Loxodes

The term Loxodes derives from the ancient greek (), meaning "oblique, tilted". 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. == Type species ==

Loxodes rostrum (Müller, 1773) Ehrenberg, 1830 == History of knowledge ==

Kolpoda rostrum was the first loxodid discovered and described. The genus Loxodes was first proposed in 1830. Advances in molecular phylogeny have also provided deeper insights into the phylogeny of Loxodes . Over time, new species and subspecies have been described, including the recent discovery of the new species L. tziscaensis in Mexico. == Habitat and ecology ==

Loxodes lives in freshwater habitats such as lakes and ponds, unlike other karyorelictean ciliates such as the other loxodid genus Remanella, which live in brackish-water or marine habitats. They feed on bacteria and protists such as microalgae. It is microaerobic, 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 use nitrate instead of oxygen as an electron acceptor for respiration. Nitrate respiration is rare among eukaryotes, and Loxodes was the first eukaryote known to have this capability. Loxodes is also sensitive to light. Members of Loxodes are found in freshwater environments. Experiments have shown that in oxygen-poor conditions Loxodes produce nitrate reductase and have increased electron transport chain activity, indicating a switch in terminal electron acceptors from oxygen to nitrate. Other researchers have reported observing a different unidentified alga within L. rostrum. However, the nature of the relationship with P. daitoensis has not been resolved, with more recent research questioning if L. rostrum is merely feeding on this alga or if this is indeed some form of endosymbiosis. 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. Research has shown that when L. stratus was consumed by the catenulid flatworm Stenostomum sphagnetorum, it was regurgitated, and when it was extrusome deficient, it was regurgitated far less. Furthermore, extracted toxins from L. stratus extrusomes were shown to be lethal to S. sphagnetorum. == Description of organism ==

Morphology Loxodes are roughly oval and range in length from 70–657 μm and are typically between 20–220 μm in width. Nuclear and cellular division Like other Ciliates, Loxodes have macronuclei and micronuclei. However, in Loxodes, 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. Different Loxodes species have different numbers and groupings of the two nuclei types. In species such as L. 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 as L. 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. 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. Pigmentocysts Loxodes have also been shown to contain yellow-brown pigment granules that are thought to act as photoreceptors that perceive increased light. In addition to the experiments with S. sphagnetorum discussed above, Loxodes have been shown to release the contents of the pigment granules when they are attacked by the predatory ciliate Dileptus. Furthermore, it has been experimentally demonstrated that the contents of the granules damage the proboscis of Dileptus and can even be fatal. UAA and UAG incorporate glutamine, and UGA is assigned to tryptophan or as a stop codon. This variant code is also used by other ciliates. == Phylogeny ==

The most recent phylogenetic work using 18S rRNA has confirmed that Loxodes form a clade separate from the sister genus of Remanella within the Loxodidae family . File:Müller vesicle.png| Detail showing a Müller vesicle (top arrow). File:Инфузория Loxodes съедает длинную цианобактерию.webm| Loxodes ciliate eating a long cyanobacterium == Cell cycle ==

Unlike other ciliates, the macronuclei of karyorelicteans do not divide. This was first observed in Loxodes by Otto Bütschli in the 1870s. It was later shown to be a distinctive feature of the class Karyorelictea in general. Experiments on Loxodes have shown that little or no DNA synthesis occurs in their macronuclei, and that the DNA content of a macronucleus is only slightly more than that of a diploid micronucleus ("paradiploid"). == List of species ==

• Loxodes kahli Dragesco and Njiné, 1971 • Loxodes magnus Stokes, 1887 • Loxodes penardi Kahl, 1931 • Loxodes rex Dragesco, 1970 • Loxodes rostrum (Müller, 1773) Ehrenberg, 1830 • Loxodes striatus (Engelman, 1862) Penard, 1917 • Loxodes tziscaensis Mendez-Sanchez, Mayen-Estrada and Ramirez-Corona, 2022 • Loxodes vorax Stokes 1884 == Practical importance ==

Research into Loxodes has been important as a model organism for increasing scientific understanding of Karyorelictids. These advancements demonstrate the usefulness of Loxodes as a model organism for investigating new scientific methods for working with other organisms that are difficult to culture, such as other marine Ciliates. == References ==