Male cone morphogenesis, pollen development and pollen dispersal mechanism in Ginkgo biloba L.

Lu, Y., Wang, L., Wang, D., Wang, Y., Zhang, M., Jin, B. and Chen, P. 2011. Male cone morphogenesis, pollen development and pollen dispersal mechanism in Ginkgo biloba L. Can. J. Plant Sci. 91: 971–981. Ginkgo biloba L. is one of the oldest gymnosperms. Male cone morphogenesis, pollen development and dispersal are important for successful pollination and reproduction. In this study, we investigated the development of male cone, pollen and the sporangial wall in detail. The results indicate that: (1) The primordia of male cones and leaves begin to differentiate in early June and remain open until the following March. The male cones then mature and release pollen in mid-April. The male cones are drooped and approximately perpendicular to the leaves during pollination. (2) The microsporocytes develop from the sporogenous cell and form a tetrahedral tetrad after two simultaneous asymmetrically meioses, then produce a matured four-cell pollen after three polar mitotic divisions. The matured pollen is hemispheric in shape with a large aperture area and three pollen wall layers; once released from the microsporangia, the pollen becomes boat-like in shape. (3) The sporangial walls are eusporangiate and consist of epidermis, endothecium and tapetum. The differentiation of the tapetum occurs separately from that of the epidermis and endothecium, and originates from the outermost layer of sporogenous cells. The sporangial walls exhibit shrinkage of the epidermis, fibrous thickening of the endothecium, and enzymic dissolution of the tapetum during pollen dispersal, which contributes to microsporangia opening. Based on these results, we conclude that there many unique and primitive characteristics of the development of the male cones, pollen and sporangial wall of G. biloba. In addition, we also found that the male cones, pollen and sporangial walls have evolved efficient structural and morphological adaptations to anemophily.

Ultrastructure of spermatia and spermatium ontogeny in the rust fungus Cronartium quercuum f.sp. fusiforme

Spermogonia of Cronartium quercuum f.sp. fusiforme developed just beneath the bark on galled regions of infected pine seedlings. Spermogonia consist of flattened, spreading, island-like masses of fungal tissue covered with a thin layer of liquid containing large numbers of spermatia. Spermatia arose in an annellophoric fashion from the tips of long, slender sporogenous cells produced in a distinct layer. Each sporogenous cell contained a large prominent nucleus that underwent mitosis as each spermatium initial developed. One of the resulting nuclei moved into the initial while the other remained in the sporogenous cell. Once a spermatium was delimited, it was pushed away from the tip of the sporogenous cell as another spermatium initial developed below it. Once delimited, a spermatium underwent specific morphological changes as it matured. A mature spermatium was subpyriform in shape and surrounded by a thin wall. In addition to a single large nucleus each spermatium contained ribosomes, mitochondria, lipid bodies, strands of endoplasmic reticulum, vacuole-like inclusions, and many small vesicles that packed its base. Keywords: transmission electron microscopy, pycnidia, pycnidiospores, spermogonia.

Ontogeny of the uredium of Melampsora lini

Light microscopy and transmission and scanning electron microscopy were used to study the development of uredia of Melampsora lini. Uredia were produced 6–10 days after inoculation with urediospores of M. lini. Uredium ontogeny began with the formation of a uredium initial from a hyphal plexus in a substomatal cavity. The hyphae oriented vertically and expanded; the enlarged ends divided to form a palisade of uredial initial cells. These cells divided into basal and terminal cells. Each terminal cell divided transversely to form a peridial and an intercalary cell; the basal cell became the sporogenous cell. The intercalary cells disintegrated early in the expansion of the uredium and released their contents into the intercellular matrix. The sporogenous cell, usually swollen at one end, gave rise by budding to successive sympodially produced spore buds which elongated and divided transversely to form pedicels and immature spore cells. As the spores matured the pedicels shriveled and became separated from the urediospores. Elongated, often capitate, paraphyses formed throughout the uredium and functioned to rupture the peridium and epidermis which covered the immature uredium. The peridium and the intercalary cells formed only once during ontogeny of the uredium, this being associated with only the first generation of urediospores. The intercalary cells were disrupted during uredium ontogeny, and most of the peridial cell layer was sloughed off, along with the overlying epidermis, upon rupture of the latter. The paraphyses were permanent and remained in the uredium throughout its functional life. Successive generations of urediospores arose within the same uredium from spore buds produced sympodially from the original sporogenous cell but without forming additional peridial or intercalary cells.

