Heterokont
| Heterokonts | |
|---|---|
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| Pacific rockweed, Fucus distichus, in Olympic National Park | |
| Scientific classification | |
| Domain: | Eukarya |
| Kingdom: | Chromalveolata |
| Phylum: | Heterokontophyta |
| Typical classes | |
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The heterokonts or stramenopiles are a major line of eukaryotes[1] currently containing more than 100,000 known species, most of them diatoms.[2] 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 infamy and Pythium which causes seed rot and damping off.
The name heterokonts refers to the motile life cycle stage, in which the flagellate cells possess two different shaped flagella.
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Chloroplasts
Heterokont algae are chromists with chloroplasts surrounded by four membranes, which are counted from the outermost to the innermost membrane. The first membrane is continuous with the host's 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 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. This suggests that they diverged before acquisition of chloroplasts within the group. However, fucoxanthin-containing chloroplasts are also found among the haptophytes. These two groups may have a common ancestry, and possibly also a common phylogenetic history with cryptomonads. This may be interpreted as suggesting that the ancestral heterokont was an alga, and all colorless groups arose through loss of the secondary endosymbiont and its chloroplast.
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.
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 hyphochytrids 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.
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 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.
Some sources divide the heterokonts into the autotrophic Ochrophyta and heterotrophic Bigyra and Pseudofungi.[3] However, some modifications to these classifications have been suggested.
Rationale for "stramenopile"
The origin of the name stramenopile is explained by Adl and coauthors:
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)[4] 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.[5]
References
- ↑ "stramenopiles". http://www.uniprot.org/taxonomy/33634. Retrieved 2009-03-08.
- ↑ Hoek, C. van den; D. G. Mann; H. M. Jahns (1995). Algae: An Introduction to Phycology. Cambridge: Cambridge University Press. pp. 104, 124, 134, 166. ISBN 0-521-31687-1.
- ↑ Riisberg I, Orr RJ, Kluge R, et al. (May 2009). "Seven gene phylogeny of heterokonts". Protist 160 (2): 191–204. doi:10.1016/j.protis.2008.11.004. PMID 19213601. http://linkinghub.elsevier.com/retrieve/pii/S1434-4610(08)00075-8.
- ↑ Patterson DJ (October 1999). "The Diversity of Eukaryotes". Am. Nat. 154 (S4): S96–S124. doi:10.1086/303287. PMID 10527921. http://www.journals.uchicago.edu/cgi-bin/resolve?AN991002.
- ↑ Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". J. Eukaryot. Microbiol. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.
External links
- Tree of Life Web Project: Stramenopiles
- Penn State University: Stramenopila (also Rhodophyta, Chlorophyta)
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