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Five distinct types of zoospore have been revealed through ultrastructural analysis of multiple isolates in the Rhizophydium clade.
The schematically represented Rhizophydium-type zoospore (Barr and Hadland-Hartmann 1978; Barr 1980) has a single microtubular root extending from the kinetosome to a fenestrated cisterna (=rumposome) that partially covers a single lipid globule. The root is composed of 2-5 (typically 4-5) stacked microtubules. Random longitudinal sections through additional isolates have revealed multiple microtubular roots. Number and configuration of multiple roots may be important as taxonomic markers at the generic or species level. Barr and Hadland-Hartmann (1978) examined the zoospores of 12 isolates of Rhizophydium and schematically represented the Rhizophydium-type zoospore (Barr 1980). As multiple character states could not be represented in a single schematic, certain ultrastructural features were excluded. Examination of the zoospores of numerous isolates in the Rhizophydium clade (Letcher, Powell, et al 2004) revealed distinct zoospore types. Many isolates have an electron-opaque kinetosome-associated structure (KAS), referred to as a spur, on the side of the kinetosome distal to the nfc. Preliminary ultrastructural analyses indicate that spur morphology may be variable, representing multiple character states, although serial sectioning is necessary to reveal precise configuration of the structure. Among revealed zoospore types, when present the spur protrudes into a vesiculated region adjacent to the ribosomal core. The vesiculated region is not present in the zoospore type that lacks a spur.
Fibrillar Bridge Morphological Variation In the transitional zone of the flagellar axoneme, the kinetosome (K) and non-flagellated centriole (nfc) are bridged and connected by fibrillar material. Character states of the precise configuration of the fibrillar bridge may be useful as taxonomic markers. Although serial sectioning is necessary to elucidate precise configuration, preliminary studies of zoospores of members of the Rhizophydium clade indicate significant morphological variation in fibrillar bridge construction. For example, members of the genus Kappamyces have a single, diagonal fibrillar bridge that distinguishes the genus from other isolates in the clade.
Rhizophydium is characterized as monocentric and inoperculate (Sparrow 1960), with the mature sporangium having one to several discharge papillae (pores). At the species level, the character state of number of discharge papillae appears to be either: 1) a single pore or papilla, or 2) a variable number of papillae, from one to many. In the second character state, number of papillae may be related to sporangial size, with more papillae occurring with larger sporangia. Morphology of discharge papillae has not been examined in detail, but preliminary studies reveal a complexity in wall material and construction. The discharge papilla in isolate JEL 348 (Rhizophydium clade) may be considered endoperculate, in which wall-like material is contained within the discharge papilla. Ultrastructural analysis of discharge papillae may reveal character states that facilitate generic or species delineation.
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