Heterokont
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224692543
2008-07-10T00:13:30Z
Kupirijo
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/* Classification */ hyphochytrid
{{Taxobox
| name = Heterokonts
| image = pacific rockweed, Olympic National Park, USA.jpg
| image_caption = [[Pacific rockweed]], ''[[Fucus distichus]]'', in [[Olympic National Park]]
| domain = [[Eukaryota]]
| regnum = [[Chromalveolata]]
| phylum = '''Heterokontophyta'''
| subdivision_ranks = Typical classes
| subdivision =
*'''Colored groups''' (alga like)
**[[Axodine|Actinochrysophyceae]] (axodines)
**[[Diatom|Bacillariophyceae]] (diatoms)
**[[Bolidophyte|Bolidophyceae]]
**[[Golden alga|Chrysophyceae]] (golden algae)
**[[Eustigmatophyte|Eustigmatophyceae]]
**[[Pelagophyte|Pelagophyceae]]
**[[Brown alga|Phaeophyceae]] (brown algae)
**[[Phaeothamnid|Phaeothamniophyceae]]
**[[Raphidophyte|Raphidophyceae]]
**[[Synurid|Synurophyceae]]
**[[Yellow-green alga|Xanthophyceae]] (yellow-green algae)
*'''Colorless groups''' (fungus like)
**[[Bicosoecid|Bicosoecea]]
**[[Hyphochytridiomycetes]]
**[[Labyrinthulomycetes]] (slime nets)
**[[Water mould|Oomycetes]] (water moulds)
**[[Opaline]]a
**[[Proteromonad]]ea
}}
The '''heterokonts''' or '''stramenopiles''' are a major line of [[eukaryote]]s presently containing about 10,500 known species.[http://encarta.msn.com/media_461543986/Major_Algae_Phyla.html] Most are [[alga]]e, ranging from the giant multicellular [[kelp]] to the unicellular [[diatom]]s, which are a primary component of [[plankton]]. Other notable members of the Stramenopila include the (generally [[parasitic]]) [[oomycete]]s, including ''[[Phytophthora]]'' of Irish potato famine infamy and ''[[Pythium]]'' which causes seed rot and damping off.
== Chloroplasts ==
Heterokont algae are [[chromista|chromists]] with [[chloroplast|chloroplasts]] surrounded by four membranes, which are counted from the outermost to the innermost membrane. The first membrane is continuous with the host's [[Endoplasmic reticulum|chloroplast endoplasmic reticulum]], or cER. The second membrane presents a barrier between the lumen of the endoplasmic reticulum and the primary endosymbiont or [[chloroplast]], which represents the next two membranes, within which the thylakoid membranes are found. This arrangement of membranes suggest that heterokont chloroplasts were obtained from the reduction of a symbiotic [[red algae|red algal]] eukaryote, which had arisen by evolutionary divergence from the monophyletic primary endosymbiotic ancestor that is thought to have given rise to all eukaryotic [[photoautotrophs]]. The chloroplasts characteristically contain [[chlorophyll a]] and [[chlorophyll c]], and usually the accessory pigment [[fucoxanthin]], giving them a golden-brown or brownish-green color.
Most basal heterokonts are colorless, suggesting they diverged before aqcuisition of chloroplasts within the group. However, fucoxanthin-containing chloroplasts are also found among the [[haptophyte]]s, and evidence suggests that the two groups have a common ancestry, as well as possible a common [[phylogeny|phylogenetic]] history with [[cryptomonad]]s. In this case the ancestral heterokont was an alga, and all colorless groups arose through loss of the secondary endosymbiont and hence its chloroplast.
== Motile cells ==
Many heterokonts are unicellular [[flagellate]]s, and most others produce flagellate cells at some point in their life-cycle, for instance as [[gamete]]s or [[zoospore]]s. 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 [[glycoprotein]]s 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.
== 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<br>
Class Chrysophyceae ([[golden alga]]e)<br>
Class Bacillariophyceae ([[diatom]]s)<br>
Division Phaeophyta ([[brown alga]]e)
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 [[oomycete]]s and [[hyphochytrid]]s 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, [[mitochondrion|mitochondria]] with tubular [[crista]]e, 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.
[[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 [[Chromalveolata]], together with the haptophytes, cryptomonads and alveolates. This is one of the most common revisions to the [[kingdom (biology)|five-kingdom system]], but has not been generally adopted, partly because some biologists doubt their monophyly. A few treat the Chromalveolata as identical in composition with the heterokonts, or list them as a kingdom Stramenopila.
===Rationale for "stramenopile"===
The origin of the name stramenopile is explained by Adl and coauthors:
<blockquote>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)<ref>'''Patterson, D. J.''' (1999). The diversity of eukaryotes. ''American Naturalist'' '''154''': S96-S124</ref> 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.<ref>'''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.</ref></blockquote>
== References ==
<references/>
==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
==External links==
* [http://tolweb.org/tree?group=Stramenopiles&contgroup=Eukaryotes Tree of Life Web Project: Stramenopiles]
* [http://courses.bio.psu.edu/fall2005/biol110/tutorials/tutorial30.htm Penn State University: Stramenopila (also Rhodophyta, Chlorophyta)]
[[Category:Chromista]]
[[Category:Chromalveolata]]
[[Category:Algae]]
[[Category:Heterokonts|*]]
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[[et:Heterokondid]]
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[[sl:Rumenkaste alge]]
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[[zh:不等鞭毛類]]