Heterokont
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The heterokonts or stramenopiles are a major line of eukaryotes containing about 10,500 species.[1] Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton. Other notable members of the Stramenopila include the (generally parasitic) oomycetes, including Phytophthora of Irish potato famine fame and Pythium which causes seed rot and damping off.
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[edit] Chloroplasts
Heterokont algae have chloroplasts surrounded by four membranes, first it is surrounded by two membranes of the chloroplast envelope, then two membranes of chloroplast endoplasmic reticulum, the final one of these is continuous with the endoplasmic reticulum that is continuous with the nuclear envelope. This arrangement of membranes suggest that heterokont chloroplasts were derived from a symbiotic eukaryote, presumably a red alga. The chloroplasts characteristically contain chlorophyll a and c, and usually the accessory pigment fucoxanthin, giving them a golden-brown or brownish-green color.
Most basal heterokonts are colorless, suggesting they branched off before the appearance of chloroplasts within the group. However, fucoxanthin-containing chloroplasts are also found among the haptophytes, and there is some evidence the two groups share a common origin, and possibly the cryptomonads as well. In that case the ancestral heterokont was an alga, and all colorless groups arose through chloroplast loss.
[edit] Motile cells
Many heterokonts are unicellular flagellates, and most others produce flagellate cells at some point in their life-cycle, for instance as gametes or zoospores. The name heterokont refers to the characteristic form of these cells, which typically have two unequal flagella. The anterior or tinsel flagellum is covered with lateral bristles or mastigonemes, while the other flagellum is whiplash, smooth and usually shorter, or sometimes reduced to a basal body. The flagella are inserted subapically or laterally, and are usually supported by four microtubule roots in a distinctive pattern.
Mastigonemes are manufactured from glycoproteins in the cell's endoplasmic reticulum before being transported to its surface. When the tinsel flagellum moves, these create a backwards current, pulling the cell through the water or bringing in food. The mastigonemes have a peculiar tripartite structure, which may be taken as the defining characteristic of the group, thereby including a few protists that do not produce cells with the typical heterokont form. They have been lost in a few lines, most notably the diatoms.
[edit] Classification
As noted above, classification varies considerably. Originally the heterokont algae were treated as two divisions, first within the kingdom Plantae and later the Protista:
Division Chrysophyta
Class Chrysophyceae (golden algae)
Class Bacillariophyceae (diatoms)
Division Phaeophyta (brown algae)
In this scheme, however, the Chrysophyceae are paraphyletic to both other groups. As a result, various members have been given their own classes and often divisions. Recent systems often treat these as classes within a single division, called the Heterokontophyta, Chromophyta or Ochrophyta. This is not universal, however - for instance Round et al. treat the diatoms as a division.
The discovery that oomycetes and hypochytrids are related to these algae, rather than fungi as previously thought, has led many authors to include them among the heterokonts. Should it turn out that they evolved from colored ancestors, the group would be paraphyletic in their absence. Once again, however, usage varies. David J. Patterson named this extended group the stramenopiles, characterized by the presence of tripartite mastigonemes, mitochondria with tubular cristae, and open mitosis. He used the stramenopiles as a prototype for a classification without Linnaean ranks. Their composition has been essentially stable, but their use within ranked systems varies.
The origin of the name stramenopile is explicited in the following quote from Adl et al. (2005):[1]
"Regarding the spelling of stramenopile, it was originally spelled stramenopile. The Latin word for ‘‘straw’’ is stramine-us, -a, -um, adj. [stramen], made of straw—thus, it should have been spelled straminopile. However, Patterson (1989)[2] clearly stated that this is a common name (hence, lower case, not capitalized) and as a common name, it can be spelled as Patterson chooses. If he had stipulated that the name was a formal name, governed by rules of nomenclature, then his spelling would have been an orthogonal mutation and one would simply correct the spelling in subsequent publications (e.g. Straminopiles). But, it was not Patterson’s desire to use the term in a formal sense. Thus, if we use it in a formal sense, it must be formally described (and in addition, in Latin, if it is to be used botanically). However, and here is the strange part of this, many people liked the name, but wanted it to be used formally. So they capitalized the first letter, and made it Stramenopiles; others corrected the Latin spelling to Straminopiles."
Thomas Cavalier-Smith treats the heterokonts as identical in composition with the stramenopiles; this is the definition followed here. He has proposed placing them in a separate kingdom Chromista, together with the haptophytes and cryptomonads. This is one of the most common revisions to the five-kingdom system, but has not been generally adopted, partly because some biologists doubt their monophyly. A few treat the Chromista as identical in composition with the heterokonts, or list them as a kingdom Stramenopila.
[edit] References
- ^ Adl, S. M. et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology 52: 399-451.
- ^ Patterson, D. J. (1999). The diversity of eukaryotes. American Naturalist 154: S96-S124
[edit] Further reading
- Fletcher, R.L.1987. Seaweeds of the British Isles. Volume 3 Fucophyceae (Phaeophyceae) Part 1. British Museum (Natural History, London. ISBN 0 565 00992 3
