Zoospore

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. One common feature of zoospores is their asymmetrical shape; a result of the ventral groove housing the flagella base. Certain zoospores progress through different phases, the first phase commonly referred to as 'the initial'. Others form cysts that vary tremendously in volume (14–4905 cubic micrometers) and shape, each with distinctive hair structures.

Flagella types

Zoospores may possess one or more distinct types of flagella – tinsel or "decorated", and whiplash, in various combinations.

• Tinsellated (straminipilous) 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.

Morphological types

In eukaryotes, the four main types of zoospore are illustrated in Fig. 1 at right:

1. 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).
2. Anisokonts are biflagellated zoospores with two whip types flagella of unequal length (Fig. 1c). These are found in some of the Myxomycota and Plasmodiophoromycota.
3. Zoospores with a single anterior flagellum (Fig. 1d) of the tinsel type are characteristic of Hyphochytriomycetes.
4. 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.