Jeffrey M. Osborn

The importance of fungi in shaping the paleoecosystem

Although fungi have a long geologic history, many aspects regarding their origins and subsequent evolution remain impossible to document from the fossil record. As heterotrophs, fungi must interact with other organisms, and it is here that the fossil record can provide an important source of biological and paleoecological information about fungal interactions. Saprophytic, parasitic and biotrophic interactions among fungi and other organisms are ancient; examples of these interrelationships are discussed as they relate to the establishment and evolution of the biological and physical paleoecosystem.

Pollen and anther development in Nelumbo (Nelumbonaceae)

The Nelumbonaceae are a small family of aquatic angiosperms comprising Nelumbo nucifera and Nelumbo lutea. Historically, the genus has been considered to be closely related to Nymphaeales, however new systematic work has allied Nelumbo with lower eudicots, particularly Platanus. In recent years, studies of pollen development have contributed greatly to the understanding of phylogenetic relationships, but little has been known about these events in Nelumbo. In this paper, pollen and anther development are morphologically described for the first time in N. lutea. A comprehensive ontogenetic sequence is documented, including the sporogenous tissue, microspore mother cell, tetrad, free spore, and mature pollen grain stages. The deposition of a microspore mother cell coat and callose wall, the co-occurrence of both tetrahedral and tetragonal tetrads, the formation of a primexine in tetrads, and primexine persistence into the late free spore stage are shown. The majority of exine development occurs during the free spore stage with the deposition of a tectate-columellate ectexine, a lamellate endexine, and an unusual granular layer below and intermixed with the endexine lamellae. A two-layered intine forms rapidly during the earliest mature pollen stage. Major events of anther development documented include the degradation of a secretory-type tapetum during the free spore stage and the rapid formation of U-shaped endothecial thickenings in the mature pollen grain stage. The majority of mature pollen grains are tricolpate, however less common monosulcate and diaperturate grains also develop. Co-occurring aperture types in Nelumbo have been suggested to be an important transition in angiosperm aperture number. However, aperture variability in Nelumbo may be correlated with the lateness of aperture ontogeny in the genus, which occurs in the early free spore stage. This character, as well as other details of pollen and anther ontogeny in Nelumbo, are compared to those of Nymphaeales and Platanus in an effort to provide additional insight into systematic and phylogenetic relationships. Although Nelumbo is similar to both groups in several characters, the ontogenetic sequence of the genus is different in many ways.

The ultrastructure of fossil ephedroid pollen with gnetalean affinities from the Lower Cretaceous of Brazil

Abstract The micromorphology and fine structure of several dispersed polyplicate, “ephedroid” palynomorphs, originally assigned to Equisetosporites spp., are described from the Lower Cretaceous (Aptian-Albian) Santana Formation of northeastern Brazil. Pollen grains are elliptic in shape and have a variable number of longitudinally oriented plicae, but lack a distinct aperture. Plicae do not reach the grain tips and are psilate to faintly scabrate in surface ornamentation. The exine is distinctly two-parted with the sexine typically staining more lightly than the nexine. The sexine is on average 1.5 times thicker than the nexine, and has a tectate granular fine structure with the granular infrastructural layer having a graded organization. Small granules characterize the lower region of the infratectum and are directly contiguous with nexine; these gradually grade into larger granules that appear to fuse with a thick, homogeneous tectum. The nexine is thick and lamellate throughout. Both sexine and nexine sporoderm components are markedly thinner between the plicae (i.e. within the wall furrows), with the granular infratectum laterally thinning until completely absent. An additional wall layer, external to the tectum, is also present within sporoderm furrows; both the tectum and this layer typically occur in a folded, hinge-like fashion in the furrow. This unique feature as well as several other structural characters of the sporoderm are discussed with regard to their possible functional significance. Exine architecture is also compared with that of the only other three fossil polyplicate palynomorphs known at the ultrastructural level ( Ephedripites sp., Equisetosporites chinleana , and Cornetipollis reticulata ) as well as with that of the pollen of the extant Gnetales ( Ephedra, Welwitschia , and Gnetum ). By comparison, the Brazilian grains are most similar to Ephedripites using fine structural features.

Aerodynamics of saccate pollen and its implications for wind pollination.

