Links between early pollen development and aperture pattern in monocots

Summary. Although the pollen grains produced in monocots are predominantly monosulcate (or monoporate), other aperture types are also found within this taxonomic group, such as the trichotomosulcate, inaperturate, zonaperturate, di-, or triaperturate types. The aperture pattern is determined during the young-tetrad stage of pollen development and it is known that some features of microsporogenesis can constrain the aperture type. For example, trichotomosulcate pollen is always associated with simultaneous cytokinesis, a condition considered as derived in the monocots. Our observations of the microsporogenesis pathway in a range of monocot species show that this pathway is surprisingly variable. Our results, however preliminary, reveal that variation in microsporogenesis concerns not only cytokinesis but also callose deposition among the microspores and shape of the tetrads. The role played by these features in aperture pattern determination is discussed. Correspondence and reprints: Laboratoire Ecologie, Systématique et Evolution, Université Paris-Sud, 91405 Orsay Cedex, France.     

Multiple developmental pathways leading to a single morph: monosulcate pollen (examples from the Asparagales)

BACKGROUND AND AIMS: 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. METHODS: 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. KEY RESULTS: 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. CONCLUSIONS: 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.     

Using the biomass-ratio and idiosyncratic hypotheses to predict mixed-species litter decomposition

Background and Aims

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.

Methods

Microsporogenesis is described for 30 species belonging to eight families of the monocots: Acoraceae, Amaryllidaceae, Alstroemeriaceae, Asparagaceae, Butomaceae, Commelinaceae, Liliaceae and Xanthorrhoeaceae.

Key Results

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.

Conclusions

In all these different microsporogenesis pathways, aperture location seems to be linked to the last point of callose deposition.     

Microsporogenesis and microgametogenesis of Excentrodendron hsienmu (Malvaceae s.l.) and their systematic implications

Flowers, microsporogenesis and microgametogenesis of Excentrodendron hsienmu in opening-functional flowers and non-opening flowers were studied to investigate the evolutionary relationships of Excentrodendron . E. hsienmu is a dioecious species that blossoms every 3–4 years, although large numbers of flower buds develop every year. The anther is tetrasporangiate, the tapetum is of the secretory type, the microspore tetrads are mainly tetrahedral, and the pollen grains are two-celled when shed. Four to six microsporocytes are seen on the transverse section of the anthers, and cytokinesis is simultaneous. The development of the anther wall conforms to the basic type and the anther wall is five or six cells thick, with a fibrous endothecium. The difference between the opening-functional and the non-opening flowers is mainly in the thickness of the anther wall. Early megasporogenesis in staminate flowers up to megaspore mother cell or megaspore tetrads has been observed. Excentrodendron shares with Dombeyeae only plesiomorphic features, but differs in anther wall development type and thickness. Most features of Excentrodendron are shared with Pterospermum , including such synapomorphic features as basic type of anther wall development, five- to six-cell-thick anther wall, biseriate tapetum at some places, and degeneration of microsporocytes, suggesting placement near Pterospermum .   © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 447–457.     

Pollen morphology of Dioscorea (Dioscoreaceae) and its relation to systematics

This paper adds new data on the pollen and orbicule morphology of 61 Dioscorea L. (Dioscoreaceae) species to the survey of Schols et al . (2001). The results indicate that pollen characters may be significant in infrageneric systematics in Dioscorea . Pollen and orbicule characters are described based on observations with light microscopy, and scanning and transmission electron microscopy, and are critically evaluated and discussed in the context of existing hypotheses of systematic relationships within the genus. Pollen is mostly disulculate (sometimes monosulcate) with a perforate, microreticulate, striate, gemmate, rugulate, or cerebroid perforate sexine. The basal section Stenophora is one of the few sections with monosulcate pollen. Brachyandra , Cardiocapsa , and Seriflorae , three Malagasy sections, are characterized by striate pollen. Pollen morphology strongly supports section Enantiophyllum as a monophyletic group. The correlation between pollen size and tuber type, as suggested previously by P. Su (1987), is confirmed by our data. As found in our earlier survey, orbicules in Dioscorea are mostly spherical with a smooth or spinulose surface.  © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society , 2003, 143 , 375–390.     

