Stramenopiles Temporal range: Late Mesoproterozoic-present, | |
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Diversity of stramenopiles | |
Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
Clade: | SAR |
Clade: | Stramenopiles Patterson 1989 [2] emend. Adl et al. 2005 [3] |
Phyla and subphyla [4] | |
Diversity | |
>100000 species [1] | |
Synonyms | |
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The stramenopiles, also called heterokonts, are a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have been secondarily lost (in which case relatedness to stramenopile ancestors is evident from other shared cytological features or from genetic similarity). Stramenopiles represent one of the three major clades in the SAR supergroup, along with Alveolata and Rhizaria.
Stramenopiles are eukaryotes; most are single-celled, but some are multicellular including some large seaweeds, the brown algae. The group includes a variety of algal protists, heterotrophic flagellates, opalines and closely related proteromonad flagellates (all endobionts in other organisms); the actinophryid Heliozoa, and oomycetes. The tripartite hairs characteristic of the group have been lost in some of the included taxa – for example in most diatoms.
Many stramenopiles are unicellular flagellates, and most others produce flagellated cells at some point in their lifecycles, for instance as gametes or zoospores. Most flagellated heterokonts have two flagella; the anterior flagellum has one or two rows of stiff hairs or mastigonemes, and the posterior flagellum is without such embellishments, being smooth, usually shorter, or in a few cases not projecting from the cell.
The term 'heterokont' is used both as an adjective – indicating that a cell has two dissimilar flagella, and as the name of a taxon. The groups included in that taxon have however varied widely, creating the 'heterokont problem', now resolved by the definition of the stramenopiles.
The term 'stramenopile' was introduced by D. J. Patterson in 1989, defining a group that overlapped with the ambiguously defined heterokonts. [12] [13] The name "stramenopile" has been discussed by J. C. David. [14]
The term 'heterokont' is used as both an adjective – indicating that a cell has two dissimilar flagella – and as the name of a taxon. The taxon 'Heterokontae' was introduced in 1899 by Alexander Luther for algae that are now considered the Xanthophyceae. [15] But the same term was used for other groupings of algae. For example, in 1956, Copeland [16] used it to include the xanthophytes (using the name Vaucheriacea), a group that included what became known as the chrysophytes, the silicoflagellates, and the hyphochytrids. Copeland also included the unrelated collar flagellates (as the choanoflagellates) in which he placed the bicosoecids. He also included the not-closely related haptophytes. The consequence of associating multiple concepts to the taxon 'heterokont' is that the meaning of 'heterokont' can only be made clear by making reference to its usage: Heterokontae sensu Luther 1899; Heterokontae sensu Copeland 1956, etc. This contextual clarification is rare, such that when the taxon name is used, it is unclear how it should be understood. The term 'heterokont' has lost its usefulness in critical discussions about the identity, nature, character and relatedness of the group. [17] The term 'stramenopile' sought to identify a clade (monophyletic and holophyletic lineage) using the approach developed by transformed cladists of pointing to a defining innovative characteristic or apomorphy. [18]
Over time, the scope of application has changed, especially when in the 1970s ultrastructural studies revealed greater diversity among the algae with chromoplasts (chlorophylls a and c) than had previously been recognized. At the same time, a protistological perspective was replacing the 19th century one based on the division of unicellular eukaryotes into animals and plants. One consequence was that an array of heterotrophic organisms, many not previously considered as 'heterokonts', were seen as related to the 'core heterokonts' (those having anterior flagella with stiff hairs). Newly recognized relatives included the parasitic opalines, proteromonads, and actinophryid Heliozoa. They joined other heterotrophic protists, such as bicosoecids, labyrinthulids, and oomycete fungi, that were included by some as heterokonts and excluded by others. Rather than continue to use a name whose meaning had changed over time and was hence ambiguous, the name 'stramenopile' was introduced to refer to the clade of protists that had tripartite stiff (usually flagellar) hairs and all their descendants. Molecular studies confirm that the genes that code for the proteins of these hairs are exclusive to stramenopiles. [19]
The presumed apomorphy of tripartite flagellar hairs in stramenopiles is well characterized. The basal part of the hair is flexible and inserts into the cell membrane; the second part is dominated by a long stiff tube (the 'straw' or 'stramen'); and finally the tube is tipped by many delicate hairs called mastigonemes. [20] The proteins that code for the mastigonemes appear to be exclusive to the stramenopile clade, and are present even in taxa (such as diatoms) that no longer have such hairs. [21]
Most stramenopiles have two flagella near the apex. [22] They are usually supported by four microtubule roots in a distinctive pattern. There is a transitional helix inside the flagellum where the beating axoneme with its distinctive geometric pattern of nine peripheral couplets around two central microtubules changes into the nine-triplet structure of the basal body. [23]
