Wilson A. Taylor

Sporopollenin exhibiting colloidal organization in spore walls

Abstract Certain megaspores from the genus Selaginella (Lycophyta) and similar fossil genera of Cretaceous and Tertiary age are known to demonstrate a remarkable, iridescent appearance. Recent work on the spore wall ultrastructure shows that this iridescence is produced by a complex, ordered, particulate organization which is analogous to that of iridescent virus aggregates, precious opal and other synthetic materials. Our results suggest that this effect is produced by the accumulation of the spore wall material in the form of a monodisperse colloidal crystal. Colloidal organizations can also account for adjacent non-iridescent wall layers, the transition zones between these and the colloidal crystal and for spore walls with no hint of regular organization. The brief time in which the distinctive ordered exine ultrastructure develops and its occurrence around non-viable spore protoplasts add weight to the hypothesis that the exine is largely self-assembling. This new model of spore wall formation may wel...

Ultrastructure of lower Paleozoic dyads from southern Ohio

The uppermost Ordovician Drakes and the lowermost Silurian Centerville Formation in southern Ohio contain a diverse and well preserved microflora including naked and enclosed monads, dyads and tetrads. The least common but, thus far, most interesting elements from an ultrastructural perspective, are the dyads. Pseudodyadospora sp. has a homogeneous spore wall. Two thin, but completely separate crosswalls separate the dyad members. All “pseudodyads” examined to date with transmission electron microscopy (TEM) possess thin crosswalls. The spore wall of the enclosed dyad Segestrespora membranifera is homogeneous to somewhat spongy throughout most of its thickness, but with some irregular globular units and lamellae in its outermost extent. Its envelope stains more darkly than the spore wall and is continuous over the outer surfaces of both spores of the dyad, but not between them. These dyads bear a striking ultrastructural resemblance to enclosed tetrades from this same locality. Both may have been produced by the same or closely related plants. Dyadospora murusdensa is a large naked dyad with a two-layered spore wall. The outer layer is homogeneous, while the inner consists exclusively of a series of parallel lamellae. These are the oldest lamellae reported to date; apparently they can survive in the fossil record up to 430 Myr. Fossil spores with similar ultrastructure to D. murusdensa include: the spores of Naiadita lanceolata from the Triassic of England (thought to be an extinct member of the Sphaerocarpales), the spores of Parka decipiens and the spores of Protosalvinia. In all cases, similarities include a two-parted spore wall with a homogeneous layer and a lamellated layer. Among modern plants, this spore wall organization is known only from the hepatic order Sphaerocarpales. The discovery of these Ordovician lamellations places an early limit on the first appearance of a mechanism of sporopollenin deposition common to all modern land plants.

Ultrastructure of Tetrahedraletes medinensis (Strother and Traverse) Wellman and Richardson, from the Upper Ordovician of southern Ohio

Unmetamorphosed sediments from the Upper Ordovician Drakes Formation of southern Ohio provide excellently preserved permanent spore tetrads that are amenable to analysis using electron microscopy. The exospores of the four tetrad members of Tetrahedraletes medinensis are distinguishable as separate from one another, but show extremely tight association over 60–80% of the proximal faces. Proximal exospores are consistently thinner than distal exospores. Equatorial exospore thickenings at the edges of the contact faces create the “knot-like” configuration seen when these tetrads are viewed with the light microscope. Ultrastructurally, these exospores are essentially homogeneous. This is interpreted to represent the original configuration. With one exception, living bryophytes that consistently produce permanent tetrads have extensively lamellated exospore ultrastructure.

Aeroterrestrial Coleochaete (Streptophyta, Coleochaetales) models early plant adaptation to land

PREMISE OF THE STUDY The streptophyte water-to-land transition was a pivotal, but poorly understood event in Earth history. While some early-diverging modern streptophyte algae are aeroterrestrial (living in subaerial habitats), aeroterrestrial survival had not been tested for Coleochaete, widely regarded as obligately aquatic and one of the extant green algal genera most closely related to embryophytes. This relationship motivated a comparison of aeroterrestrial Coleochaete to lower Paleozoic microfossils whose relationships have been uncertain. METHODS We tested the ability of two species of the experimentally tractable, complex streptophyte algal genus Coleochaete Bréb. to (1) grow and reproduce when cultivated under conditions that mimic humid subaerial habitats, (2) survive desiccation for some period of time, and (3) produce degradation-resistant remains comparable to enigmatic Cambrian microfossils. KEY RESULTS When grown on mineral agar media or on quartz sand, both species displayed bodies structurally distinct from those expressed in aquatic habitats. Aeroterrestrial Coleochaete occurred as hairless, multistratose, hemispherical bodies having unistratose lobes or irregular clusters of cells with thick, layered, and chemically resistant walls that resemble certain enigmatic lower Paleozoic microfossils. Whether grown under humid conditions or air-dried for a week, then exposed to liquid water, aeroterrestrial Coleochaete produced typical asexual zoospores and germlings. Cells that had been air-dried for periods up to several months maintained their integrity and green pigmentation. CONCLUSIONS Features of modern aeroterrestrial Coleochaete suggest that ancient complex streptophyte algae could grow and reproduce in moist subaerial habitats, persist through periods of desiccation, and leave behind distinctive microfossil remains.

