| Loxodes | |
|---|---|
.jpg) | |
| Illustration ofLoxodes rostrum | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Sar |
| Clade: | Alveolata |
| Phylum: | Ciliophora |
| Subphylum: | Postciliodesmatophora |
| Class: | Karyorelictea |
| Order: | Loxodida |
| Family: | Loxodidae |
| Genus: | Loxodes Ehrenberg, 1830 |
| Species | |
Several, including:[1] | |
Loxodes is a genus ofkaryorelicteanciliates, belonging toLoxodidae.[1] It is the only known karyorelictean ciliate that lives in freshwater habitats. The genus is known for its distinctive morphology, including a relatively large, flattened body and unique nuclear structures. It is also known to exhibit fascinating behaviours, such as geotaxis and light sensitivity.[2]
The termLoxodes derives from theancient greekλοξός (loxós), meaning "oblique, tilted".[3][4] According to Ehrenberg, who first described the genus, Loxodes means oblique and refers to the anterior margin of the ciliated lip of the organisms in this genus.[5]
Loxodes rostrum (Müller, 1773) Ehrenberg, 1830
Kolpoda rostrum was the first loxodid discovered and described.[6] The genusLoxodes was first proposed in 1830.[5] Ehrenberg movedKolpoda rostrum into the genusLoxodes and set it as the type species. Research has explored the geotaxis ofLoxodes species, while subsequent work has investigated their photosensitivity.[2][7] Advances in molecular phylogeny have also provided deeper insights into the phylogeny ofLoxodes .[8][9] Over time, new species and subspecies have been described, including the recent discovery of the new speciesL.tziscaensis in Mexico.[8] Discoveries, such as that ofL. rex in both Florida and Thailand, previously only observed in Africa, have challenged the understanding of the distribution of what were thought to be endemic species.[10][11]
Loxodes lives infreshwater habitats such as lakes and ponds, unlike other karyorelictean ciliates such as the other loxodid genusRemanella, which live in brackish-water or marine habitats.[12] They feed on bacteria and protists such asmicroalgae.[13] It ismicroaerobic, preferring low concentrations of oxygen, below 5% atmospheric saturation. It can also survive extended periods in anoxic water, where oxygen is absent. Under such conditions,Loxodes is able to usenitrate instead of oxygen as an electron acceptor forrespiration.[14]Nitrate respiration is rare among eukaryotes, andLoxodes was the first eukaryote known to have this capability.Loxodes is also sensitive to light.
Members ofLoxodes are found in freshwater environments.[15][8] They typically inhabit aquatic microhabitats, including sediment and water columns, where they feed on small microorganisms. They live in the hypolimnion and range from areas of low O2 concentration to anoxic conditions.[15] They will move downwards toward lower O2 concentrations, where it is hypothesized that there will be less competition for food.[7]
Loxodes have been shown to be able to conduct nitrate reduction, allowing them to switch from aerobic respiration to nitrogen respiration in anoxic conditions.[16][17][18] Experiments have shown that in oxygen-poor conditionsLoxodes produce nitrate reductase and have increased electron transport chain activity, indicating a switch in terminal electron acceptors from oxygen to nitrate.[17] Evidence of high nitrite concentration colocalized withLoxodes further supports this conclusion.[17]
It has been suggested thatL. rostrum may form symbiotic relationships with the green algaePediludiella daitoensis based on observations ofL. rostrum containing many cells of this algal species.[19] Other researchers have reported observing a different unidentified alga withinL. rostrum.[20] However, the nature of the relationship withP. daitoensis has not been resolved, with more recent research questioning ifL. rostrum is merely feeding on this alga or if this is indeed some form of endosymbiosis.[8] It has been established that many freshwater ciliates can contain as many as hundreds of algae within them, but determining if these organisms are symbiotic or are temporarily retained food has been difficult due to the difficulties in culturing many ciliate species, includingLoxodes.[21] Due to the difficulties in culturing these species, the specimens observed with algal cells within them have solely been collected from the wild already containing algal cells. It is thus not fully understood where they encounter the algae or how long they maintain them. It has been speculated that the alga may be maintained as UV protection or an O2 source in anoxic conditions.[20]
There is evidence that in some cases, more than one species ofLoxodes coexist in the same habitat where they have the same distribution in the water column.[15] Observations suggest that they partition food resources by size, where the largerLoxodes species consume food particles that are too large for the smallerLoxodes species.
At least one species ofLoxodes has been shown to release toxic compounds from its extrusomes as a defence mechanism to avoid being eaten by predators.[22] Research has shown that whenL. stratus was consumed by the catenulid flatwormStenostomum sphagnetorum, it was regurgitated, and when it was extrusome deficient, it was regurgitated far less. Furthermore, extracted toxins fromL. stratus extrusomes were shown to be lethal toS. sphagnetorum.
