Light and electron microscopy of oospore maturation in Saprolegnia furcata

The development, ultrastructural cytology, and molecular phylogeny of the basal oomycete Eurychasma dicksonii, infecting the filamentous phaeophyte algae Ectocarpus siliculosus and Pylaiella littoralis

The morphological development, ultrastructural cytology, and molecular phylogeny of Eurychasma dicksonii, a holocarpic oomycete endoparasite of phaeophyte algae, were investigated in laboratory cultures. Infection of the host algae by E. dicksonii is initiated by an adhesorium-like infection apparatus. First non-walled, the parasite cell developed a cell wall and numerous large vacuoles once it had almost completely filled the infected host cell (foamy stage). Large-scale cytoplasmic changes led to the differentiation of a sporangium with peripheral primary cysts. Secondary zoospores appeared to be liberated from the primary cysts in the internal space left after the peripheral spores differentiated. These zoospores contained two phases of peripheral vesicles, most likely homologous to the dorsal encystment vesicles and K-bodies observed in other oomycetes. Following zoospore liberation the walls of the empty cyst were left behind, forming the so-called net sporangium, a distinctive morphological feature of this genus. The morphological and ultrastructural features of Eurychasma were discussed in relation to similarities with other oomycetes. Both SSU rRNA and COII trees pointed to a basal position of Eurychasma among the Oomycetes. The cox2 sequences also revealed that the UGA codon encoded tryptophan, constituting the first report of stop codon reassignment in an oomycete mitochondrion.

Sexual reproduction in thePeronosporomycetes(chromistan fungi)

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.