Many stramenopiles have plastids which enable them to photosynthesise, using light to make their own food. Those plastids are coloured off-green, orange, golden or brown because of the presence of chlorophyll a, chlorophyll c, and fucoxanthin. This form of plastid is called a stramenochrome or chromoplast. [a] The most significant autotrophic stramenopiles are the brown algae (wracks and many other seaweeds), and the diatoms. The latter are among the most significant primary producers in marine and freshwater ecosystems. [24] Most molecular analyses suggest that the most basal stramenopiles lacked plastids and were accordingly colourless heterotrophs, feeding on other organisms. This implies that the stramenopiles arose as heterotrophs, diversified, and then some of them acquired chromoplasts. Some lineages (such as the axodine lineage that included the chromophytic pedinellids, colourless ciliophryids, and colourless actinophryid heliozoa) have secondarily reverted to heterotrophy. [25] [26]
Some stramenopiles are significant as autotrophs and as heterotrophs in natural ecosystems; others are parasitic. Blastocystis is a gastrointestinal parasite of humans; [27] opalines and proteromonads live in the intestines of cold-blooded vertebrates and have been described as parasitic; [28] oomycetes include some significant plant pathogens such as the cause of potato blight, Phytophthora infestans . [29] Diatoms are major contributors to global carbon cycles because they are the most important autotrophs in most marine habitats. [30] The brown algae, including familiar seaweeds like wrack and kelp, are major autotrophs of the intertidal and subtidal marine habitats. [31] Some of the bacterivorous stramenopiles, such as Cafeteria, are common and widespread consumers of bacteria, and thus play a major role in recycling carbon and nutrients within microbial food webs. [32] [33]
Stramenopiles are most closely related to alveolates and Rhizaria, all of which have tubular mitochondrial cristae and collectively form the SAR supergroup, whose name is formed from their initials. [34] [26] [35] The ancestor of the SAR supergroup appears to have captured a unicellular photosynthetic red alga, and many stramenopiles, as well as members of other SAR groups such as the Rhizaria, still have plastids which retain the double membrane of the red alga and a double membrane surrounding it, for a total of four membranes. [36] In addition, species of Telonemia, the sister group to SAR, exhibit heterokont flagella with tripartite mastigonemes, implying a more ancient origin of stramenopile characteristics. [37]
The following cladogram summarizes the evolutionary relationships between stramenopiles. The phylogenetic relationships of Bigyra vary greatly from one analysis to the next: it has been recovered as either monophyletic [38] [39] or paraphyletic. When paraphyletic, the branching order of the bigyran groups also varies: in some studies, Sagenista is the most basal-branching clade, [38] [40] [41] while in others Opalozoa is the most basal. [42] Nonetheless, Platysulcea is consistently recovered as the sister clade to all other stramenopiles. [39] [40] In addition, a flagellate species discovered in 2023, Kaonashia insperata , remains in an uncertain phylogenetic position, but more closely related to Gyrista than to other clades. [41]
Stramenopiles | ||
The classification of the stramenopiles according to Adl et al. (2019), with additions from newer research: [43] [4]
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word flagellate also describes a particular construction characteristic of many prokaryotes and eukaryotes and their means of motion. The term presently does not imply any specific relationship or classification of the organisms that possess flagella. However, several derivations of the term "flagellate" are more formally characterized.
The alveolates are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondria into the SAR supergroup.
The cryptomonads are a group of algae, most of which have plastids. They are traditionally considered a division of algae among phycologists, under the name of Cryptophyta. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, with an anterior groove or pocket. At the edge of the pocket there are typically two slightly unequal flagella. Some may exhibit mixotrophy. They are classified as clade Cryptomonada, which is divided into two classes: heterotrophic Goniomonadea and phototrophic Cryptophyceae. The two groups are united under three shared morphological characteristics: presence of a periplast, ejectisomes with secondary scroll, and mitochondrial cristae with flat tubules. Genetic studies as early as 1994 also supported the hypothesis that Goniomonas was sister to Cryptophyceae. A study in 2018 found strong evidence that the common ancestor of Cryptomonada was an autotrophic protist.
The axodines are a group of unicellular stramenopiles that includes silicoflagellate and rhizochromulinid algae, actinomonad heterotrophic flagellates and actinophryid heliozoa. Alternative classifications treat the dictyochophytes as heterokont algae, or as Chrysophyceae. Other overlapping taxonomic concepts include the Actinochrysophyceae, Actinochrysea or Dictyochophyceae sensu lato. The grouping was proposed on the basis of ultrastructural similarities, and is consistent with subsequent molecular comparisons.
Chromista is a proposed but polyphyletic biological kingdom, refined from the Chromalveolata, consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all eukaryotes whose plastids contain chlorophyll c and are surrounded by four membranes. If the ancestor already possessed chloroplasts derived by endosymbiosis from red algae, all non-photosynthetic Chromista have secondarily lost the ability to photosynthesise. Its members might have arisen independently as separate evolutionary groups from the last eukaryotic common ancestor.
