Structure and function in angiosperm pollen wall evolution

Observations and measurements from angiosperm species in more than 150 genera from 15 monocotyledonous and 69 dicotyledonous families show that harmomegathy, changes in size, shape, and volume of pollen grains as a result of water loss or intake are characteristic for flowering plants. Size changes as measured from the exterior wall surfaces average 46%. No particular pollen types or kinds or arrangements of apertures seem to be superior for providing mechanisms to accomodate these volumetric changes and the associated stress. As water is lost from a grain, contractions and invaginations of thinner areas of the wall establish geometric forms related to the positions and numbers of germinal apertures, and to internal and sculptural elements of the exine. As a result of these invaginations and accompanying flexion and arcuation (arching) of interapertural wall sections, configurations are created that permit the grain to develop and withstand enormous stress without breaking. The stress is a measure of negative wall pressure that helps establish a water deficit within the grain that brings the living substance of the protoplast into equilibrium with the desiccating forces of the atmosphere. Stress is accommodated through the inherent strength and resiliency of sporopollenin, enhanced by cellularization (baculation, lamination, plication, and alveolation), and by strutting and reinforcement systems involving massive deposits of sporopollenin at special bearing and stress points, such as costal and oral margins, or the poles. Architectural features that distribute and bear the stress include mechanisms for tension, arcuation, and suspension that carry stress to bearing points. The effect of these accommodations is to permit the pollen grain to arrive at the stigma alive and in condition to respond and grow quickly. It is hypothesized that evolution of structural features that meet harmomegathal stress requirements provides the principal explanation for pollen wall form, composition, organization, and architecture.     

The endexine: a frequently overlooked pollen wall layer and a simple method for detection

For the detection of pollen wall layers, the use of different staining methods for one and the same species is highly recommended. The usage of standard transmission electron microscopy (TEM) staining methods showed that the ektexine-layers have always the same contrast behaviour, while the endexine changes its electron opaqueness depending on the method used. However, the endexine can often not be discriminated from the other wall layers. A simple method to detect the endexine is the use of potassium permanganate, which stains the layer electron dense, producing a distinct contrast.     

OsLTP47 may function in a lipid transfer relay essential for pollen wall development in rice

In plants, lipid transfer proteins(LTPs) transport pollen wall constituents from the tapetum to the exine, a process essential for pollen wall development. However, the functional cooperation of different LTPs in pollen wall development is not well understood. In this study, we have identified and characterized a grassspecific LTP gene, Os LTP47, an important regulator of pollen wall formation in rice(Oryza sativa). Os LTP47 encodes a membrane-localized LTP and in vitro lipid-binding assays conf...     

Plasmodial tapetum and pollen wall development in Butomus umbellatus (Butomaceae)

This report describes the ultrastructural development of plasmodial tapetum and pollen wall of Butomus umbellatus. The tapetum contains extensive arrays of rough endoplasmic reticulum, vesicles from which are responsible for the formation of sporopollenin-like bodies. The tapetum is also involved in the formation of other forms of sporopollenin precursors. Development of pollen wall continues after microspores are released from their callosic walls; they are then enclosed by plasmodial tapetum. The activity and products of the plasmodial tapetum show substantial correlation with pollen wall development, particularly ektexine formation. In B. umbellatus, the tapetum intrudes into the anther locule at early microspore stage. This timing of plasmodial intrusion occurs at a later stage of pollen development as compared to those in the advanced monocotyledons. We report the rough endoplasmic reticulum origin of sporopollenin-like bodies and their occurrence in the plasmodial tapeta of B. umbellatus.     

A clarification of pollen discounting and its joint effects with inbreeding depression on mating system evolution

Given the predominance of outcrossing by angiosperms, large costs must often overwhelm the genetic benefit of selfing derived from contributing two haploid genomes to each off-spring rather than one. In addition to the well-studied genetic cost of inbreeding depression, selfing imposes a mating cost whenever self-pollination reduces opportunities for pollen export. Because self-pollination is a heterogeneous process, pollen discounting and its evolutionary consequences vary with pollination conditions. In this article we model self-pollination as comprising discounting and nondiscounting components, and we consider the consequences of this heterogeneity for outcross siring success. Aided by this depiction of pollination, we then compare previous theoretical representations of pollen discounting and consider their relative virtues. Finally, we consider conditions that would allow a population to be invaded by a variant with different pollination characteristics. This analysis exposes the pollination conditions implicit in standard results of mating system theory. It also identifies associations between four possible changes in pollination expected in different reproductive environments, including the incidence of positive or negative correlations between self-pollination and pollen export. These results emphasize the benefits of expanding the theory of plant reproduction to recognize explicitly when and how pollination mechanisms affect mating outcomes.     

