Straminipilous fungi : systematics ofthe peronosporomycetes, including accounts of the marine straminipilous protests, the plasmodiophorids, and similar organisms / by Michael W. Dick. p.cm. Includes bibliographical references (p. ).

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An overview of sexual reproduction in the Peronosporomycetes leads to the recognition of constant features and variable traits. Variation in characters is accorded different weighting depending on its use for taxonomic identification or phylogenetic systematics. The phylogenetic significance of features of gametangial morphogenesis, such as vacuolation and the patterns of nuclear meioses and abortions, can be related to morphological characters such as the persistence of periplasm and oogonial and oospore wall deposition. The cellular reorganization occurring in the oogonium and oospores, particularly in relation to the possible functions of the dense body vesicle system and the glucan, phosphate, and lipid oospore reserves, may be critical for the internal morphology of the mature oospore and its function. Such differences are valuable for morphometric analysis, which lends itself to new identification procedures. The ecological implications of these differences in the production of oogonia and oospores are considered; sexual reproduction (its frequency or absence) may be crucial for successful life-history strategies in biodiversity. Key words: meiosis, karyogamy, syngamy, morphogenesis, morphometry, wall membranes.

Cultures of the causal fungus of Kikuyu Yellows disease of Pennisetum clandestinum isolated from Australia have been examined. The fungus is described. The oospore has three noteworthy features: it is plerotic even into the oogonial neck; the ooplast appears to develop a condensed core; the relative thicknesses of the oospore wall layers appear to differ from those of other Oomycetes. Oogonial characters and parasitism indicate an affinity with the graminicolous downy mildews. However, asexual sporulation is not immediately reconcilable with that of these mildews. A major re‐appraisal of the classification of the peronosporalean fungi has been necessary to accommodate this fungus. Attention has been drawn to mycelial morphology. Three new orders and two new families are recognized. The division between the peronosporalean fungi and saprolegnialean fungi is recognized at the sub‐class level.

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Summary Problems of intraspecific variability and the delimitation of taxa are discussed in relation to the patterns of inheritance in Oomycetes made possible by the combination of a haplobiontic‐B life cycle with multinucleate gametangia.

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PeronosporalesObligate parasites of dicotyledons (very rarely monocotyledons). Thallus mycelial and intercellular with haustoria; zoosporogenesis, when present, by internal cleavage, otherwise asexual reproduction by deciduous conidiosporangia; conidiosporangiophores well-differentiated, persistent; oogonia thin-walled, oospore single, aplerotic with a well-defined exospore wall layer derived from persistent periplasm. Possibility that all are dependent on exogenous sources of sterols. Peronosporaceae: Myceliar fungi with large, lobate haustoria. Asexual reproduction by deciduous PythialesParasites or saprotrophs; parasites mostly in axenic culture. Some members parasitic on fungi and some on animals. Thallus mycelial with little evidence of cytoplasmic streaming; zoosporangium formation terminal, less frequently sequential, then percurrent or by internal or sympodial proliferation; sporangiophores rarely differentiated; oogonial periplasm minimal and not persistent; oospore usually single, plerotic or aplerotic. Evidence of partial dependence on exogenous sterol precursors. Pythiaceae: Thallus mycelial or monocentric and pseudomycelial; hyphae diameter;conidiosporangia or conidia borne on conidiosporangiophores; conidiosporangia pedicellate; conidiosporangiophores dichotomously branched, monopodially branched, or unbranched and clavate; conidiogenesis simultaneous; zoosporogenesis, when present, internal within a plasmamembranic membrane, zoospore release by operculate or poroid discharge. deciduous sporangia, conidiosporangia or conidia borne on unbranched conidiophores. Conidiogenesis sequential and percurrent. Zoosporogenesis internal with papillate discharge. Albuginaceae:Myceliar fungi with small, spherical or peg-like haustoria. Asexual reproduction by Genus: Albugo.zoosporogenesis either by internal cleavage without vesicular discharge or with a plasmamembranic vesicle or by external cleavage in a homohylic vesicle; oogonia (with very few exceptions) thin-walled; oospores never strictly plerotic. Aerobic metabolism. Freshwater or marine.