Pollen grains of many wind-pollinated plants contain 1-3 air-filled bladders, or sacci. Sacci are thought to help orient the pollen grain in the pollination droplet. Sacci also increase surface area of the pollen grain, yet add minimal mass, thereby increasing dispersal distance; however, this aerodynamic hypothesis has not been tested in a published study. Using scanning electron and transmission electron microscopy, mathematical modeling, and the saccate pollen of three extant conifers with structurally different pollen grains (Pinus, Falcatifolium, Dacrydium), we developed a computational model to investigate pollen flight. The model calculates terminal settling velocity based on structural characters of the pollen grain, including lengths, widths, and depths of the main body and sacci; angle of saccus rotation; and thicknesses of the saccus wall, endoreticulations, intine, and exine. The settling speeds predicted by the model were empirically validated by stroboscopic photography. This study is the first to quantitatively demonstrate the adaptive significance of sacci for the aerodynamics of wind pollination. Modeling pollen both with and without sacci indicated that sacci can reduce pollen settling speeds, thereby increasing dispersal distance, with the exception of pollen grains having robust endoreticulations and those with thick saccus walls. Furthermore, because the mathematical model is based on structural characters and error propagation methods show that the model yields valid results when sample sizes are small, the flight dynamics of fossil pollen can be investigated. Several fossils were studied, including bisaccate (Pinus, Pteruchus, Caytonanthus), monosaccate (Gothania), and nonsaccate (Monoletes) pollen types.

Pollen ontogeny in Brasenia (Cabombaceae, Nymphaeales)

Brasenia is a monotypic genus sporadically distributed throughout the Americas, Asia, Australia, and Africa. It is one of eight genera that comprise the two families of Nymphaeales, or water lilies: Cabombaceae (Brasenia, Cabomba) and Nymphaeaceae (Victoria, Euryale, Nymphaea, Ondinea, Barclaya, Nuphar). Evidence from a range of studies indicates that Nymphaeales are among the most primitive angiosperms. Despite their phylogenetic utility, pollen developmental characters are not well known in Brasenia. This paper is the first to describe the complete pollen developmental sequence in Brasenia schreberi. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using combined scanning electron, transmission electron, and light microscopy. Both tetragonal and decussate tetrads have been identified in Brasenia, indicating successive microsporogenesis. The exine is tectate-columellate. The tetrad stage proceeds rapidly, and the infratectal columellae are the first exine elements to form. Development of the tectum and the foot layer is initiated later during the tetrad stage, with the tectum forming discontinuously. The endexine lamellae form during the free microspore stage, and their development varies in the apertural and non-apertural regions of the pollen wall. Degradation of the secretory tapetum also occurs during the free microspore stage. Unlike other water lilies, Brasenia is wind-pollinated, and several pollen characters appear to be correlated with this pollination syndrome. The adaptive significance of these characters, in contrast to those of the fly-pollinated genus Cabomba, has been considered. Brasenia does not produce pollenkitt nor develop tectal microchannels as does Cabomba. Instead, the discontinuity of the tectum reduces the amount of sporopollenin in the wall, which may allow for more effective wind dispersal. The importance of reassessing palynological characters in light of new ontogenetic data and the phylogenetic implications of this reevaluation are also discussed.

Morphological and ultrastructural studies of plant cuticular membranes. I. Sun and shade leaves of Quercus velutina (Fagaceae).

Sun and shade leaves of Quercus velutina Lam. were evaluated with respect to differences in gross anatomy, morphology, and cuticle (cuticular membrane [CM]) ultrastructure and micromorphology. Sun leaves are smaller, with more deeply lobed margins, and have more stomata, thicker mesophylls, and thicker CMs when compared with shade leaves. Cuticular membranes are thicker on both the adaxial and abaxial surfaces of sun leaves as a result of deposition of more cuticular components and scaly epicuticular wax. Both the adaxial and abaxial epidermises have the same basic fine structure in sun and shade leaves with respect to the outer periclinal cell wall and overlying CM. The cell wall is lamellate and the CM is composed of a two-zoned, reticulate cuticular layer and an amorphous cuticle proper. The outer periclinal wall and associated CM of the adaxial epidermis is thicker than that of the abaxial epidermis with both epidermal layers thicker in sun leaves compared with shade leaves. Difference in thickness of both epidermal layers, between sun and shade leaves, can be attributed to an increase in the inner reticulate region of the CM of sun leaves. Cells of the abaxial epidermis have ultrastructurally different CMs. Nonstomatal epidermal cells have a distinct amorphous cuticle proper whereas subsidiary cells have reticulations that traverse most of the outer CM. Guard cells have radially aligned reticulations through the entire outer CM and, therefore, lack an amorphous cuticle proper. Moreover, an internal CM, which is only sparsely reticulate, lines substomatal chambers. The internal CM of sun leaves is thicker and extends considerably deeper into substomatal chambers.

