The stramenopiles, also called heterokonts, are Protist 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 supergroup, along with Alveolate and Rhizaria.
Stramenopiles are Eukaryote; most are single-celled, but some are multicellular including some large seaweeds, the brown algae. The group includes a variety of algal , heterotrophic flagellates, and closely related proteromonad flagellates (all Endosymbiont in other organisms); the actinophryid Heliozoa, and . The tripartite hairs characteristic of the group have been lost in some of the included taxa – for example in most .
Many stramenopiles are unicellular , and most others produce flagellated cells at some point in their lifecycles, for instance as or . Most flagellated heterokonts have two flagella; the anterior flagellum has one or two rows of stiff hairs or Mastigoneme, 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.
Nomenclature
The term 'stramenopile' was introduced by D. J. Patterson in 1989, defining a group that overlapped with the ambiguously defined heterokonts.
The name "stramenopile" has been discussed by J. C. David.
Patterson's term was formalized as a taxonomic name,
Stramenopiles, in 2005 by the International Society of Protistologists.
It has since been the most widely accepted name for this group of organisms.
Several alternative names have been proposed since Patterson's publication, such as Straminipila,
Straminopiles,
and Stramenopila,
which is the valid name under the
PhyloCode.
The heterokont problem
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.
But the same term was used for other groupings of algae. For example, in 1956, Copeland
used it to include the xanthophytes (using the name Vaucheriacea), a group that included what became known as the
, the
Silicoflagellata, and the
. Copeland also included the unrelated collar flagellates (as the
) in which he placed the
. He also included the not-closely related
. 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.
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.
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 opalinidae, , and actinophryid Heliozoa. They joined other heterotrophic protists, such as , , 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.
Characteristics
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
.
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.
Most stramenopiles have two flagella near the apex.
Plastids
Many stramenopiles have
which enable them to
, using light to
autotroph. 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. The most significant autotrophic stramenopiles are the
brown algae (wracks and many other seaweeds), and the
. The latter are among the most significant primary producers in marine and freshwater ecosystems.
Most molecular analyses suggest that the most basal stramenopiles lacked plastids and were accordingly colourless
, 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
, colourless ciliophryids, and colourless actinophryid heliozoa) have secondarily reverted to heterotrophy.
Ecology
Some stramenopiles are significant as autotrophs and as heterotrophs in natural ecosystems; others are parasitic.
Blastocystis is a gastrointestinal parasite of humans;
opalines and proteromonads live in the intestines of cold-blooded vertebrates and have been described as parasitic;
oomycetes include some significant plant pathogens such as the cause of potato blight,
Phytophthora infestans.
Diatoms are major contributors to global carbon cycles because they are the most important autotrophs in most marine habitats.
The brown algae, including familiar seaweeds like wrack and kelp, are major autotrophs of the intertidal and subtidal marine habitats.
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.
Evolution
External
Stramenopiles are most closely related to alveolates and Rhizaria, all of which have tubular mitochondrial
Crista and collectively form the
Sar supergroup supergroup, whose name is formed from their initials.
[Krylov, M. V.; Dobrovolskii, A. A.; Issi, I. V.; Michaelevich, B. I.; Podlipaev, S. A.; Reshetnyak, V. V.; Seravin, L. N.; et al. 1980. New concepts for the system of unicellular organisms. Trudy Zoologischkei Institut Akademiya Nayuk, SSSR 94:122–132.] The ancestor of the SAR supergroup appears to have captured a unicellular photosynthetic
Red algae, 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.
In addition, species of
Telonemia, the sister group to SAR, exhibit heterokont flagella with tripartite mastigonemes, implying a more ancient origin of stramenopile characteristics.
Internal
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 or
paraphyletic. When paraphyletic, the branching order of the bigyran groups also varies: in some studies,
Sagenista is the most basal-branching clade,
while in others
Opalozoa is the most basal.
Nonetheless,
Platysulcea is consistently recovered as the
sister clade to all other stramenopiles.
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.
Classification
The classification of the stramenopiles according to Adl
et al. (2019), with additions from newer research:
Notes
External links
