Airborne‐pollen pool and mating pattern in a hybrid zone between Pinus pumila and P. parviflora var. pentaphylla

The reproductive isolation barriers and the mating patterns among Pinus pumila, P. parviflora var. pentaphylla and their hybrids were examined by flowering phenology and genetic assays of three life stages: airborne‐pollen grains, adults and seeds, in a hybrid zone on Mount Apoi, Hokkaido, Japan. Chloroplast DNA composition of the airborne‐pollen was determined by single‐pollen polymerase chain reaction. Mating patterns were analysed by estimating the molecular hybrid index of the seed parent, their seed embryos and pollen parents. The observation of flowering phenology showed that the flowering of P. pumila precedes that of P. parviflora var. pentaphylla by about 6 to 10 days within the same altitudinal ranges. Although this prezygotic isolation barrier is effective, the genetic assay of airborne‐pollen showed that the two pine species, particularly P. pumila, still have chances to form F₁ hybrid seeds. Both parental species showed a strong assortative mating pattern; F₁ seeds were found in only 1.4% of seeds from P. pumila mother trees and not at all in P. parviflora var. pentaphylla. The assortative mating was concluded as the combined result of flowering time differentiation and cross‐incompatibility. In contrast to the parental species, hybrids were fertilized evenly by the two parental species and themselves. The breakdown of prezygotic barriers (intermediate flowering phenology) and cross‐incompatibility may account for the unselective mating. It is suggested that introgression is ongoing on Mount Apoi through backcrossing between hybrids and parental species, despite strong isolation barriers between the parental species.     

Natural hybridization in the Andropogon lateralis complex (Andropogoneae,Poaceae) and its impact on taxonomic literature

In north‐eastern Argentina, Paraguay and south‐eastern Brazil, morphologically intermediate plants involving Andropogon lateralis, A. bicornis, A. glaziovii, A. arenarius and A. hypogynus were found. The possibility that they were natural hybrids was tested in two ways: (1) where they were sterile, their morphology was compared with that of the putative parents, and their meiosis and reproductive behaviour were studied; (2) where they were fertile, studies of artificial hybrids were also made. Most of the hybrids were sterile. The only fully fertile combination, generating recombination and hybrid swarms, was A. lateralis × A. hypogynus. In spite of apparently normal chromosome pairing, fertility was low in all other combinations on both the male and female sides. Sterility is probably a result of ‘cryptic’ or ‘gametic sterility’, which produces complete sterility of the gametes. Many of the hybrids survive and compete successfully with the parental species in natural populations, but their sterility maintains the genetic isolation of the majority of the taxa involved. Meiotic chromosome behaviour in all the hybrids indicates that the group of species shares slightly different forms of three basic genomes. Several specimens of natural hybrids were found in historical herbarium collections. In the past, they were given the status of type specimens of at least five taxonomic entities (A. lindmanii, A. coloratus, A. lateralis var. subtilior, A. multiflorus and A. lateralis var. bogotensis). The taxonomic consequences of these findings are discussed. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 136–154.     

Characteristics of pollen from natural triploid Betula hybrids

Birch has a key role in the Holocene vegetation history of northern Europe and in sub‐arctic climates dwarf birch and tree birch co‐exist. In Iceland, triploid hybrids between diploid Betula nana (dwarf birch) and tetraploid B. pubescens (downy birch) are common and therefore likely to contribute to pollen deposition. Pollen from 22 triploid trees/shrubs from ten woodlands in Iceland was examined and its size and shape compared with pollen from the parental species. The mean diameter of pollen grains from the triploid hybrids was not statistically different from that of B. nana pollen, but was significantly smaller than the mean value of B. pubescens pollen. On the contrary, the size of the vestibulum was similar to that of B. pubescens, which was significantly greater than that of B. nana, and therefore the diameter‐pore depth ratio was lower than the values from either species. The pattern of size distribution within plants indicated that triploid hybrids might have produced two sizes of triporate pollen grains, but the small B. nana size was far more prevalent than the larger B. pubescens size. Several anomalies in pollen morphology were common among the hybrid pollen grains: four or more pores were the most frequent type of abnormality. Characteristics of the pollen of triploid Betula hybrids, especially structural anomalies, may provide a means to reveal periods of interspecific hybridisation in the analysis of sub‐fossil pollen.     

