|Ochromonas sp. (Chrysophyceae), with two unequal (heterokont) flagella. Mastigonemes not represented.|
Colored groups (alga-like)
The heterokonts or stramenopiles (formally, Heterokonta or Stramenopiles) are a major line of eukaryotes currently containing more than 25,000 known species. 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 Stramenopiles include the (generally) parasitic oomycetes, including Phytophthora of Irish potato famine infamy and Pythium which causes seed rot and damping off.
In 1899, Luther created "Heterokontae" for some algae with unequal flagella, today called Xanthophyceae. Later, some authors (e.g., Copeland, 1956) would include other groups in Heterokonta, expanding its sense.
The origin of the other name of the group, "stramenopile", is explained by David (2002) and Adl et al. (2005):
Many heterokonts are unicellular flagellates, and most others produce flagellated cells at some point in their lifecycles, 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 straminipilous flagellum is covered with one or two rows of lateral bristles or mastigonemes, which are tripartite (with three regions each), while the posterior flagellum is whiplike, 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 the anterior flagella's surface. When the straminipilous flagellum moves, the mastigonemes create a retrograde 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 heterokonts, thereby including a few protists that do not produce cells with the typical heterokont form. Mastigonemes have been lost in a few heterkont lines, most notably the diatoms.
Many heterokonts are algae 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 cER and the primary endosymbiont or chloroplast, which represents the next two membranes, within which the thylakoid membranes are found. This arrangement of membranes suggests 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 the 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, being grouped by some authors in the Chromista. 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.
As noted above, classification varies considerably. Originally, the heterokont algae were treated as two divisions, first within the kingdom Plantae and later the Protista:
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; 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 these two groups among the heterokonts. Should it turn out that they evolved from colored ancestors, the heterokont 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 rank. 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, the 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 adopted, because Chromalveolata is not a monophyletic group. A few treat the Chromalveolata as identical in composition with the heterokonts, or list them as a kingdom Stramenopila.
The name Heterokonta can be confused with the (much older) name Heterokontae, which is generally equivalent to the Xanthophyceae, a limited subset of the Heterokonta.
The simplified classification of the group according to Adl et al. (2012) is:
Based on the following works of Ruggiero et al. 2015 & Silar 2016.
- ^ Cavalier-Smith, T. (1999). "The kingdom Chromista, origin and systematics". In Round, F.E.; Chapman, D.J. Progress in Phycological Research. 4. Elsevier. pp. 309–347. ISBN 978-0-948737-00-8.
- ^ Patterson, D.J. (1989). "Stramenopiles: Chromophytes from a protistan perspective". In Green, J.C.; Leadbeater, B.S.C.; Diver, W.L. The chromophyte algae: Problems and perspectives. Clarendon Press. ISBN 0198577133.
- ^ Vørs, N (1993). "Marine heterotrophic amoebae, flagellates and heliozoa from Belize (Central America) and Tenerife". Journal of Eukaryotic Microbiology. 40: 272–287. doi:10.1111/j.1550-7408.1993.tb04917.x.
- ^ David, J.C. (2002). "A preliminary catalogue of the names of fungi above the rank of order". Constancea. 83: 1–30.
- ^ van den Hoek, C.; Mann, D.G.; Jahns, H.M. (1995). Algae An Introduction to Phycology. Cambridge University Press. ISBN 0-521-30419-9.
- ^ Alexopoulos, C.J.; Mims, C.W.; Blackwell, M. (1996). Introductory Mycology (4th ed.). Wiley. ISBN 0471522295.
- ^ Dick, M.W. (2013). Straminipilous Fungi: Systematics of the Peronosporomycetes Including Accounts of the Marine Straminipilous Protists, the Plasmodiophorids and Similar Organisms. Springer. ISBN 978-94-015-9733-3.
- ^ "Stramenipila M.W. Dick (2001)". MycoBank. International Mycological Association.
- ^ "stramenopiles". 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.
- ^ Chapman, A. D. (2009). Numbers of living species in Australia and the world.
- ^ 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.
- ^ 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.
- ^ Blackwell, W. H. (2009). "Chromista revisited: A dilemma of overlapping putative kingdoms, and the attempted application of the botanical code of nomenclature" (PDF). Phytologia. 91 (2).
- ^ "The Revised Classification of Eukaryotes". Journal of Eukaryotic Microbiology. 59: 429–514. doi:10.1111/j.1550-7408.2012.00644.x.
- ^ Ruggiero; et al. (2015). "Higher Level Classification of All Living Organisms". PLoS ONE. 10 (4): e0119248. doi:10.1371/journal.pone.0119248. PMC 4418965 . PMID 25923521.
- ^ Silar, Philippe (2016). "Protistes Eucaryotes: Origine, Evolution et Biologie des Microbes Eucaryotes". HAL archives-ouvertes: 1–462.
- ^ Cavalier-Smith, Thomas; Scoble, Josephine Margaret (2013). "Phylogeny of Heterokonta: Incisomonas marina, a uniciliate gliding opalozoan related to Solenicola (Nanomonadea), and evidence that Actinophryida evolved from raphidophytes". European Journal of Protistology. 49: 328–353. doi:10.1016/j.ejop.2012.09.002.
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