The fine structure of ascospore wall formation in Sordaria fimicola

Two Urosoma species (Ciliophora, Hypotrichia): A multidisciplinary approach provides new insights into their ultrastructure and systematics

The general morphology and ultrastructure of two soil hypotrichous ciliates, Urosoma emarginata and U. salmastra, were investigated using light microscopy, scanning electron microscopy and transmission electron microscopy. Phylogenetic analyses, based on the newly sequenced small subunit ribosomal (SSU) rRNA genes, were conducted on three U. emarginata populations and one U. salmastra population. Our findings support for the validity of Perilemmaphora Berger, 2008, a rankless taxon comprising spirotrich ciliates having a perilemma. The cortical granules of both species are extrusomes representing a new type of mucocyst in U. emarginata and possibly a new type of pigmentocyst in U. salmastra. Additionally, the lithosomes were revealed as subglobose structures composed of a low electron-dense, homogeneous inner part and an electron-dense outer part. The ultrastructural features of the cortical granules, together with ontogenetic and molecular phylogenetic data, suggest that the genus Urosoma might need to be divided. It is posited that ultrastructural features of hypotrichous ciliates in general may have important taxonomic value warranting further investigation.

Ultrastructural observations on ascosporogenesis in Ciboria betulae (Helotiales : Sclerotiniaceae)

The ultrastructure of ascospore ontogeny including spore delimitation and spore wall formation is examined in Ciboria betulae. A peripheral delimiting cisterna is derived from the ascus plasmalemma apparently by the self-assembly of subunits in the cytosol. The cisterna is discontinuous and delimits each of eight incipient ascospores in the ascus. The particular, verrucose ascospore ornamentation in C. betulae appears early during primary wall formation. The electron-lucent primary wall differentiates into an endospore of finely fibrillar texture and a two-layered, fibrillar epispore. The ultrastructural changes of ascus epiplasm, sporoplasm, and spore wall are described and compared with previous studies on Ascomycetes. Key words: ascosporogenesis, ascospore ontogeny, Ciboria betulae, ascospore wall differentiation.

Ultrastructure of developing ascospores in Sordaria brevicollis

The ultrastructure of ascospore wall formation in the pyrenomycete Sordaria brevicollis was studied in developing asci at progressive time intervals. From early spore delimitation through final stage of maturation, the wall of the ascospore differentiated into four composite layers, the periascosporium the delineation ascosporium, the subascosproium, and the endoascosproium, While ascospores were at the hyaline stage of development,they possessed only the periascosporium and delineation ascosporium as their wall components. At about 7 to 8 days from the initiation of the cross, the spores developed a yellow color, and this coloration was always associated with the elaboration of the subascorsporium just internal to the ascosporium. Asthe spores continued to progressively darken in color, the subascosporium was seen to increase in complexity, electron density, and thickness. Soon after the formation of the subascosporium, the endoascosporium began to develop de novo and was, therefore, the last wall layer formed as the spore approached maturity.

Ascospore wall development in Saccharomyces cerevisiae

Ascospore delimitation in Saccharomyces cerevisiae is initiated by a pair of unit membranes between which the spore wall is subsequently laid down.

Changes in the lipid composition and fine structure of Saccharomyces cerevisiae during ascus formation

Eighty to ninety percent of vegetative cells of Saccharomyces cerevisiae DCL 740 incubated in KCl-acetate medium form asci, the majority of which are four-spored. Ascospores are visible in asci after about 24 hr, and spore formation is complete after about 48 hr. The dry weight of the cells increases by about 75% during 48 hr of incubation, while the lipid content of the cells increases by a factor of four. The increase in lipid content is attributed mainly to an increased synthesis of sterol esters and triacylglycerols and to a lesser extent of phospholipids. The phospholipid and sterol compositions do not change appreciably, but there is a marked increase in the proportion of unsaturated fatty acid residues in ascan lipids. Uniformly labeled (14)C-acetate is incorporated mainly into sterol esters and triacylglycerols and phospholipids. Pulse-labeling by adding acetate-U-(14)C to sporulating cultures and harvesting after a further 6 hr of incubation reveal two main periods of acetate incorporation, namely between 0 and 18 hr, and between 24 and 30 hr. Electron micrographs of thin sections through developing asci show that the principal changes in fine structure occur between 18 and 24 hr and include the appearance of numerous electron-transparent vesicles which become aligned around the meiotic nucleus, and the laying down of extensive endoplasmic reticulum membranes. Changes in fine structure are discussed in relation to the alterations in lipid content and composition of asci.