Chromista
The heterokonts are a major line of algae. They vary from the multicellular brown algae, such as kelp, to the unicellular diatoms which are a primary component in plankton. The name comes from the distinctive structure of motile cells, which have two unequal flagella, found at some point in the life cycle of most heterokonts, for instance as zoospores or gametes if not as the normal vegetative form.
The two flagella are inserted subapically or laterally, and are generally supported by four microtubule roots in a distinctive pattern. The anterior or tinsel flagellum is covered with lateral bristles or mastigonemes, which create a backwards current when it moves, pulling the cell forward. The other flagellum is smooth, and usually smaller, sometimes reduced to a basal body. There are a number of variations on this basic pattern, however. Mastigonemes have a peculiar tripartite structure, and are produced in the endoplasmic reticulum from glycoproteins before being transported to the cell surface.
The classification of heterokonts varies. Originally the brown algae were placed in a division Phaeophyta and the others, which are unicellular or colonial, in a division Chrysophyta with two classes: the Bacillariophyceae, or diatoms, and the Chrysophyceae, or golden algae. The last group is paraphyletic, however, and as a result various members have been removed to separate classes and divisions. Recent systems sometimes unite all the heterokonts as classes in a single division, called the Heterokontophyta, Chromophyta, or Ochrophyta, as given at right.
Heterokont algae have chloroplasts surrounded by four membranes, the last of which is continuous with the endoplasmic reticulum, and containing chlorophylls a, c, and usually the accessory pigment fucoxanthin, giving them a golden-brown color. Heterokont cells are also found among several colorless groups: the water molds and hypochytrids, superficially similar to fungi, and the flagellate bicosoecids. Some other colorless groups do not produce heterokont cells, but are nevertheless close relatives of those that do. Most have tripartite mastigonemes, and all have mitochondria with tubular cristae and undergo open mitosis.
This extended group is called the stramenopiles by Patterson, and essentially corresponds to the Kingdom Chromista defined by Cavalier-Smith, except this also includes the haptophytes and cryptomonads. The chloroplasts in those groups are closely related to those of heterokont algae, and so they may share a common origin. However, there is little evidence of this from the host cells, and separate origins are suggested by the lack of chloroplasts in basal stramenopiles.
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