New natural hybrids of Convolvulus (Convolvulaceae) from Turkey

The present study introduces two new nothospecies from Turkey, Convolvulus×pseudocompactus C. Aykurt & Sümbül (C. oleifolius Desr. var. deserti Pamp.×C. compactus Boiss.) and Convolvulus×peshmenii C. Aykurt & Sümbül (C. holosericeus Bieb. subsp. macrocalycinus Hausskn. & Bornm.×C. compactus Boiss.). Morphological differences and similarities between the hybrids and their parents are discussed; in addition, habit and sepals are illustrated as diagnostic characters, and the geographic distribution of the hybrids and their parents are mapped. Pollen characteristics of Convolvulus×pseudocompactus, Convolvulus×peshmenii and their parents were examined by means of light microscopy and SEM. Detailed morphological and palynological investigations have been performed for hybrid individuals and parental species. As a result of the palynological investigations, the rate of dehydrated pollen grains was established as an important criterion to determine the hybrid status of Convolvulus taxa.     

The role of parental and hybrid species in multiple introgression events: evidence of homoploid hybrid speciation in Centaurea (Cardueae,Asteraceae)

Molecular approaches have greatly increased the number of confirmed homoploid hybrids, which suggests that the frequency of this phenomenon was underestimated in the past because it was much more difficult to detect than allopolyploidy. Centaurea is a suitable model group for studying homoploid speciation, as hybridization events have been commonly reported for this genus. Based on this, here we study Centaurea × forsythiana, a naturally occurring homoploid hybrid between two Sardinian endemics, C. horrida and C. filiformis, using a molecular approach involving nuclear and plastid markers, to understand the underlying population dynamics between homoploid hybrids and their parents. Our results confirm that C. × forsythiana is a hybrid between the above‐mentioned species and define the roles of the parents. Plastid markers point towards C. horrida as the maternal progenitor, and nuclear markers reveal that the other parental species, C. filiformis, is itself an old, stabilized homoploid hybrid related to the C. paniculata complex from the Italian mainland. Homoploid hybrid speciation is discussed and C. × forsythiana and C. filiformis are compared with other similar examples. The study confirms the importance of introgression between parental species mediated by hybrids and its potential implications in conservation. Furthermore, it shows how hybridization studies become even more complex when the parents are themselves of probable hybrid origin. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 453–467.     

Patterns of hybridization in a multispecies hybrid zone in the Ranunculus cantoniensis complex (Ranunculaceae)

Simultaneous hybridizations among three or more parental taxa occur in nature, but these have rarely been analysed. The present study investigates the natural hybridization of diploid members of the Ranunculus cantoniensis complex over a microgeographical area in south‐western China. Sequence information from maternally inherited plastid DNA (trnQ‐rps16 and rpL32‐trnL) and biparentally inherited nuclear ribosomal DNA was used to identify hybrids, and these were further confirmed by a dramatic reduction in pollen viability. The populations from the contact zone contained individuals that had more than two nuclear ribosomal internal transcribed spacer (nrITS) types; in addition, a network analysis revealed the presence of hard conflicts between the nuclear and plastid DNA data. These results prove that R. trigonus and R. silerifolius var. dolicanthus have relatively little reproductive isolation from each other, leading to a high gene flow rate in contact populations. The four investigated species can hybridize with each other in this contact zone, forming seven hybrid types. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 174 , 227–239.     