Anther wall and pollen development in Ophrys mammosa L. (Orchidaceae)

The objective of this study was to investigate anther wall and pollen development in Ophrys mammosa. Primary sporogen tissue resembles longitudinal cells with divided archeosporal cells. Thereafter these primary sporogen tissue cells re-divide anticlinally and periclinally forming secondary sporogen tissue. Microsporogenesis was successive type. Microgametogenesis occurred at the distal poles of the microspores. In addition, dense starch accumulation was detected in the pollen. Pollinia and massulae are separated from each other by dead cells filled with callose, according to histochemical preparations. The anther wall was a four-layered “monocotyledon” type. There was ring-like wall thickening in the endothecium. The tapetum is of the glandular type. When these two developmental processes are compared, it is seen that the anther wall has become mature by the sporogen tissue phase and is composed of only epidermis and endothecium at the beginning of microgametogenesis.     

Merging concepts: The role of self-assembly in the development of pollen wall structure

In this article, we analyse established details of exine development from a perspective that favours the integration of self-assembly. We isolate those intervals in development in which genomic control is exercised and offer a number of scenarios, which show how self-assembly can build upon a genetic basis to give rise to the fundamental pollen exine structure. This paper is a synthesis of a new concept and a detailed review of achievements in the field of developmental palynology. It seeks to link what is known regarding development with the liquid crystal realm of colloid chemistry.     

The anthropoid postcranial axial skeleton: comments on development, variation, and evolution

Within-species phenotypic variation is the raw material on which natural selection acts to shape evolutionary change, and understanding more about the developmental genetics of intraspecific as well as interspecific phenotypic variation is an important component of the Evo-Devo agenda. The axial skeleton is a useful system to analyze from such a perspective. Its development is increasingly well understood, and between-species differences in functionally important developmental parameters are well documented. I present data on intraspecific variation in the axial postcranial skeleton of some Primates, including hominoids (apes and humans). Hominoid species are particularly valuable, because counts of total numbers of vertebrae, and hence original somite numbers, are available for large samples. Evolutionary changes in the axial skeleton of various primate lineages, including bipedal humans, are reviewed, and hypotheses presented to explain the changes in terms of developmental genetics. Further relevant experiments on model organisms are suggested in order to explore more fully the differences in developmental processes between primate species, and hence to test these hypotheses.     

Bombacaceae pollen from the Indian Tertiary sediments and its bearing on evolution and migration

Several pollen species recovered from the Indian Tertiary sediments have close similarities with pollen of extant Bombacaceae. These morpho species are included in the morphogenera Lakiapollis Venkatachala and Kar, Dermatobrevicolporites Kar, Bombacacidites Couper, Tricolporocolumellites Kar and Retitribrevicolporites Kar. Nine species are re-described on the basis of original type material or other specimens. The fossil pollen belongs to the Bombax- and Durio-types. The small-sized Bombax-type pollen (e.g., Eriotheca pollen) has been documented in India from the early Palaeocene to the Miocene while large-sized pollen similar to that of Bombax appeared in the Oligocene and continued up to the Present. The lobed variety of the Bombax-type is rare and occurs only in the Pleistocene. Palynological evidence suggests a migration of Bombacaceae producing Bombax-type pollen to India from Africa. Durio-type pollen appeared in the late Palaeocene and continued to dominate until the early Eocene. In southern India, this pollen remained frequent during Oligocene/Miocene times, whereas in other areas, it became rare at the end of the Eocene. The earliest (late Palaeocene) record suggests an origination of Bombacaceae producing Durio-type pollen in India and a subsequent migration to Southeast Asia.     

