of N. bipartita with ginkgoalean leaves of Eretmophyllum obtusum (Velenovský) Kvaček, J., ginkgoalean short shoots of Pecinovicladus kvacekii Falcon-Lang, and ginkgoalean trunk wood of Ginkgoxylon gruettii Pons and Vozenin-Serra in monodominant taphocoenoses at four geographically distant localities suggests that these remains all belong to one plant. This is supported by the close morphological and anatomical similarity between the different organs. Facies analysis of plant assemblages indicates that our Cretaceous tree occupied a water-stressed coastal salt marsh environment. It therefore represents the first unequivocal halophyte among the Ginkgoales.

Multiple Developmental Pathways Leading to a Single Morph: Monosulcate Pollen (Examples From the Asparagales)

Early developmental events in microsporogenesis are known to play a role in pollen morphology: variation in cytokinesis type, cell wall formation, tetrad shape and aperture polarity are responsible for pollen aperture patterning. Despite the existence of other morphologies, monosulcate pollen is one of the most common aperture types in monocots, and is also considered as the ancestral condition in this group. It is known to occur from either a successive or a simultaneous cytokinesis. In the present study, the developmental sequence of microsporogenesis is investigated in several species of Asparagales that produce such monosulcate pollen, representing most families of this important monocot clade. The developmental pathway of microsporogenesis was investigated using light transmission and epifluorescence microscopy for all species studied. Confocal microscopy was used to confirm centripetal cell plate formation. Microsporogenesis is diverse in Asparagales, and most variation is generally found between families. It is confirmed that the whole higher Asparagales clade has a very conserved microsporogenesis, with a successive cytokinesis and centrifugal cell plate formation. Centripetal cell wall formation is described in Tecophilaeaceae and Iridaceae, a feature that had so far only been reported for eudicots. Monosulcate pollen can be obtained from several developmental pathways, leading thus to homoplasy in the monosulcate character state. Monosulcate pollen should not therefore be considered as the ancestral state unless it is produced through the ancestral developmental pathway. The question about the ancestral developmental pathway leading to monosulcy remains open.

Fine structure of Rossipollis reticulatus from the Upper Cretaceous of southern Sweden

Dispersed monosulcate pollen identified as Rossipollis reticulatus Ross and similar pollen in two pollen sacs from a single anther were recovered from Upper Cretaceous fluvial sediments in southern Sweden. In most pollen grains the wall consists of a foot layer, an inner solid reticulum, crowned by columellae, which appear as an infratectal reticulum, and a tectum with irregularly spaced fine perforations. An additional papillate innermost layer, interpreted as a transitory endexin, is present in some pollen grains. A monocotyledonous origin is indicated by the configuration of exine and aperture along with the presence of papillate endexine structures in grains considered to have aborted at earlier stages of development.Key words: Cretaceous, fossil pollen, endexine, monocots.