Pollen production of the genus Cupressus

The study of potential pollen emission in the genus Cupressus was carried out in the city of Córdoba (Spain). Pollen production of 227 trees of the represented species of Cupressus in the area (Cupressus sempervirens, C. macrocarpa and C. arizonica) was studied by counting the number of pollen grains per flower, the number of flowers per branch, and the number of branches per m² of surface area. Total surface area was estimated by considering the shape of the tree as a cone and estimating the average height and radius of each species. Results indicated that there was little variation in the number of pollen grains produced by a flower within each species. Equally, there were few differences in the number of grains per flower among the species, although C. arizonica had slightly fewer than the other species because it possessed fewer stamens per flower. However, there were clear differences in the number of flowers per branch, with C. macrocarpa having the highest floral density, considerably lower in C. arizonica and even lower in C. sempervirens. All of this indicates that the pollen contribution of C. macrocarpa to the atmosphere is far greater than that of C. arizonica and C. sempervirens. The results indicate a different potential emission in each species, and this must be taken into account when determining the quantity of pollen present in the atmosphere. The maximum amount of pollen grains that one individual can spread is provided in this study and would provide useful data in the development of an airborne forecasting system.     

Evolution of pollen,stigmas and ovule numbers at the caesalpinioid–mimosoid interface (Fabaceae)

In this study we examine the pollen, stigmas and ovaries from 62 collections of herbarium material representing 16 genera, using light and scanning electron microscopy. The caesalpinioid Dimorphandra group (Burkea, Dimorphandra, Erythrophleum, Mora, Pachyelasma, Stachyothyrsus and Sympetalandra) pollen grains are small, tricolporate monads, with perforate or psilate ornamentation. Dinizia, Pentaclethra and Aubrevillea have morphological characters that have suggested either a mimosoid or caesalpinioid placement. Dinizia pollen is in permanent tetrads with clavate ornamentation. Pentaclethra pollen grains are monads, two species have tricolporate pollen and the third is porate. Aubrevillea has tricolporate, finely reticulate monads. All ten genera have variable, non‐predictable stigma type and ovule number. The mimosoid Adenanthera group (Adenanthera, Tetrapleura, Amblygonocarpus, Pseudoprosopis, Calpocalyx and Xylia) pollen grains are in 8‐ to 16‐grain polyads. In all Adenanthera group species, the stigmatic cavity is only large enough to accommodate one polyad. In addition, the number of ovules present matches the number of pollen units in one polyad. Polyads have porate, operculate apertures that differ in layout, aperture morphology and development when compared with caesalpinioid and other eudicot pollen. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 162 , 594–615.     

Palynological analysis of five selected <Emphasis Type="Italic">Onosma</Emphasis> taxa

Pollen size and exine structure of the following five Onosma L. species from Slovakia and Italy were studied by light and scanning electron microscopy: Onosma echioides, O. tornensis, O. visianii, O. arenaria, and O. pseudoarenaria. Among the taxa studied, O. echioides has the smallest pollen grains in average (10.5–17.5 × 8.0–14.5 μm), and O. pseudoarenaria the largest grains in average (15.3–20.5 × 12.3–16.3 μm). Pollen grains of all 5 species are small sized, 3-syncolporate, subprolate, heteropolar, with ovate equatorial outlines and circular to rounded triangular polar outlines; the tectum is microechinate. A positive correlation is found between pollen size and chromosome number. The value of pollen characters for taxonomic purposes and the position of the taxa studied within the genus Onosma are discussed.     

Cytogenetic implications of artificial hybrids between the hawaiian silversword alliance and North American tarweeds (Asteraceae: Heliantheae—Madiinae)

Two vigorous transoceanic, bigeneric hybrids, Dubautia laevigata (n = 14) × Raillardiopsis muirii (n = 8) and [Dubautia knudsenii × laxa] (n = 14) × Madia bolanderi (n = 6), and one vigorous transoceanic trigeneric hybrid, Dubautia scabra (n = 14) × [M. bolanderi × R. muirii], between mainland tarweeds and Hawaiian silversword allies were artificially produced and subjected to cytogenetic analysis. In addition to univalents, ≈46–80% of the microsporocytes scored from these hybrids exhibited from one to four bivalents. However, some of the bivalents scored in the second bigeneric hybrid represented infragenomic association of Dubautia chromosomes. Stainable pollen of these hybrids ranged from 4.4 to 49%, mostly comprising large, tetracolporate, apparently diploid grains. The functionality of such grains was demonstrated in the primary hybrid M. bolanderi × R. muirii that was used to produce the trigeneric hybrid, and suggests the possible mode of origin of the Hawaiian genome via allopolyploidy. Illustrations of parents and F₁s indicate that the hybrids produced in this study generally exhibit intermediate character states. However, the phenotypes of the “ray” flowers in hybrids between discoid and radiate species were noticeably unpredictable; in one case intermediacy appeared to be expressed largely in quantitative terms, while in two others intermediacy appeared to be expressed largely in qualitative terms.     