Diversity and evolution of microsporogenesis in Bromeliaceae

In this study, we explore the features of microsporogenesis in Bromeliaceae and, in particular, the diversity and evolution of additional callose deposits. Cytokinesis type, cell wall formation, tetrad form and patterns of additional callose deposition after intersporal wall formation were studied in 12 species of Bromeliaceae (each from a different genus) presenting four different aperture patterns. Microsporogenesis is highly conserved, with successive cytokinesis, centrifugal cell plate formation and predominantly tetragonal and decussate tetrads, as in many monocots, but five different patterns of additional callose deposition were recorded. The optimization of patterns of additional callose deposition on the phylogeny of Bromeliaceae reveals convergences. Additional callose deposition is a variable and labile feature of microsporogenesis in Bromeliaceae and is linked, to some extent, to aperture pattern. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 36–45.     

Taxonomic significance of pollen types in the Guyana Highland-centred composite genera of Mutisioideae (Asteraceae)

The Guyana Highland-centred genera, or Stenopadus group, are a complex of species that belong to the tribes Mutisieae and Stifftieae of Asteraceae. The pollen morphologies of 29 species, from 12 of 13 genera of this complex, are described and illustrated using light and scanning electron microscopy. The exine sculpture and structure are highly significant. Four exine types, previously characterized, were distinguished: Gongylolepis , Wunderlichia , Mutisia , and Stenopadus . The characterization of the Stenopadus exine type is enlarged here. These exine types led to the recognition of four well-defined pollen types, whereas the spine length and exine thickness characterized six subtypes. Pollen types circumscribe genera or groups of genera, and some subtypes distinguish species. The pollen morphology within the complex is discussed in relation to the rest of Mutisioideae and other palynologically allied tribes of Cichorioideae. There is little correlation between pollen types and tribes; only the Stenopadus exine type is exclusive to the Stifftieae tribe. The remaining types are shared by the two tribes of the complex. Pollen morphology supports the hypothesis that this group of genera is close to the Gochnatia complex and the Cardueae tribe.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 327–340.     

Triaperturate pollen in the monocotyledons: configurations and conjectures

Triaperturate pollen are known in at least twenty seven genera of monocotyledons. Differences between aperture type and polarity indicate that the development of three apertures has occurred a number of times. Mode of cytokinesis during microsporogenesis is compared with differences in aperture configuration, to assess the extent to which this appears to influence aperture arrangement. Triapertury in monocot pollen tends to fall into one or another of three situations: 1) it is the normal state, 2) it is fairly common, but pollen with more or less apertures also occur in the taxon or sample, 3) it is a rare, or abnormal state for pollen which usually has less than three apertures. The various forms of triaperturate pollen are described, as well as monosulcate pollen of the orchid genera Cypripedium and Paphiopedilum, often misinterpreted as tri-sulcate, and the unusual extended trichotomosulcate pollen of Agrostocrinum (Hemerocallidaceae). Monosulcy, trichotomosulcy, and zonasulcy, with unusual and rare exceptions of zonasulcy in the eudicots, are aperture states shared exclusively with the basal dicots. Furthermore, to some extent all have links with the triaperturate condition in monocots and basal dicotyledons. This is discussed, as well as the association of tripory with polypory in monocots and basal dicots. The fossil pollen record is considered.This paper is dedicated to Klaus Kubitzki in recognition, not only for his extensive contribution to systematic botany, but also for his firm belief that pollen characteristics contribute to a better understanding of plant systematics and evolution.     

Pollen studies in the genus Echinops L. and Xeranthemum group (Asteraceae)

The phylogeny of the genus Echinops and the Xeranthemum complex were the subject of a previous molecular study of the tribe Cardueae. An electronic microscope survey of pollen morphology and ultrastructure is presented for all the genera belonging to these groups, allowing four pollen types to be identified. The exine pattern indicates that there are two pollen types in the genus Echinops (including Acantholepis ), although no differences have been found between sections in Echinops . First, the annual species of Echinops and Acantholepis orientalis show a pollen type that differs from that of perennial species of the former genus and, second, the genera Chardinia and Xeranthemum exhibit a pollen type distinct from that Amphoricarpos and Siebera . The internal structure of the pollen wall is the same for all the genera. Only slight differences were observed in the tectum, but in all cases these were of the anthemoid type. Statistically significant correlations between pollen size and the life cycle were found.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 154 , 549–557.     