Ultrastructure of lower Paleozoic dyads from southern Ohio II: Dyadospora murusattenuata, functional and evolutionary considerations

Fossil cryptospore dyads consistent with published descriptions of Dyadospora murusattenuata fall into two general categories when examined with the TEM. Type I dyads are enclosed, and have a wall consisting of (1) an outer homogeneous layer that is present on non-contact surfaces, but absent in large part over the contact surface, (2) a middle layer composed of plate-shaped units, and (3) an innermost granular layer. The latter two layers are continuous over each of the dyad members. Type II dyads show a range of ultrastructural types, differing primarily with respect to the extent of a region of spongy wall material that occurs at the edge of the contact face. Thin regions and breaks present in the walls of many dyads and pseudodyads may represent primitive sutures. Ultrastractural analysis suggests the existence of at least two separate lineages of plants in the Late Ordovician and Early Silurian, but the phylogenetic relationship between these groups and to more recent land plants remains uncertain.

Ultrastructural analysis of selected Cretaceous megaspores from Argentina

Detailed ultrastructural analysis of an assemblage of Lower Cretaceous megaspores from the Baqueró Formation of Argentina allows a complete description of two new taxa as well as additional information on the structure of several previously described types. Erlansonisporites sparassis displays a complex wall stratification, a separable surface reticulum, and attached microspores. Horstisporites iridodea sp. nov. possesses an elaborately patterned wall like that seen in several extant species of the modern lycopod genus Selaginella. On the basis of developmental information from modern heterosporous lycopod megaspores, the mesospore bearing Bacutriletes triangulatus sp. nov. is recognised as an aborted product of a meiotic tetrad. Also reported from this locality are Hughesisporites patagonicus, Bacutriletes sp. A, and an additional loosely defined megaspore type with characteristic clavae. Transmission electron microscopy provides the opportunity to infer previously unattainable biological information from dispersed spores. Such studies provide new insights as a foundation for the further elucidation of the developmental and evolutionary significance of wall construction in heterosporous pteridophytes.

Megaspore morphology in the Selaginellaceae in a phylogenetic context: A study of the megaspore surface and wall structure using scanning electron microscopy

Megaspore morphology in the lycopod family Selaginellaceae is studied using scanning electron microscopy. The 52 species examined correspond to taxa included in a molecular phylogenetic analysis of the family, based on rbcL gene sequences, and represent the morphological and geographical variation found within the group. Surface sculpture and wall structure are described. A revised terminology to describe the continuum seen in wall structures is presented. The morphological variation is discussed in a phylogenetic context and several putative synapomorphies of clades within the family are identified. This study allows us to assign extant and fossil megaspores to monophyletic groups within the Selaginellaceae.

SEM Analysis of Spongiophyton Interpreted as a Fossil Lichen

Spongiophyton is an enigmatic Lower to Middle Devonian fossil that has been the subject of much debate since its initial description. The most recent hypothesis based on newly described permineralized material and chemical analysis posits it as the remains of a fossil lichen. Detailed analysis with the SEM is generally supportive of this interpretation. The main structural characteristics include a series of variously sized, embedded linear chains of cells aligned perpendicular to the surface and pores that traverse the entire thicker upper half of the thallus. Aside from a size disparity of the cell chains (wider than that seen in modern lichens), both characteristics are reminiscent of the cortex of a lichen (the medulla having been lost during fossilization) with embedded gelatinized hyphae.

Megaspore wall development in Isoetes melanopoda: morphogenetic post-initiation changes accompanying spore enlargement

Abstract This study traces megaspore wall development in Isoetes , comparing results with those obtained from a previous study of Selaginella . As is the case in Selaginella , the shape, number, and arrangement of the exospore units of the megaspores of Isoetes melanopoda is established at an early stage of development. Analysis of subsequent stages leading to mature megaspores reveals a trend of enlarging wall unit size uniformly throughout the exospore resulting in an increase in overall exospore thickness. Megaspores of Isoetes have an inner separable layer like that in Selaginella , as well as a siliceous surface deposit, also a consistent feature of Selaginella megaspores. However, the relative proportions of silica to sporopollenin are much higher in megaspores of Isoetes than in those of Selaginella . Compressional forces affect the entire inner half of the exospore of Isoetes as opposed to only the inner 10% in Selaginella .

Studies in cryptospore ultrastructure: variability in the tetrad genus Tetrahedraletes and type material of the dyad Dyadospora murusattenuata

Abstract Dispersed permanent tetrads assignable to Tetrahedraletes range from the Middle Ordovician to the Lower Devonian. Ultrastructural examination of the wall of these cryptospores reveals considerable diversity in characters such as wall thickness (overall and between proximal and distal walls), wall construction (homogeneous, spongy, globular or some combination thereof), degree of fusion between adjacent tetrad members and presence of a tightly adherent envelope undetectable with the light microscope. When assessing ultrastructural diversity, particular attention must be paid to changes that may be the result of diagenetic and preparatory processes. While ultrastructural factors will probably never be part of routine palynological analyses, such studies can serve to focus attention on characters that might be usable for separating diverse taxa using the light microscope. Paratype material of Dyadospora murusattenuata has an unlamellated spore wall like that identified as type 2 in [Taylor, Rev. Palaeobot. Palynol. 97 (1997)1–8].