Morphology and variability of pollen of the genus <Emphasis Type="Italic">Spiraea</Emphasis> L. (Rosaceae) in Siberia and the Far East

Pollen of 12 species of the genus Spirea L. growing in different regions of Siberia and the Far East was investigated. The ultrasculpture of pollen grains is described. On the basis of aperture morphology and exine sculpture, pollen grains are typified, with key indices given for their identification. The species of the Spiraria section are distinguished by large grains. Pollen grains of S. media and S. alpina considerably vary in size. The level of natural polymorphism of palynological indices has been established.     

Palynological study of Cupressus sempervirens L. var. pyramidalis and var. horizontalis

ABSTRACT

Cupressus sempervirens L. var. pyramidalis and C. sempervirens var. horizontalis were examined for their pollen morphometry and ultrastructure to verify whether, in addition to phenotype, pollen could also show characteristics useful to discriminate the two infraspecific taxa. C. sempervirens var. pyramidalis compared to C. sempervirens var. horizontalis has mainly larger, subcircular pollen with a higher percentage of apolar and germinating grains, and a wall with thicker intine; the cytoplasm has a greater number of callose-containing vesicles. C. sempervirens var. horizontalis has pollen distributed in five classes of shape. In addition it is smaller and shows a higher percentage of ruptured grains, and a lower percentage of germination in vitro. The cytoplasm contains vesicles rich in callose and also a higher amount of osmophilic bodies. Pollen grains in both varietas contain only one cell at dispersal. The reported differences in pollen morphometry, structure and biology, together with the phenotypic characteristics of the tree, support the validity of two infraspecific taxa.     

Effects of environmental factors on pollen production in anemophilous woody species

The aim of this study was to estimate the amount of pollen produced by anemophilous woody taxa with allergenic properties and with considerable contribution in the concentration of pollen in the air of a Mediterranean city (Thessaloniki, Greece). The taxa selected are Corylus avellana, Cupressus sempervirens var. horizontalis and var. pyramidalis, Olea europaea and Platanus orientalis; each was studied in more than one sampling stations differing in elevation, direction or both. O. europaea produced the highest number of pollen grains per flower (1.3 × 10⁵ ± 0.1 × 10⁵) and P. orientalis the highest per inflorescence (3.3 × 10⁶ ± 0.2 × 10⁶). At the level of crown, pollen grains produced were of the order of 10⁹ per surface/volume unit for O. europaea and the two C. sempervirens varieties; for the other two taxa, they were of the order of 10⁶. Pollen production was lower at higher elevation and northern direction and depended on the size of the floral unit sampled (flower for O. europaea, inflorescence for all other species): the bigger the floral unit, the more pollen it contained. Our results and reports from other areas, where C. sempervirens and O. europaea grow, show that these two Mediterranean species produce comparable amounts of pollen at the levels of inflorescence or flower, respectively, wherever they occur.     

Comparative longevity of Australian orchid (Orchidaceae) seeds under experimental and low temperature storage conditions

