Flagellates swim with microtubule whips built nine-plus-two
A flagellate is a cell or organism bearing one or more whip-like flagella, also naming a body plan common among protists. Eukaryotic flagella ride microtubules in a nine fused pairs around two central singlets, rising from a basal body. Sperm of most animal phyla are flagellate cells.
The whip usually drives the cell forward, though in some species it instead sweeps food toward a mouth-like cytostome. In most flagellates one or more flagella sit at or near the front of the cell, as in Euglena, often with one pointing ahead and another trailing. Several parasites that burden human health or farming, including Naegleria, Trichomonas and Plasmodium, carry flagella during at least one stage of life.
The trait turns up across the eukaryote tree. Flowering plants never make flagellate cells, but ferns, mosses, most green algae, cycads and Ginkgo produce swimming male gametes. Most fungi have lost them, yet the primitive chytrids keep swimming zoospores. Brown algae, oomycetes and centric diatoms use them in spores or gametes, foraminiferans in gametes, and in animals they drive the sperm of most phyla and line epithelia and the collar cells of sponges.
Older schemes lumped flagellated protozoa into a class called Flagellata or Mastigophora, split into mostly autotrophic Phytoflagellata and heterotrophic Zooflagellata, and sometimes merged with amoebae as Sarcomastigophora. Colourless forms were sorted into three admittedly artificial groups by where they absorbed food, one of which, the binucleate Distomatineae with paired mouths, roughly matches today’s diplomonads.
Those groupings proved highly polyphyletic. Modern classifications scatter flagellated taxa among Archaeplastida, stramenopiles, Cercozoa, Discoba, Metamonada and others, from trypanosomes and euglenids to parabasalids, with about a hundred genera still unplaced. The word survives as a level of organisation, sometimes called monadoid, and amoeboflagellates such as Cercomonas can show flagella and pseudopods together or in turn.
Source: Flagellate
