A zoospore is a
motile asexual
spore that uses a
flagellum for
locomotion in aqueous or moist environments.[1] Also called a swarm spore, these spores are created by some
protists,
bacteria, and
fungi to propagate themselves. Certain zoospores are infectious and transmittable,[1] such as Batrachochytrium dendrobatidis, a fungal zoospore that causes high rates of mortality in
amphibians.
Diversity
General morphology
Zoospores are composed of a microtubular cytoskeleton base which extends from the base of the flagellum. The complexity and structure of this cytoskeleton is variable and is largely dependent on volume and size.[2] One common feature of zoospores is their asymmetrical shape; a result of the ventral grove housing the flagella base. Certain zoospores progress through different phases, the first phase commonly referred to as 'the initial'.[2] Others form cysts that vary tremendously in volume (14-4905 cubic micrometers) and shape, each with distinctive hair structures.[2]
Flagella types
Zoospores may possess one or more distinct types of flagella - tinsel or "decorated", and whiplash, in various combinations.
Tinsellated (straminipilous[3]) flagella have lateral
filaments known as
mastigonemes perpendicular to their main axis, which allow for more surface area, and disturbance of the medium, giving them the property of a
rudder, that is, used for steering.
Whiplash flagella are straight, to power the zoospore through its medium. Also, the "default" zoospore only has the propelling, whiplash flagella.
Both tinsel and whiplash flagella beat in a
sinusoidal wave pattern, but when both are present, the tinsel beats in the opposite direction of the whiplash, to give two axes of control of
motility.
Attachment to the base of the zoospore is variable between taxa and may help with identification of species.[2]
Morphological types
In
eukaryotes, the four main types of zoospore are[3] illustrated in Fig. 1 at right:
Posterior whiplash flagella are a characteristic of the
Chytridiomycota, and a proposed uniting trait of the
opisthokonts, a large clade of eukaryotes containing animals and fungi. Most of these have a single posterior flagellum (Fig. 1a), but the
Neocallimastigales have up to 16 (Fig. 1b).
Anisokonts are biflagellated zoospores with two whip types flagella of unequal length (Fig. 1c). These are found in some of the
Myxomycota and
Plasmodiophoromycota.
Zoospores with a single anterior flagellum (Fig. 1d) of the tinsel type are characteristic of
Hyphochytriomycetes.
Heterokont are biflagellated zoospores (Fig. 1e, f) with both whiplash (smooth) and tinsel type (fine outgrowths called mastigonemes) flagella attached anteriorly or laterally. These zoospores are characteristic of the
Oomycota and other
heterokonts.
Zoosporangium
A zoosporangium is the asexual structure (
sporangium) in which the zoospores develop in plants, fungi, or protists (such as the
Oomycota). Developing sporangia of oomycetes go through a process of cleavage in which a
protein kinase, in the case of Phytophthora infestans, induces the sporangial cytoplasm to split and release the various zoospores.[4] Release of the spores can occur either inside of the zoosporangium (intrasporangial zoosporogenesis) or exteriorly (extrasporangial zoosporogenesis).[2] Spores absorb water and travel through the
cell membrane for excretion.[2]
Infectivity
Different fungal zoospores may infect different taxa of organisms. Due to zoospores' aquatic lifestyle,[1] fish and amphibians are ideal hosts. Some colonize exposed injuries in fish which may cause
epidermal damage, leading to death in certain cases.[5] Others may utilize species of frogs (such as Bufo marinus and Rana catesbieana) as carriers, allowing extended ranges of travel.[1]
^
abcdO’Rourke, Dorcas P.; Rosenbaum, Matthew D. (2015),
"Biology and Diseases of Amphibians", Laboratory Animal Medicine, Elsevier, pp. 931–965, retrieved 2023-11-08
^Fox, James G.; Anderson, Lynn C.; Otto, Glen M.; Pritchett-Corning, Kathleen R.; Whary, Mark T. (2015). Laboratory animal medicine. American College of Laboratory Animal Medicine series. American college of laboratory animal medicine (3rd ed.). Amsterdam: Elsevier/Academic Press.
ISBN978-0-12-409527-4.
Alexopoulos, C. J.; Mims, Charles W.; Blackwell, M.; et al. (2004). Introductory Mycology (4th ed.). Hoboken, New Jersey: John Wiley and Sons.
ISBN0-471-52229-5.