Rhizophydium isolate #136


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Appearance of various developmental stages


General Description

Rhizophydium #136 is an un-named representative of the Chytridiales (Chytridiomycetes), which colonized pollen bait in a water culture containing garden soil collected in Penobscot County, Maine. The fungus was isolated by mechanically wiping mature sporangia through agar and allowing cleaned sporangia to discharge zoospores on PmTG (Barr 1986) nutrient agar.

Development on nutrient agar. Isolate #136 had a generation time of 3 days at ambient laboratory conditions. The zoospore encysted and within 24 hrs branched rhizoids formed and the cyst began to enlarge to form a zoosporangium (Fig. 1). By day 2 the rhizoidal system became more complex and branched and the sporangium increased in diameter (Fig. 2). Note the enlarged bases of rhizoids where they attach to the sporangium. Rhizoids taper to tips that are approximately 0.5 µm dia. In 3 days zoosporangia were mature and exhibited dome-shaped discharge papillae (Figs. 3, 4). These papillae deliquesced and zoospores escaped through pores, which had slightly out-turned edges (Fig. 5). Newly emerged zoospores contained a single prominent lipid globule and were variously shaped (Fig. 6), but became more or less spherical when in motion.

Sporangia growing on spruce pollen (Figs. 7, 8) were smaller than those on PmTG agar and generation time on pollen was less than 2 days. Resting spores developed in old cultures on pine pollen and PmTG agar, but their mode of formation has not been determined.


Identity

Rhizophydium is the largest genus in the Chytridiales. Its members are monocentric (having one reproductive center per thallus), eucarpic (having rhizoids), and develop endogenously (the zoospore cyst enlarges to form the zoosporangium). Species of Rhizophydium release zoospores through one or more pores or, in some species, a large part of the sporangial wall deliquesces to release zoospores. Isolates of Rhizophydium that discharge zoospores through many pores have classically been referred to R. sphaerotheca Zopf if they develop on pollen and to R. globosum (Braun) Rabenhorst if they develop on algae (Sparrow 1960).

Barr found 6 isolates from soil that could be assigned to R. sphaerotheca. The isolates, however, had quite different morphologies when studied in pure culture and Barr gave new names to 4 of these isolates (3 are now members of the Spizellomycetales) and assigned pictures of one of them as neotype material for R. sphaerotheca (Barr 1969). The zoospore of the neotype isolate was among those studied with transmission electron microscopy and used to describe the Rhizophydium subtype of zoospore (Barr and Hadland-Hartmann 1978). The developmental morphology and zoospore ultrastructure of #136 is different from the R. sphaerotheca neotype material, different from R. sphaerotheca sensu Booth (Barr and Hadland-Hartmann 1978) and different from other Rhizophydium and Phlyctochytrium species with multiple discharge pores. Consequently, isolate #136 will be described as a new species, and possibly, based on its zoospore ultrastructure, be placed in a new genus.


For comments, or for more information about this strain you may contact the author of this contribution, Dr. Joyce Longcore by e-mail: LONGCORE@MAINE.MAINE.EDU