kimerajamm
Joined: 28 Nov 2010
Posts: 785
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 Posted: Sun Jan 23, 2011 2:18 pm Post subject: Varying forms |
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Life cycle
Bd has two primary life stages - a sessile, reproductive zoosporangium and a motile, uniflagellated zoospore released from the zoosporangium. The zoospores are known be active only for a short period of time, and can travel short distances of one to two centimeters.[5] However, the zoospores are capable of chemotaxis, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, proteins and amino acids.[6] Bd also contains a variety of proteolytic enzymes and esterases that help it digest amphibian cells and use amphibian skin as a nutrient source.[7] Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. The encysted zoospores develop into zoosporangia, which may produce more zoospores that can reinfect the host, or be released into the surrounding aquatic environment.[8] The amphibians infected with these zoospores are shown to die from cardiac arrest.
[edit]Varying forms
Bd has occasionally been found in forms distinct from its traditional zoospore and sporangia stages. For example, before the 2003 European heatwave that decimated populations of the water frog Rana lessonae through chytridiomycosis, the fungus existed on the amphibians as spherical, unicellular organisms, confined to minute patches (80-120 micrometers across). These organisms, unknown at the time, were subsequently identified as Bd. Characteristics of the organisms were suggestive of encysted zoospores; they may have embodied a resting spore, a saprobe, or a parasitic form of the fungus that is conditionally non-pathogenic [9]. Once the heatwave began, the organisms assumedly changed into the more familiar disease-causing zoospores. This suggests that some populations afflicted with Bd may be free of chytridiomycosis not because of some inherent immunity, but because environmental conditions have not altered the organism into its more common, pathogenic form.
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