Seeds from ten terrestrial orchid species, nine from the south‐west Australian biodiversity hotspot (Caladenia arenicola, Caladenia flava, Caladenia huegelii, Diuris laxiflora, Microtis media ssp. media, Pterostylis recurva, Pterostylis sanguinea, Thelymitra crinita and Thelymitra macrophylla) and one from south‐east Australia (Diuris fragrantissima), were placed into experimental storage to assess their relative longevity and likely optimal conditions for long‐term conservation seed banking. Seeds from all species were desiccation tolerant, germinating after drying at 23% relative humidity (C. arenicola, C. huegelii, P. sanguinea and T. macrophylla) or 5% relative humidity (C. flava, D. laxiflora, M. media ssp. media, P. recurva and T. crinita) at 23 °C. From automatedly determined moisture adsorption and desorption isotherms at 23 °C, these equate to tolerance of drying to 0.03–0.06 g water g^?1 dry weight or 0.013–0.028 g water g^?1 dry weight, respectively. Results of storage experiments at a range of moisture contents and temperatures suggest conventional seed bank storage at ?18 °C after equilibration at c. 23% relative humidity (at 23 °C) may be suitable for most of the species, although there was higher germination of P. recurva seeds stored at ?80 °C and of M. media ssp. media seeds equilibrated at 75% relative humidity. However, there was considerable variation in germination of seeds sampled after different storage periods, making it difficult to identify optimal storage conditions definitively. Results of comparative longevity storage experiments at 60% relative humidity and 40 °C suggest seeds from these orchid species are short‐lived compared with non‐orchid species, and with Australian species in particular. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 164 , 26–41.     

Palynological characters of Glycyrrhiza,Glycyrrhizopsis, and Meristotropis (Leguminosae), with special reference to their taxonomic significance

Abstract The pollen morphology of 11 species of the genus Glycyrrhiza L. with one from each of the genera Glycyrrhizopsis Boiss. & Bal. and Meristotropis Fisch. & C. A. Mey. was investigated by scanning electron microscopy. In pollen morphology, the main differences between Glycyrrhizopsis and Glycyrrhiza are: Glycyrrhizopsis—pollen grains 36.63 × 40.42 μm in size, oblate spheroidal in shape; and Glycyrrhiza—pollen grains 24.47–33.18 × 23.82–31.83 μm in size, prolate spheroidal in shape. Glycyrrhizopsis and Glycyrrhiza should be recognized as two distinct genera based on palynological and morphological characters. Meristotropis and Glycyrrhiza are similar in many important palynological and morphological characters, suggesting that the two should be merged. In Glycyrrhiza, two types of pollen grains, 3‐lobed‐circular or subtriangular in polar view, are found in different species, in accordance with morphological differences in the two groups, shedding light on the classification and evolution of the genus.     

Palynological survey of subtribe Pithecocteniinae (Bignonieae,Bignoniaceae)

The pollen morphology of subtribe Pithecocteniinae was reviewed. Thirty species of the six genera currently recognized, namely Amphilophium, Distictis, Distictella, Glaziovia, Haplolophium and Pithecoctenium, were considered. All the species surveyed fell into one of the two pollen groups: (1) inaperturate, spheroid pollen; and (2) stephanocolpate, prolate pollen. The former group included the studied species of Distictis, Distictella and Pithecoctenium, the latter species of Amphilophium, Glaziovia and Haplolophium. The variation of exine sculpture and thickness did not show any taxonomic relationships. An argument for considering pollen features, together with other morphological characters, to elucidate monophyletic units within Pithecocteniinae is presented. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 155–162.     

Palynotaxonomy of Brazilian species of Polygala subgenus Hebeclada

This study used pollen morphology to address taxonomic controversies related to several species belonging to subgenus Hebeclada of the genus Polygala (Polygalaceae). According to the last traditionally accepted revision of this subgenus, it comprises 40 species. Nevertheless, a recent taxonomic treatment applying more comprehensive criteria reduced this number to only nine species. Our work focused on 15 traditionally accepted species that occur in Brazil, fourteen of which have been considered as only six species in this recent taxonomic study. Pollen grains from floral buds at pre‐anthesis were collected from herbarium specimens and analysed using both visible light and scanning electron microscopy. Most of the traditionally accepted species were distinguished by different combinations of several pollen‐related features, in particular colpi number (13 or 15), pollen size and shape, and endoaperture type (endocingulate or not) and arrangement (parallel or sinuous). The species accepted in the recent taxonomic treatment could not be discriminated by pollen characters. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 609–619.     