Ultrastructure of teliospore formation in the cedar-apple rust fungus Gymnosporangium juniperi-virginianae

A telium of G. jttniperi-virginianae consists of a palisade-like layer of hyphae located beneath the host epidermis. The terminal cell of each hypha within the telium disintegrates before the onset of teliospore formation. The cell immediately beneath the terminal cell functions as a sporogenous cell giving rise to teliospore initials. Each sporogenous cell contains two nuclei which divide mitotically as each initial forms. Two daughter nuclei move into the teliospore initial and two remain in the sporogenous cell. The initial elongates and is delimited from the sporogenous cell by a basal septum. The nuclei within the initial then divide and a centripetally developing septum separates the initial into two binucleate cells. The lower cell dies and becomes the pedicel of the spore while the upper cell or teliospore mother cell continues to develop. The nuclei of the mother cell divide and a centripetally developing central septum divides the cell into two binucleate cells. At this stage, the young teliospore is delimited. Its wall thickens and the spore enlarges, becoming more ellipsoid. Eventually, the nuclei within each cell of the teliospore fuse. After karyogamy, synaptonemal complexes appear in the nuclei of spores still within the telium.

Electron microscopy of urediospore formation in Puccinia coronata avenae and P. graminis avenae

Electron microscopy revealed that the sporogenous cells in urediosori of Puccinia coronata avenae and P. graminis avenae were enlarged at one end, and the cytoplasm was dense, vacuolated, and showed an early accumulation of lipid droplets. Urediospore formation was initiated by the outgrowth of a spore bud from the enlarged end of a sporogenous cell. A nuclear division occurred in the spore bud, the spore bud then grew to form the urediospore initial, and a septum formed to delineate the urediospore initial from the sporogenous cell. A further nuclear division occurred in the urediospore initial followed by septation to form the pedicel and spore cell. The urediospores rapidly grew to full size and further differentiation was marked by increased density of the cytoplasm, disappearance of vacuoles, increased lipid droplet accumulation, thickening of the spore wall, and spine formation. Nucleoli were not found in nuclei of mature urediospores. Possible paraphysis cells were intermixed with urediospores near the margins of urediosori of P. coronata. These cells were characterized by small nuclei which contained densely staining patches, a fine membranous network throughout the cytoplasm, and numerous cytoplasmic inclusions of variable morphology.

Asterobolus: a new parasitic hyphomycete with a novel dispersal mechanism

The genus Asterobolus is proposed for an undescribed hyphomycete characterized by multicelled, star-shaped conidia forcibly discharged by a downfolding of the radiating appendages and rupture of the sporogenous cell. Asterobolus gaultheriae sp. nov. is the causal agent of a leaf spot of Gaultheria shallon Pursh. The fungus also infects species of Vaccinium, Pteridium, Malus, and Menziesia. Sclerotia were observed only on Gaultheria and infections of other hosts were found only near infected Gaultheria. In addition to the star-shaped conidia, a Gliocladium-like conidial stage was also observed in cultures of the fungus.

The development of the ovule and megagametophyte in Rhexia mariana

The ovules of Rhexia mariana are bitegmic and anatropous or rarely atropous. Periclinal divisions in the nucellar epidermis at the micropylar end produces a nucellar cap. A terminal pore in the nucellus is formed by the suppression of anticlinal divisions in the epidermis and subsequent separation of the cells during nucellar enlargement. A single hypodermal archesporium divides, producing a primary parietal cell and primary sporogenous cell. The primary parietal cell establishes a prominent parietal tissue as the primary sporogenous cell differentiates into the megasporocyte. Two prominent nucleoli that consistently appear in the archesporium persist in the primary sporogenous cell and the megasporocyte. Meiosis produces a tetrad of megaspores in either linear or approximately T-shaped arrangement. Cells of the nucellus adjacent to the sporogenous cell, megasporocyte, and tetrad rarely give rise to megagametophytes aposporically. The chalazal megaspore functions in megagametogenesis. Shortly after the first nuclear division, vacuoles migrate and coalesce between the nuclei. Two more nuclear divisions establish the four- and eight-nucleate megagametophytes. The chalazal nuclei are situated laterally and are noticeably smaller than the micropylar nuclei in both four- and eight-nucleate stages. The two polar nuclei remain in close contact, usually near the egg apparatus, but sometimes near the center of the megagametophyte. The three antipodal nuclei are ephemeral with degeneration completed by the time the egg apparatus is established. Orientation of the egg apparatus places the synergids on the side of the megagametophyte adjacent to the funiculus and the egg cell on the opposite side. The similarities and differences in ovule and megagametophyte development between Rhexia mariana and the tropical representatives of Melastomataceae previously investigated are discussed.