Development of the exineless pollen wall in Callitriche truncata (Callitrichaceae) and the evolution of underwater pollination

 The Callitrichaceae are a monogeneric family of aquatic angiosperms comprising approximately 50 terrestrial, amphibious, and obligately submersed species. Callitriche is unique in being the only known genus with co-occurring aerial and underwater pollination systems. Mature pollen structure is correlated with growth habit, pollination biology, and phylogeny within the genus. In the present study, development of exineless pollen in the obligately submersed species Callitriche truncata was examined, with particular emphasis on the tetrad stage. Pollen ontogeny occurred rapidly and non-synchronously; tetrads, free microspores, and two-celled pollen grains were identified within the same anthers. Formation of the intine also occurred relatively early, during the tetrad stage. Tetrads were surrounded by a structurally distinct envelope, and its ultrastructure and histochemistry indicate that this callose-like envelope is in a transitional state. Reduction or complete loss of the exine has evolved at least twice in Callitrichaceae, and the new ontogenetic data indicate that exine loss evolves more quickly than the loss of callose. In addition, developmental information on exineless pollen in C. truncata coupled with other palynological data for the exine-bearing terrestrial and amphibious growth forms provide support for the hypothesis that underwater pollination has had a relatively recent origin in the family. Received January 2, 2001 Accepted March 27, 2001     

Development of the pollen grain wall in Canna

During the tetrad period spinules form on the Canna L. plasma membrane at intervals of 1–2 μm on a microspore surface of ca. 100 μm². The isolated spinules represent all that there is of a primexine-like nature. Immediately following loss of the callosic tetrad envelope a channeled, oncus-like zone forms on the plasma membrane over the entire microspore surface, elevating the spinules. The oncus-like zone becomes ca. 4 μm thick by microspore mitosis. Intine introduction during the pollen grain period coincides with substantial thinning of the oncus-like zone and pollen grain enlargement. In the final phases of maturation grains increase further in diameter and become packed with starch and lipoidal material. The oncus-like zone more than doubles in height necessitating a migration of the boundary of the oncus-like zone and intine. At pollen grain maturity the thin (ca. 200 nm) surface layer of the oncus-like zone appears to be compacted or filled in.     

Cell wall reformation by pollen tube protoplasts of olive (Olea europaea L.): structural comparison with the pollen tube wall

. Mature pollen grains of olive (Olea europaea L.) were germinated in vitro in Brewbaker and Kwack medium, and emerging pollen tubes were then enzymatically digested in the presence of high osmoticum. This treatment resulted in simultaneous degradation of pollen tube walls and fragmentation of their cytoplasm, giving rise to numerous protoplasts of different sizes and different numbers of nuclei. After the protoplasts had been purified, they were cultured in Murashige and Skoog medium supplemented with auxin and cytokinin. The initial steps of cell wall reformation were studied after 12 h and 24 h of culture with a series of cytochemical techniques including periodic acid-Schiff reagent and phosphotungstic acid, as well as with electron microscopy and immunocytochemical techniques using monoclonal antibodies directed against pectins and #-(1̅)-glucan (callose). Among the components of new wall in the protoplasts, callose proved to be the earliest and most abundantly secreted polysaccharide, whereas the deposition of pectins recognized by the antibody JIM7 started several hours later. Pectins that bind JIM5 antibody were not detected in this early stage of development. Cell wall components deposited by protoplasts were compared with those present in growing pollen tubes. Callose secreted by protoplasts formed a relatively thicker layer than that found in the tubes, and pectins recognized by JIM7 were highly abundant, mostly within the cytoplasm and in the apical zone of the tubes.     

Passage of lanthanum through the pollen grain wall of Olea europaea L. during development

Summary In order to study the behavior of the exine as a site of passage of material from the locule of the anther, lanthanum nitrate was used to locate the possible routes of communication between the Olea europaea L. pollen grain and its external environment from the period of exine consolidation until dehiscence of the anther. In all four stages of development studied, dense lanthanum deposits occupied microchannel-like orifices, whereas these dense deposits occupied the apertural regions only in the first stages of development. Lanthanum precipitate is also present in the endexine, intine and cytoplasmic vesicles of mature pollen. The transportational function of the apertures is discussed in relation to the presence of dense lanthanum deposits in some stages of pollen grain.