The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprole... more The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprolegnia from fish hatcheries in Japan were compared. A linear relationship between the number of hairs per bundle against bundle length, on secondary zoospore cysts, was found. One isolate of S. parasitica, a particularly strong pathogen, from silver salmon, had a secondary zoospore cyst ornamentation consisting of separate bundles of long, short and intermediate length hairs. This feature was also seen to a lesser extent in other isolates, including a strong pathogen from brown trout in the U.K. Using the Japanese isolates, temperature-growth relationships at 30 °C distinguished isolates of S. parasitica, from rainbow trout, from those from other fish. A mutant of one of the rainbow trout isolates showed differences in growth rates compared with the parent isolate, the possible ecological significance of which is discussed.
The ultrastructural architecture of the oospore-oogonium walls of Phytophthora megasperma was stu... more The ultrastructural architecture of the oospore-oogonium walls of Phytophthora megasperma was studied in the soybean (f. sp. glycinea ) and alfalfa (f. sp. medicaginis ) isolates with the major objective of elucidating changes that occur during oospore germination. For comparison, ultrastructural changes in isolated oogonium-oospore walls brought about by digestion with various β-glucanases were also examined. Major emphasis was placed on determining the structure of the conspicuous inner oospore wall (IOW). This thick (2–3 μm) layer showed little staining by any of the procedures used, including staining for polysaccharides by the Thiery procedure. There were only minor differences in electron density across the IOW. Observations on sections of whole oospores and mechanically fractured, or partially digested, walls indicated that the IOW contains microfibrils embedded in an amorphous matrix which tend to be oriented parallel to the plasma membrane. During oospore germination, the IOW is extensively eroded in a characteristic ‘scallop’ pattern that creates numerous cavities separated by radial strands of undigested wall. The electron dense outer oospore wall (OOW) remained intact and unaltered and probably confines the activity of the wall digesting enzymes to the IOW. In the digested cavities, removal of the wall matrix exposed the network of underlying microfibrils together with some electron dense globular material. Treatment of isolated oogonium-oospore walls with exo-β-1,3 glucanase or a mixture of endo-β-1,3-glucanases caused substantial digestion of the oogonium wall matrix and the IOW. Following enzymatic digestion, both the IOW and, in particular, the oogonium walls showed greater reactivity with the Thiery procedure. The observed patterns of wall digestion indicate that the amorphous β-1,3-glucan component of the IOW is extensively consumed during germination and is probably the main supply of carbon and energy for germination. No evidence of a pre-formed germination wall layer in the oospore was found. As the innermost layers of the oospore wall became digested, a new wall was produced internal to the IOW.
This review discusses the taxonomy, patterns of sporogenesis and modes of infection of a group of... more This review discusses the taxonomy, patterns of sporogenesis and modes of infection of a group of little studied holocapic pathogens of bactivorous nematodes (and rotifers) from terrestrial and marine habitats. These holocarpic obligate parasites have been traditionally placed within the "Iagenidiaceous oomycetes" although Haptoglossa had been placed in the Saprolegniales. The nematode pathogens that will be discussed fall within the genera C/amydomyzium, Gonimocheate, Haptoglossa and Myzocytiopsis. The patterns of asexual and sexual sporogenesis will be described in detail in the light of recent ultrastructural studies that we have undertaken. We conclude by discussing the main infection strategies employed by these organisms which we categorise into active and passive types. In the former, zoospores actively locate their host (by chemotaxis) and encyst on the host surface immediately prior to infection. In the latter types, the zoospores or aplanospores rapidly germinate...
The origin and structure of cyst surface components in oomycete fungi are described in relation t... more The origin and structure of cyst surface components in oomycete fungi are described in relation to evaluating their use as taxonomic markers. The ontogeny, biochemistry, and morphological variations in the populations of peripheral vesicles that occur in the zoospores of members of the Peronosporomycetidae and Saprolegniomycetidae, which are discharged upon encystment to give rise to spore surface components, are reviewed. The use of the ornamentation of the cyst coat to define genera within the Saprolegniomycetidae and as an indicator of geographical and host-specific clones within a species of Saprolegnia are reviewed. The cyst coat of the fish pathogen Saprolegnia parasitica shows considerable morphological diversity that appears to be independent of the geographic origin of isolates. There is an indication that a group of isolates with variable, relatively short, spines form a separate clone from those with larger hooped bundles. The monoclonal antibody probes that have been generated to spore surface components of Saprolegnia parasitica, Pythiwn aphanidermatum, and Phytophthora cinnamomi are summarized and their use in defining taxonomic groups reviewed. The antibodies show different levels of specificity ranging from isolate- to order-specific reactions. The degree of specificity can be influenced in some cases by the immunoassay method used. These studies indicate that surface structures and molecules provide extremely versatile and valuable markers for use in oomycete taxonomy and phylogeny. Key words: Saprolegnia, Phytophthora, Achlya, oomycete, zoospore, cysts, ultrastructure, monoclonal antibodies, taxonomy.
... Sections were poststained in 2% uranyl acetate for 40 min and Reynold's lead citrate... more ... Sections were poststained in 2% uranyl acetate for 40 min and Reynold's lead citrate for 10 min in an LKB automatic stainer before examination in either a CORA or JEOL 100S electron microscope operated at 60 kV with 25-/Ш1 apertures. ...