POLLEN TRANSFER BY NATURAL HYBRIDS AND PARENTAL SPECIES IN AN IPOMOPSIS HYBRID ZONE

Models of hybrid zones differ in their assumptions about the relative fitnesses of hybrids and the parental species. These fitness relationships determine the form of selection across the hybrid zone and, along with gene flow, the evolutionary dynamics and eventual outcome of natural hybridization. We measured a component of fitness, export and receipt of pollen in single pollinator visits, for hybrids between the herbaceous plants Ipomopsis aggregata and I. tenuituba and for both parental species. In aviary experiments with captive hummingbirds, hybrid flowers outperformed flowers of both parental species by receiving more pollen on the stigma. Although hummingbirds were more effective at removing pollen from anthers of I. aggregata, hybrid flowers matched both parental species in the amount of pollen exported to stigmas of other flowers. These patterns of pollen transfer led to phenotypic stabilizing selection, during that stage of the life cycle, for a stigma position intermediate between that of the two species and to directional selection for exserted anthers. Pollen transfer between the species was high, with flowers of I. aggregata exporting pollen equally successfully to conspecific and I. tenuituba flowers. Although this study showed that natural hybrids enjoy the highest quality of pollinator visits, a previous study found that I. aggregata receives the highest quantity of pollinator visits. Thus, the relative fitness of hybrids changes over the life cycle. By combining the results of both studies, pollinator-mediated selection in this hybrid zone is predicted to be strong and directional, with hybrid fitness intermediate between that of the parental species.     

An ecomorphological framework for the coexistence of two cyprinid fish and their hybrids in a novel environment

Niche variation between hybrid taxa and their parental species has been deemed imperative to the persistence of hybrid populations in nature. However, the ecological factors promoting hybrid establishment remain poorly understood. Through the application of a multidisciplinary approach integrating genetics, morphometry, life‐history, and trophic ecology, we studied the hybrids of roach (Rutilus rutilus L.) and bream (Abramis brama L.), and their parental species inhabiting an Irish lake. The roach × bream hybrid exhibited a body shape intermediate of that of the parental species. Diet analyses depicted the hybrid as a generalist, feeding on all prey items consumed by either parental species. Stable isotope data confirm the trophic niche breadth of hybrids. A significant correlation between body shape and diet was detected, suggesting that the intermediate phenotype of hybrids might play a role in their feeding abilities, resulting in the utilization of a broader trophic spectrum than the parental species. Growth and age class structure analyses also yielded a scenario that is consistent with the ecological success of hybrids. Genetic analyses suggest that the majority of hybrids result from first‐generation crosses between the parental species; however, a potentially significant proportion of back‐crosses with bream were also detected. The recent introduction of roach and bream into Irish waters, as well as the climatic and ecological features of the colonized habitats, can explain the remarkable success of the roach × bream hybrid in Ireland. The adaptive significance of hybridization and its demographic consequences for the parental species are discussed. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 768–783.     

Pollen diversity in Aquifoliales

Aquifoliales, as currently circumscribed, comprise five families and 20 genera, most of which have not been compared with regard to their pollen. Generic relationships within the order have not been fully resolved with molecular data, but pollen can provide a potential source of characters for future phylogenetic studies. To assess diversity in the order, pollen from 19 genera was examined with light and scanning electron microscopy. Pollen is typically tricolpate to triporate, although grains with one to nine pores were observed. Grains are small to medium, with a polar axis of 6–44 μm and an equatorial axis of 10–47 μm. Irregular pollen was recorded from nine genera. Exine patterning is diverse at the generic level and includes psilate, microechinate, striate to reticulate and clavate types, and is quite complex in some genera. All but four genera of Aquifoliales can be readily distinguished by their pollen, if heavy deposits of pollenkitt (present in 11 genera) are removed during and after acetolysis. Pollen from multiple taxa of Gomphandra, the second most diverse genus in the order, was surveyed to investigate species boundaries. Specimens of Gomphandra from continental Asia exhibited seven different pollen morphologies, suggesting that exine patterns may be useful for the recognition of species in that region. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 169–190.