Periplasma , gen. nov., a new oomycete lineage with isogamous sexual reproduction.

This paper describes the taxonomy, developmental morphology, and phylogeny ofPeriplasma isogametum, a new monotypic member of the Leptomitales (Oomycota). In phylogenetic trees inferred from concatenated and separate nuc 18S rDNA (18S) and nuc 28S rDNA (28S) sequences,P. isogametumforms a well-supported clade related to but distinct fromApodachlya, a member of the Leptomitales. The organism is a holocarpic facultative saprotroph of moribund aquatic insects but grows well on a variety of mycological media in which it produces large eucarpic hyphae with a peripheral layer of protoplasm surrounding a central vacuole. In zoosporogenesis, a peripheral network of zoospore initials collapses to the center of the zoosporangium and is partitioned into individual heterokont zoospores. Sexual reproduction involves morphological isogamy in which male gametes produce elongate fertilization tubes, which fuse with female gametes, which are subsequently converted into thick-walled oospores. Developmental morphology is detailed in photomicrographs and pointillism drawings.