Morphology, ultrastructure and mating of sporidia of a wheat-bunt fungus, Tilletia caries (DC.) Tul. Public Deposited

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  • The teliospore of Tilletia caries germinates to produce a promycelium with a whorl of primary sporidia at its tip. The time between emergence of the promycelium and the development of primary sporidia averaged 2.3 h. Fusion between compatible sporidia occurred within an additional 3.8 h. Primary sporidia appeared to be blastic and at maturity were 37-57 pm long and c. 0.7, 1.8 and 1.2 μm wide at the tip, middle and base respectively. While still attached to the promycelium sporidia usually fused in pairs with a short tube near their midpoints. Sporidia in individual whorls were usually packed tightly together, and fused sporidia that were dislodged also remained close together along their entire length. Promycelia contained many small vacuoles and mitochondria, and numerous lipid bodies. As the primary sporidia developed, the promycelial cytoplasm passed into the nascent cells. Septa developed between the bases of mature sporidia and the tips of the denticles. Sporidia that abscised from the denticles commonly had prominent birth scars at their bases. The sporidia had very thin walls, few vacuoles, attenuated mitochondria, and numerous lipid bodies. Conjugation pegs were generally produced by both members of a conjugating pair of primary sporidia and there were bud scars where they emerged. The sporidial walls were apparently hydrolyzed during emergence of the pegs. Vesicles were sometimes present at the tips of the conjugation pegs and electron-dense accumulations were sometimes observed between the tips of advancing pegs just before fusion. The approaching conjugation pegs were precisely aligned, suggesting intercellular communication. The walls of the conjugation pegs fused and then were hydrolyzed. Fused sporidia were relatively homogeneous in content. The nucleus in a sporidium was often close to the conjugation tube and occasionally was partly within the tube. When lunate secondary sporidia of opposite mating types were paired on agar, conjugation pegs developed as hyphal protuberances from sporidia or their germination tubes. Sometimes, germination tubes developed into conjugation pegs. A conjugation peg from one sporidium sometimes appeared to induce a peg in its partner, after contact. Conjugation pegs met precisely tip-to-tip before fusion. This often required, or caused, curvature of the pegs. Secondary sporidia of opposite mating types initially adjacent to each other were pushed apart during conjugation. Conjugation tubes were formed by the fusion of conjugation pegs from sporidia of opposite mating types or by fusion of a conjugation peg from a sporidium of one mating type with the body or germination tube of another sporidium of opposite mating type. The times required for development of conjugation pegs and formation of conjugation tubes depended on the relative orientations of the sporidia in a mating pair. Conjugation was most rapid with sporidia positioned end-to-end. Conjugation was favored by a pH of 4.5-6 and at pH 6, conjugation occurred as readily on water agar as on a nutrient medium. The time required for conjugation increased as the initial distance between mating sporidia was increased. On agar, there were most intersporidial contacts in pairs of sporidia of opposite (+ -) mating types than in pairs of like (+ + or - -) mating types. Pre-conjugation pegs were produced by sporidia after displacement of stimulating (opposite mating type) sporidia and the pegs continued to elongate. Sporidia of opposite mating types conjugated when subjected to a treatment that would disrupt fimbrial connections. Multiple hyphal tips were produced by differentiating sporidia separated from monokaryons (mycelia and sporidia) of opposite mating type by distances of 70-200 pm. The response was sex-specific. In some experiments the factors causing these responses were retained in the agar substrate. These factors may initiate the formation of conjugation pegs.
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