Loxodes are roughly oval and range in length from 70–657 μm and are typically between 20–220 μm in width.[8][9] They are laterally flattened with the right side ciliated with dikinetids in bipolar kineties in anywhere from 5 to 84 lateral rows depending on species.[11][8] There are only two bipolar kineties on their left side. There is an anterior oral aperture on the ventral side leading into a cytostome at the posterior of the aperture. The oral aperture forms a distinctive hook shape at the cell's anterior end, and the cell's posterior end is rounded. The oral aperture contains a single buccal kinety on the right and an inner and outer buccal kinety on the left. Additionally, there is a rectilinear intrabuccal kinety.[8]Loxodes are transparent with yellow to brownish pigmentation.[23]
Like other Ciliates,Loxodes have macronuclei and micronuclei.[24][25] However, inLoxodes, the fates and transfer of these nuclei during asexual division are unique compared to other Ciliates. In the sister class, Heterotrichea, both the macronucleus and micronucleus divide during asexual cellular division. However, in Karyorelictea, the macronuclei do not divide and are transferred wholly to daughter cells. DifferentLoxodes species have different numbers and groupings of the two nuclei types. In species such asL. rostrum, each cell has two macronuclei and one micronucleus. The two macronuclei are separated into the daughter cells when the cell divides. The micronucleus then goes through two rounds of mitosis. The two micronuclei from the second division differentiate into macronuclei. These divisions provide each daughter cell with a micronucleus and a second macronucleus. Similarly, species such asL. striatus have two pairs of nuclei, each pair made of one macronucleus and one micronucleus. When these cells divide, each micronucleus is divided into one micronucleus and one macronucleus. The remaining micronuclei then divide again to produce another micronucleus, thus providing each daughter cell with two new pairs of nuclei.[24][25]
Loxodes species may have as many as 181 macronuclei and 138 micronuclei in similar arrangements, all following similar patterns of nuclear division.[8] The nuclei can be found in groups containing at least one macronucleus and one micronucleus but often include many more of each.[25] The macronuclei can range from being equal in number to micronuclei to being twice as numerous. As in other Ciliates, the micronuclei are diploid; however, the macronuclei are paradiploid instead of the usual ampliploid state in other Ciliates.[8] The macronuclei degrade over time and are lost after relatively few rounds of cellular division.[24] For example, inL. magnus, the macronucleus only lasts 3-7 divisions. It has been suggested that maintaining multiple macronuclei allowsLoxodes to have higher numbers of transcripts that may be needed to have larger cells.[24]
Loxode cells divide transversally, with the nuclei roughly evenly distributed between the daughter cells.[25] Division timing is correlated with the number of macronuclei reaching a certain quantity within the cell.[25]
Both genera in the family Loxodidae have organelles known asMüller (or Müllerian) vesicles, which are involved in the sensing of gravity. They are about 7 μm across, and contain a membrane-covered mineral body known as a statolith. InLoxodes, the statolith is mostly composed ofbarium salts, compared toRemanella, where they are mostly strontium. Its structure and function resembles thestatocyst of some animals.[12][26] These vesicles are located on the dorsal rim on the left side of the cell in the cytoplasm and range in number from 2 to as many as 60.[8] The vesicle is associated with basal granules, with one granule attached by cilia to the Müller vesicle like a stalk. It is thought that movement of the barium grain inside the vesicle is perceived by the cilia, allowingLoxodes to distinguish between up and down (geotaxis orgravitaxis), which it uses as a stimulus in addition to the oxygen concentration to orient itself in the water column. When oxygen concentrations are high,Loxodes tends to swim downwards, and vice versa.[27]
Loxodes have also been shown to contain yellow-brown pigment granules that are thought to act as photoreceptors that perceive increased light.[7] Research has linked light perception inLoxodes with the partial pressure of O2.Loxodes are thought to integrate light perception with geotaxis to position themselves in the water column.
The pigment granules have also been demonstrated to act as extrusomes that contain defence toxins.[28] In addition to the experiments withS. sphagnetorum discussed above,Loxodes have been shown to release the contents of the pigment granules when they are attacked by the predatory ciliateDileptus. Furthermore, it has been experimentally demonstrated that the contents of the granules damage the proboscis ofDileptus and can even be fatal.[28]
Recent research has shown that members of the Loxodidae family, of whichLoxodes is one of two genera, use a variation of the standard genetic code where the stop codons have alternate uses.[29] UAA and UAG incorporate glutamine, and UGA is assigned to tryptophan or as a stop codon. Thisvariant code is also used by other ciliates.[30]
The most recent phylogenetic work using 18S rRNA has confirmed thatLoxodes form a clade separate from the sister genus ofRemanella within the Loxodidae family .[8] This work has also shown thatLoxodes species cluster in correlation with nuclear organization, with the species that have two macronuclei and a single micronucleus clustered in one group and the rest clustered in another.[8]
![Species of Loxodes containing significantly large green-colored algae. Scale bar: 10 μm.[31]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aLoxodes.png)
![Detail showing a Müller vesicle (top arrow).[31]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aM%25C3%25BCller_vesicle.png)
Unlike other ciliates, the macronuclei ofkaryorelicteans do not divide. This was first observed inLoxodes byOtto Bütschli in the 1870s.[32] It was later shown to be a distinctive feature of the class Karyorelictea in general. Experiments onLoxodes have shown that little or noDNA synthesis occurs in their macronuclei, and that the DNA content of a macronucleus is only slightly more than that of adiploid micronucleus ("paradiploid").[32]
Research intoLoxodes has been important as a model organism for increasing scientific understanding of Karyorelictids.[23]Loxodes' adaptation to freshwater makes them easier to culture, thus making them useful for studying the cellular features and behaviours of these Karyorelictids as well as other related Ciliates.[9] Despite the comparative ease of culturing compared to other karyorelictids,Loxodes are still quite difficult to culture. Research continues to work to improve culturing, fixation, and staining techniques to aid future research.[33][34] These advancements demonstrate the usefulness ofLoxodes as a model organism for investigating new scientific methods for working with other organisms that are difficult to culture, such as other marine Ciliates.
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Data related toLoxodes at Wikispecies