A new Coryphoid palm genus from Madagascar

Tahina J.Dransf. & Rakotoarinivo, gen. nov. (Arecaceae) is described as a new genus from north-western Madagascar, with a single species T. spectabilis J.Dransf. & Rakotoarinivo, sp. nov. Tahina is included within tribe Chuniophoeniceae of subfamily Coryphoideae, based on the strictly tubular imbricate rachilla bracts, the flowers grouped in cincinni with tubular bracteoles, and the stalk-like base to the corolla. This position is corroborated by evidence from plastid DNA. Lamina anatomy is discussed in detail, and similarities with and differences from the other members of Chuniophoeniceae are discussed. Based on the ecological characteristics of the single locality, predictions are made on where else it may occur in Madagascar.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 79–91.     

Historical legacies in the geographical diversity patterns of New World palm (Arecaceae) subfamilies

The extent to which species richness patterns of the major palm subfamilies in the Americas are controlled by lineage history was studied. Based on the fossil record, we suggest that the subfamily Coryphoideae has followed a boreotropical dispersal route into Central and South America, whereas Calamoideae (tribe Lepidocaryeae), Ceroxyloideae and Arecoideae have Gondwana/South America-biased histories. However, Arecoideae has been present and diverse in both South and Central America at least since the early Tertiary. We used regression analyses to evaluate the relative importance of environmental factors and spatial variables (as substitutes for historical or other non-environmental factors) as determinants of geographical variation in species richness for each subfamily. Given the different lineage histories, we hypothesized that: (1) coryphoid richness should be least strongly controlled by the modern environment and exhibit a strong non-environmental bias towards Central and North America, reflecting its boreotropical invasion route, (2) calamoid species richness should exhibit a non-environmental bias towards South America, reflecting its long African–South American history, and (3) arecoid species richness should be most strongly environmentally determined, reflecting the long arecoid residency in both Central and South America. The regression analyses confirmed the hypothesized effects of lineage history on the geographical patterns in species richness. Hence, modern species richness patterns in the New World palm subfamilies strongly reflect their divergent biogeographical histories.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 113–125.     

Character divergences and convergences in canopy-dwelling Loranthaceae

The Loranthaceae is the largest plant family with aerial branch parasites termed mistletoes. Three genera of Loranthaceae are terrestrial root parasites and the remaining 72 genera are aerial parasites. Several characters, including habit, haustorial type, germination pattern, pollen morphology, chromosome number, inflorescence morphology and flower merosity, fusion, symmetry and size, are considered to reflect evolutionary relationships within the family. Convergence is a common evolutionary pattern and can confound interpretations of evolution. We investigated character evolution by mapping character states onto a phylogenetic tree based on the nuclear ITS and chloroplast trnL–trnF regions. Convergences in form were found in several characters, including habit, haustorial type, flower symmetry and merosity. These convergences typically correspond to ecological parameters such as pollination syndrome or stresses associated with the canopy habit. Other characters such as chromosome number and germination pattern illustrate divergent evolution among clades.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 101–113.     

Studies on the Pollen Morphology of the Chinese Palmae

This paper deals with morphology of the recent pollen grains of 26 species representing 16 genera and some fossil pollen of the Chinese Palmae. They are examined with the light microscope and scanning electron microscope. On the basis of the types of aperture and the distribution of fossil pollen and lithophyll in the strata, the trends of the evolution of the family Palmae are discussed. This paper points out that the monosulcate pollen belongs to the most predominent and more primitive aperture type in this family and that there is a trend in which the trichotomosulcate, annulocolpate, disulcate etc. are derived from the monosulcate pollen.     

Microsporogenesis and systematics of Aristolochiaceae

Within Aristolochiaceae, a secretory tapetum and orbicules are ubiquitous, but both simultaneous and successive types of microsporogenesis occur. Simultaneous cytokinesis is apparently plesiomorphic within the order Piperales, in which Aristolochiaceae are now placed. Successive microsporogenesis was found only in species of Aristolochia confined to a crown clade in the proposed phylogeny of this genus. In contrast to many other taxa, within Aristolochiaceae there is no strict relationship between microsporogenesis type and tetrad configuration, which is strongly influenced by spindle orientation, especially during meiosis II. There is also no direct correlation between microsporogenesis type and the aperture of mature pollen grains.     

