Observations on the Life Stages of Sphaerothecum destruens n. g., n. sp., a Mesomycetozoean Fish Pathogen Formally Referred to as the Rosette Agent

Authors

  • KRISTEN D. ARKUSH,

    Corresponding author
    1. Bodega Marine Laboratory, University of California, P.O. Box 247, Bodega Bay, California 94923 USA
      Corresponding Author: K. Arkush—Telephone number: 707–875-2062; FAX number: 707–875–2009; E-mail: kdarkush@ucdavis.edu
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  • LEONEL MENDOZA,

    1. Medical Technology Program, Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan 48824–1030 USA
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  • MARK A. ADKISON,

    1. Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California at Davis, Davis, California 95616 USA
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  • RONALD P. HEDRICK

    1. Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California at Davis, Davis, California 95616 USA
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Corresponding Author: K. Arkush—Telephone number: 707–875-2062; FAX number: 707–875–2009; E-mail: kdarkush@ucdavis.edu

Abstract

ABSTRACT. The rosette agent is an obligate intracellular parasite that causes morbidity and mortality in salmonid fish. In laboratory cultures, the spore stage (2–6 uμm diam.) replicates in a salmonid cell line by sequential asexual division, giving rise to daughter cells. If infected cell cultures are transferred to distilled water, the spore stage undergoes internal division to give rise to at least 5 cells each of which develops into a uniflagellated zoospore with a body of approximately 2 μm and a flagellum approximately 10 μm long. Zoosporulation does not occur in cell culture medium alone, artificial seawater, or phosphate-buffered saline. This parasite is currently classified as a member of the Class Mesomycetozoea (formerly Ichthyosporea) based on phylogenetic analyses of the small subunit ribosomal DNA of three different isolates from fish. Given these new morphological observations combined with the available molecular phylogenetic data on other mesomycetozoeans, we propose to classify the rosette agent as Sphaerothecum destruens, n. g., n. sp. This new genus has unique features including (1) intracellular development of spore stages in various organs eliciting a host granulomatous response; and (2) the differentiation of mature spores into multiple, flagellated zoospores. Taken together, these characteristics clearly distinguish it from the closely related genera Dermocystidium and Rhinosporidium.

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