Zoospores are spores that are self-propelled by means of flagella. Propulsion is often coupled with chemotactic movement, zoospores having the ability to sense chemicals diffusing from suitable substrata and to move towards them, or gametes detecting and following extremely low concentrations of hormones.
In some cases, oxygen or light are also stimuli for tactic movement. The fungal groups which possess flagella are mostly aquatic or, if terrestrial, rely on water for dispersal or infection.
zoospores are of four kinds.
1) Posteriorly flagellate zoospores
Posteriorly flagellate zoospores with flagella of the whiplash-type are characteristic of the Chytridiomycota. Each whiplash flagellum has 11 microtubules arranged in the 9 + 2 pattern typical of eukaryotes.
The microtubules are enclosed in a smooth, membranous axoneme sheath continuous with the plasma membrane. In most members of the Chytridiomycota, there is a single posterior flagellum, but in the rumen-inhabiting Neocallimastigomycota, there may be up to 16 flagella.
Such spores are driven forward by the sinusoidal rhythmic beating of the flagellum. This type of zoospore flagellation is termed Opisthokont (Gr. Opisthen = behind, at the back; kontos= a pole).
2. Biflagellate zoospores
Biflagellate zoospores with two whiplash flagella of unequal length are called anisokont and are found in some Myxomycota and the Plasmodiophoromycota, both now classified among the Protozoa.
3. Anteriorly uniflagellate zoospores
Anteriorly uniflagellate zoospores with a flagellum of the tinsel type are characteristic of the Hyphochytriomycota. The axoneme sheath of the tinsel or straminipilous flagellum (Lat. stramen = straw; pilus = hair) is adorned by two rows of fine hairs.
These are called tripartite tubular hairs or mastigonemes (Gr. mastigion = a small whip; nema = a thread). Rhythmic sinusoidal beating of the tinsel type flagellum pulls the zoospore along, in contrast to the pushing action of whiplash flagellum.
4. Biflagellate zoospores
Biflagellate zoospores with anteriorly or laterally attached flagella, one of which is of the whiplash type and the other of the tinsel type, are characteristic of the Oomycota.
Zoospores with the two different kinds of flagellum are heterokont. Where the two types of flagellum are attached anteriorly, as in the first-released zoospores of Saprolegnia, their propulsive actions tend to work against each other and the zoospore is a very poor swimmer.
However, the secondary zoospore (termed the principal zoospore) in Saprolegnia and in many other Oomycota has laterally attached flagella, with the tinsel-type (pulling action) flagellum pointing forwards and the whiplash-type (pushing action) flagellum directed backwards and possibly acting as a rudder, jointly providing much more effective propulsion.