Pollen morphology and ultrastructure of the corystospermales: permineralized in situ grains from the triassic of Antarctica

Abstract Corystosperms, represented by Dicroidium leaves and Pteruchus -like pollen organs, are major components of the Early-Middle Triassic silicified flora from the Fremouw Formation of Antarctica. The micromorphology and ultrastructure of the in situ pollen contained within these organs are described. Pollen sacs of varying ontogenetic ages have been isolated. Mature grains are monosulcate and bisaccate, with large, crescent-shaped eusacci. The exine is relatively thick in the cappa region and thins toward the distal sulcus; surface ornamentation is psilate. In medial positions of the proximal wall, the exine is homogeneous but becomes tectate-alveolate in more lateral regions of the cappa. The alveolar units extend into the sacci forming an endoreticulum; however, the endoreticulations are discontinous and only attach to the outer walls of the sacci. A wedge-shaped unit, where the sacci attach to the corpus, characterizes both the proximal and distal poles. The sulcus is broad, extends the entire width of each grain, and is longitudinally flanked by elevated lips. The structural features of these grains are discussed with respect to other fossil and extant saccate pollen. The grains are systematically compared with those of other bisaccate pollen-producing plants with which the Corystospermales have been suggested to be closely related, including Glossopteridales, Caytoniales, and angiosperms (Lactoridaceae). The permineralized in situ grains are also compared with other compressed Pteruchus species known at the ultrastructural level and with morphologically similar dispersed palynomorphs known from Antarctic sediments.

Comparative pollen morphology and ultrastructure of the Callitrichaceae

The Callitrichaceae are an aquatic family of dicots that include the single, geographically cosmopolitan genus Callitriche. Callitriche contains 40-50 terrestrial, amphibious, and obligately submersed species, and it is the only known genus in the plant kingdom with co-occurring aerial and hydrophilous pollination syndromes. Pollen morphology and ultrastructure were described for 13 Callitriche species using scanning electron and transmission electron microscopy. Representative taxa of each growth form were examined; these included three terrestrial species (C. deflexa, C. peploides, and C. nuttallii), nine amphibious species (C. brutia, C. cophocarpa, C. cophocarpa-stagnalis hybrid, C. cribrosa, C. hamulata, C. heterophylla var. heterophylla, C. lusitanica, C. marginata, and C. trochlearis), and one obligately submersed species (C. truncata). Of the amphibious taxa, C. heterophylla var. heterophylla and C. trochlearis had internal geitonogamy, a type of internal self-fertilization. Pollen from all taxa was spheroidal, small, intectate, and lacked well-defined apertures. Taxa primarily differed with respect to exine thickness, surface ornamentation, and the presence or absence of aperture-like regions. The pollen of terrestrial species, as well as that of C. marginata, had well-developed exines with thick sculptured and basal layers. In general, amphibious taxa produced pollen with distinct, but thinner, exines than that of terrestrial taxa. Pollen of the amphibious taxa with internal geitonogamy had a thicker basal layer than species without internal geitonogamy, whereas the overall exine was reduced in C. hamulata and absent in C. brutia and C. lusitanica. Pollen of the obligately submersed C. truncata also lacked an exine. These palynological data were correlated with growth habits and related pollination biologies, as well as with phylogenetic interpretations of Callitrichaceae. Exine reduction or loss has evolved at least twice in the family, and it is associated with aneuploid reduction in chromosome number.

Comparative pollen morphology of five New World genera of Podostemaceae

Abstract The Podostemaceae are the largest family of strictly aquatic angiosperms; however, relatively little is known about the palynology of the family. Pollen micromorphology and ultrastructure of five representative New World taxa are described, including Marathrum rubrum Novelo & Philbrick, Oserya coulteriana Tul., Podostemum ceratophyllum Michx., Tristicha trifaria (Bory ex Willd.) Sprengel, and Vanroyenella plumosa Novelo & Philbrick. Pollen grains from all five species are relatively small, spherical, microechinate, have a tectate-granular sexine and a thick nexine in non-apertural regions, and a semitectate sexine and a thin nexine in apertural regions. Characters that vary among the taxa include dispersal unit (monads or dyads), sculptural element morphology, infratectal granule size, and aperture morphology and ultrastructure. This is the first study to describe the pollen morphology of these five taxa in detail, and it is the first to illustrate the ultrastructure of pollen wall characters for any member of the family.

Pollen and anther ontogeny in Cabomba caroliniana (Cabombaceae, Nymphaeales).

Cabomba is a small water lily genus that is native to the New World. Studies of pollen development and associated changes in the anther yield valuable characters for considering the evolution of reproductive biology in seed plants. Here we characterized the complete ontogenetic sequence for pollen in Cabomba caroliniana. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using scanning electron, transmission electron, and light microscopy. Tetragonal and decussate tetrads both occur in C. caroliniana, indicating successive microsporogenesis. The exine is tectate-columellate, and the infratectal columellae are the first exine elements to form, followed by a continuous tectum and a thin foot layer. A lamellate endexine initiates in the early free microspore stage, but becomes compressed in mature grains. Tectal microchannels and sculptural rods also initiate during the early free microspore stage, and significant pollenkitt deposition follows, supporting the hypothesis that these elements function in entomophily. The tapetum is morphologically amoeboid, with migratory tapetal cells directly contacting developing free microspores within the anther locule. Results from this study illustrate the importance of including ontogenetic data in analyzing pollen characters and in developing evolutionary and ecological hypotheses. The new palynological data also emphasize the character plasticity that occurs in basal angiosperms.