The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprole... more The morphology and physiology (temperature-growth relationships) of seventeen isolates of Saprolegnia from fish hatcheries in Japan were compared. A linear relationship between the number of hairs per bundle against bundle length, on secondary zoospore cysts, was found. One isolate of S. parasitica, a particularly strong pathogen, from silver salmon, had a secondary zoospore cyst ornamentation consisting of separate bundles of long, short and intermediate length hairs. This feature was also seen to a lesser extent in other isolates, including a strong pathogen from brown trout in the U.K. Using the Japanese isolates, temperature-growth relationships at 30 °C distinguished isolates of S. parasitica, from rainbow trout, from those from other fish. A mutant of one of the rainbow trout isolates showed differences in growth rates compared with the parent isolate, the possible ecological significance of which is discussed.
The ultrastructural architecture of the oospore-oogonium walls of Phytophthora megasperma was stu... more The ultrastructural architecture of the oospore-oogonium walls of Phytophthora megasperma was studied in the soybean (f. sp. glycinea ) and alfalfa (f. sp. medicaginis ) isolates with the major objective of elucidating changes that occur during oospore germination. For comparison, ultrastructural changes in isolated oogonium-oospore walls brought about by digestion with various β-glucanases were also examined. Major emphasis was placed on determining the structure of the conspicuous inner oospore wall (IOW). This thick (2–3 μm) layer showed little staining by any of the procedures used, including staining for polysaccharides by the Thiery procedure. There were only minor differences in electron density across the IOW. Observations on sections of whole oospores and mechanically fractured, or partially digested, walls indicated that the IOW contains microfibrils embedded in an amorphous matrix which tend to be oriented parallel to the plasma membrane. During oospore germination, the IOW is extensively eroded in a characteristic ‘scallop’ pattern that creates numerous cavities separated by radial strands of undigested wall. The electron dense outer oospore wall (OOW) remained intact and unaltered and probably confines the activity of the wall digesting enzymes to the IOW. In the digested cavities, removal of the wall matrix exposed the network of underlying microfibrils together with some electron dense globular material. Treatment of isolated oogonium-oospore walls with exo-β-1,3 glucanase or a mixture of endo-β-1,3-glucanases caused substantial digestion of the oogonium wall matrix and the IOW. Following enzymatic digestion, both the IOW and, in particular, the oogonium walls showed greater reactivity with the Thiery procedure. The observed patterns of wall digestion indicate that the amorphous β-1,3-glucan component of the IOW is extensively consumed during germination and is probably the main supply of carbon and energy for germination. No evidence of a pre-formed germination wall layer in the oospore was found. As the innermost layers of the oospore wall became digested, a new wall was produced internal to the IOW.
This review discusses the taxonomy, patterns of sporogenesis and modes of infection of a group of... more This review discusses the taxonomy, patterns of sporogenesis and modes of infection of a group of little studied holocapic pathogens of bactivorous nematodes (and rotifers) from terrestrial and marine habitats. These holocarpic obligate parasites have been traditionally placed within the "Iagenidiaceous oomycetes" although Haptoglossa had been placed in the Saprolegniales. The nematode pathogens that will be discussed fall within the genera C/amydomyzium, Gonimocheate, Haptoglossa and Myzocytiopsis. The patterns of asexual and sexual sporogenesis will be described in detail in the light of recent ultrastructural studies that we have undertaken. We conclude by discussing the main infection strategies employed by these organisms which we categorise into active and passive types. In the former, zoospores actively locate their host (by chemotaxis) and encyst on the host surface immediately prior to infection. In the latter types, the zoospores or aplanospores rapidly germinate...
The origin and structure of cyst surface components in oomycete fungi are described in relation t... more The origin and structure of cyst surface components in oomycete fungi are described in relation to evaluating their use as taxonomic markers. The ontogeny, biochemistry, and morphological variations in the populations of peripheral vesicles that occur in the zoospores of members of the Peronosporomycetidae and Saprolegniomycetidae, which are discharged upon encystment to give rise to spore surface components, are reviewed. The use of the ornamentation of the cyst coat to define genera within the Saprolegniomycetidae and as an indicator of geographical and host-specific clones within a species of Saprolegnia are reviewed. The cyst coat of the fish pathogen Saprolegnia parasitica shows considerable morphological diversity that appears to be independent of the geographic origin of isolates. There is an indication that a group of isolates with variable, relatively short, spines form a separate clone from those with larger hooped bundles. The monoclonal antibody probes that have been generated to spore surface components of Saprolegnia parasitica, Pythiwn aphanidermatum, and Phytophthora cinnamomi are summarized and their use in defining taxonomic groups reviewed. The antibodies show different levels of specificity ranging from isolate- to order-specific reactions. The degree of specificity can be influenced in some cases by the immunoassay method used. These studies indicate that surface structures and molecules provide extremely versatile and valuable markers for use in oomycete taxonomy and phylogeny. Key words: Saprolegnia, Phytophthora, Achlya, oomycete, zoospore, cysts, ultrastructure, monoclonal antibodies, taxonomy.
... Sections were poststained in 2% uranyl acetate for 40 min and Reynold's lead citrate... more ... Sections were poststained in 2% uranyl acetate for 40 min and Reynold's lead citrate for 10 min in an LKB automatic stainer before examination in either a CORA or JEOL 100S electron microscope operated at 60 kV with 25-/Ш1 apertures. ...
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