Labyrinthulomycetes (ICBN) or Labyrinthulea (ICZN) is a class of protists that produce a network of filaments or tubes, which serve as tracks for the cells to glide along and absorb nutrients for them. The two main groups are the labyrinthulids and thraustochytrids. They are mostly marine, commonly found as parasites on algae and seagrasses or as decomposers on dead plant material. They also include some parasites of marine invertebrates and mixotrophic species that live in a symbiotic relationship with zoochlorella.
Telonemia is a phylum of microscopic eukaryotes commonly known as telonemids. They are unicellular free-living flagellates with a unique combination of cell structures, including a highly complex cytoskeleton unseen in other eukaryotes.
Ochrophytes, also known as heterokontophytes or stramenochromes, are a group of algae. They are the photosynthetic stramenopiles, a group of eukaryotes, organisms with a cell nucleus, characterized by the presence of two unequal flagella, one of which has tripartite hairs called mastigonemes. In particular, they are characterized by photosynthetic organelles or plastids enclosed by four membranes, with membrane-bound compartments called thylakoids organized in piles of three, chlorophyll a and c as their photosynthetic pigments, and additional pigments such as β-carotene and xanthophylls. Ochrophytes are one of the most diverse lineages of eukaryotes, containing ecologically important algae such as brown algae and diatoms. They are classified either as phylum Ochrophyta or Heterokontophyta, or as subphylum Ochrophytina within phylum Gyrista. Their plastids are of red algal origin.
A protist or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor.
SAR or Harosa is a highly diverse clade of eukaryotes, often considered a supergroup, that includes stramenopiles (heterokonts), alveolates, and rhizarians. It is a node-based taxon, including all descendants of the three groups' last common ancestor, and comprises most of the now-rejected Chromalveolata. Their sister group has been found to be telonemids, with which they make up the TSAR clade.
Bigyra is a phylum of microscopic eukaryotes that are found at the base of the Stramenopiles clade. It includes three well-known heterotrophic groups Bicosoecida, Opalinata and Labyrinthulomycetes, as well as several small clades initially discovered through environmental DNA samples: Nanomonadea, Placididea, Opalomonadea and Eogyrea. The classification of Bigyra has changed several times since its origin, and its monophyly remains unresolved.
The kathablepharids or katablepharids are a group of heterotrophic flagellates closely related to cryptomonads. First described by Heinrich Leonhards Skuja in 1939, kathablepharids were named after the genus Kathablepharis. This genus is corrected to Katablepharis under botanical nomenclature, but the original spelling is maintained under zoological nomenclature. They are single-celled protists with two anteriorly directed flagella, an anterior cytostome for ingesting eukaryotic prey, and a sheath that covers the cell membrane. They have extrusomes known as ejectisomes, as well as tubular mitochondrial cristae.
Haptista is a proposed group of protists made up of centrohelids and haptophytes. Phylogenomic studies indicate that Haptista, together with Ancoracysta twista, forms a sister clade to the SAR+Telonemia supergroup, but it may also be sister to the Cryptista (+Archaeplastida). It is thus one of the earliest diverging Diaphoretickes.
Platysulcus tardus is an eukaryotic microorganism that was recently discovered to be the earliest diverging lineage of the Heterokont phylogenetic tree. It is the only member of the family Platysulcidae, order Platysulcida and class Platysulcea.
Endohelea is a proposed clade of eukaryotes that are related to Archaeplastida and the SAR supergroup. They used to be considered heliozoans, but phylogenetically they belong to a group of microorganisms known as Cryptista.
Ultrastructural identity is a concept in biology. It asserts that evolutionary lineages of eukaryotes in general and protists in particular can be distinguished by complements and arrangements of cellular organelles. These ultrastructural components can be visualized by electron microscopy.
Thecamonadinae is a subfamily of heterotrophic protists. It is a monophyletic group, or clade, of apusomonads, a group of protozoa with two flagella closely related to the eukaryotic supergroup Opisthokonta. The subfamily contains two genera Chelonemonas and Thecamonas, which are found in marine habitats.
Commation is a genus of marine heterotrophic protists closely related to the actinophryids. It contains two species, Commation cryoporinum and Commation eposianum, discovered in antarctic waters and described in 1993. Currently, the genus is classified within a monotypic family Commatiidae and order Commatiida. Along with the photosynthetic raphidophytes, these organisms compose the class of stramenopiles known as Raphidomonadea.
Colponemids are free-living alveolates, unicellular flagellates related to dinoflagellates, apicomplexans and ciliates. They are predators of other small eukaryotes, found in freshwater, marine and soil environments. They do not form a solid clade, but a sparse group of deep-branching alveolate lineages.
Anisonemia is a clade of single-celled protists belonging to the phylum Euglenozoa, relatives of the Euglenophyceae algae. They are flagellates, with two flagella for locomotion. Anisonemia includes various phagotrophic species and a group of primary osmotrophic protists known as Aphagea.