On the presence of extracellular calcium oxalate crystals on the inflorescences of Araceae

This study presents a survey of the species of the Araceae where extracellular production of calcium oxalate crystals has been observed and discusses the patterns of production of the crystals in different genera. For all Araceae studied using SEM, the oxalate crystals exuding on the epidermal surface correspond to extended aggregate/druses or crystal sand and the oxalate crystals mixed with pollen correspond to raphides or styloids (prismatic crystals). The type of crystals associated with pollen varies among genera. However, the presence of crystals associated with pollen is a specific rather than a generic characteristic. Our results show that the presence of raphides mixed with pollen seems to be a widespread phenomenon in the aroid family.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 181–190.     

Development and cytology of pollen in Dahlstedtia Malme (Leguminosae: Papilionoideae)

Microsporogenesis, chromosome number, meiotic behaviour and meiotic index were investigated in Dahlstedtia pinnata and D . pentaphylla , two legume species occurring largely in Brazil, in order to ascertain whether the pollen could limit fertilization events. Archesporial cells originate primary sporogenous and anther wall precursor cells, the tapetum is uniseriate, uninucleate and glandular. Tetrads are tetrahedric or decussate, and cytokinesis is of the simultaneous type. Mature pollen grains are tricolpate and bicellular. No abnormalities in microsporogenesis were found. In both species the chromosome number is n = 11, a number not reported previously. The base number for Dahlstedtia is also 11, because cytological observations include both species of Dahlstedtia . D. pentaphylla has a higher meiotic index and lower individual variation values, and it is considered meiotically stable. Its pollen grains do not limit fertilization. D. pinnata has a lower meiotic index, and the pollen is one of the factors which limit fertilization. Furthermore, D. pinnata has numerous adventitious shoots, which suggest that vegetative propagation is important in its reproductive process. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 138 , 461–471.     

The pollen morphology of Phyllanthus (Euphorbiaceae) section Choretropsis

The pollen grain morphology of the eight South American species of the genus Phyllanthus , subgenus Phyllanthus , section Choretropsis , endemic to Brazil, was investigated using light and scanning electron microscopy. The high homogeneity in pollen shape, as well as aperture number and configuration, allowed the definition of a basic pollen type for this section, viz. prolate spheroidal or subprolate, zonocolporate with 4–5 invaginated colpi, sexine reticulate. Two subtypes have been observed, based on pollen grain apertures and exine sculpture. The first subtype is characterized by straight and narrow colpi provided with costae, lalongate endoapertures and a reticulum of rectilineous muri as in P. choretroides and P. spartioides . The second subtype has wider colpi without costae, circular endoapertures and a reticulum with curvilineous muri as in P. flagelliformis , P. goianensis and P. scoparius , or curvilineous-rectilineous muri as in P. angustissimus , P. gladiatus and P. klotzschianus . It was demonstrated that the pollen shape and the number of colpi varied within and between some of the species. The results suggest that pollen grain morphology, in association with plant architecture and the presence of phylloclades, constitute additional evidence to delimit the species of the section Choretropsis and to further understand their evolutionary origin within the genus Phyllanthus.  © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 144 , 243–250.     

Delturinae, a new loricariid catfish subfamily (Teleostei, Siluriformes), with revisions of Delturus and Hemipsilichthys

A new subfamily, Delturinae, is described to accommodate the loricariid catfish genera Delturus Egenmann & Eigenmann, 1889 and Hemipsilichthys Eigenmann & Eigenmann, 1889, a clade recently demonstrated to be the sister group of all remaining loricariids except Lithogenes . The genus Hemipsilichthys is restricted to three species, H. gobio (Lütken, 1874), its sister species H. papillatus Pereira et al ., 2000, and H. nimius Pereira et al ., 2003. Relationships among species of Delturus were not resolved and a new species, D. brevis , is described from the Rio Jequitinhonha basin in eastern Brazil. The geographical distribution of Delturinae, exclusively on the south-eastern Brazilian Shield, indicates that south-eastern Brazil acts as either a refugium for basal loricariid taxa or a point of origin for the Loricariidae. Lectotypes are designated for D. parahybae Eigenmann & Eigenmann, 1889 and Plecostomus angulicauda Steindachner, 1877. Keys are presented for subfamilies of Loricariidae and for genera and species of Delturinae. Diagnoses are provided for all delturine clades and species.  © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 147 , 277–299.