Opalina

Opalis is a very large (sometimes reaching a length of 3 mm ) unicellular organisms having from 2 to several hundred nuclei and covered with thousands of short flagella resembling cilia . Flagella are organized in longitudinal rows (kinets), and the nature of their beating is similar to the beating of cilia of ciliates . Between kinets, the surface of the cell has the appearance of ridges strengthened by microtubules . Between the longitudinal rows of microtubules numerous vesicles are found . When the reflected light passes through these ridges, the interference of light occurs, leading to beautiful opalescence , for which this group of protozoa got its name. Opalins do not have special devices for ingesting food, and nutrients are absorbed by the entire surface of the cell by pinocytosis . The resulting vesicles merge with each other in the center of the cell, forming large diameter of up to 4 microns . Flattened exocytotic vesicles are described. Cristae are tubular in mitochondria , and often lipid drops are located near mitochondria ^{[4] [1]} .

The genera of opals differ in two characteristics: the shape of the cell (cylindrical or flattened) and the number of nuclei (2 or many). Opalina (flattened cells) and Cepedea (cylindrical cells) have many nuclei, while flattened Zelleriella and cylindrical Protoopalina have two nuclei. The genus Protozelleriella was described last (in 1991) and is similar to Zelleriella (in particular, also has 2 cores), but its representatives have hyaline bordering, which is devoid of flagella. In opalins with two cores, the cores themselves are significantly larger than in opals with many cores ^[3] .

Opaline cells are divided by closed mitosis , in which the nuclear membrane does not collapse and the fission spindle forms inside the nucleus. When dividing, the division plane passes between the kinets and divides the cell into two equal parts. In Zelleriella, cytokinesis precedes karyokinesis , and initially mononuclear daughter cells are formed, the nuclei of which are then divided into two. In multicore opals, the nuclei divide asynchronously ^{[5] [6]} .

The life cycle of opals is rather complicated. Multinucleated trophons are found in adult frogs throughout the year, however, in the mating season, the number of opals sharply increases due to active cell division without subsequent growth (palintomy). It is believed that the active division of opaline is stimulated by the hormones of the host. The resulting small individuals (low-nuclear trophons) have only a few nuclei and throw fragments. Further, small-nuclear trophons are encysted, and cysts enter the external environment along with feces. Cysts have a spherical shape and reach from 20 to 45 microns in diameter; cysts contain from 4 to 8 cores. Cysts remain viable in fresh water for several more weeks. When cysts are swallowed by tadpoles , mobile low-nuclear trophons come out of them. In the intestine, they are divided several times, including meiosis, with the formation of narrow microgametes and large macrogametes. Microgametes have a narrow “tail”, lacking cilia and, apparently, having sticky properties. Microgametes float to the macrogametes with the “tail” forward and with it are fixed on the macrogametes. Gametes merge with the formation of a diploid zygote , which encysts and enters the environment with feces. If it again enters the tadpole, then the gamete formation again occurs, and if it enters the adult frog or the tadpole of the last stage before metamorphosis, large multinuclear trophons are formed through the growth and multiple fission of the nuclei of low-nuclear trophons ^{[7] [8]} .

All opals are endobionts of poikilothermic vertebrates - usually frogs, less often newts , fish and reptiles ^[6] . It is believed that reptiles become infected with opalins by eating infected frogs. Opalins inhabit the back of the intestine and eat no pathogenic commensals . Opalins, apparently, do no harm to their owners. Opaline can be found on all continents, but they are most diverse in the tropics and subtropics , where the main diversity of their hosts is concentrated. Representatives of Zelleriella are not found in the Palearctic , Opalina and Cepedea are extremely rare in Australia , and Protozelleriella is found only in Africa . There are no laboratory cultures of opals yet, but sometimes the opals of the genus Protoopalina settle in the intestines of the model object - frogs Xenopus laevis , which are grown in the laboratory. Opalins isolated from the host organism can live in the laboratory for several weeks ^[9] . Sometimes amoebas of the genus live in opal. Some other frog parasites feed on opalins, for example, the trematode Diplodiscus temperatureus destroys opal in the intestines of adult frogs ^[10] .

Apparently, for the first time, opaline was already seen by Levenguk in 1683, when he studied the feces of frogs and found in them many motile cells of Cepedea dimidiata . The genus Opalina was isolated by Purkinje and Valentine in 1835. During the 19th century, opals were studied by many scientists, in particular, Zeller, who published his work on opals in 1877. During the first half of the 20th century, Metcalfe was actively engaged in opals. He distinguished 3 genera of opaline and suggested a close relationship between opal and ciliates ^[11] .

The relationship of opal with other protists has long been a mystery. Due to the superficial resemblance to ciliates, 19th-century scholars placed opaline in this group. In 1918, it was proposed to isolate opaline into an independent subtype of Protociliata within the type of ciliates, which was opposed to “real” ciliates. Some scholars saw opal as an intermediate between flagellates and ciliates. The presence of two or multiple nuclei, the presence of kinets, and the sexual process inherent in opalins have long been considered as features that a hypothetical ancestor of ciliates possessed ^[12] . In the middle of the 20th century, works appeared stating that opal was the closest to flagella, and not to ciliates ^[11] .

The situation changed dramatically after the study of opaline cells using electron microscopy . It turned out that the protists of the genera and are the closest to opals, and it was proposed to combine the opals with the new order Slopalinida, which, by some structural features, is very close to stranopiles . The first phylogenetic analysis also confirmed the inclusion of opal in the composition of stranopil. It also turned out that protists of the genus are very close to Slopalinida ^[12] .

The close relationship between opal and frogs indicates that the main radiation of opals occurred in the Mesozoic . Binuclear forms are considered more primitive, and multicore opals are sometimes combined into a monophyletic group . The relationship between binuclear opalins is unclear, but there is a hypothesis that Protozelleriella is the most primitive opaline ^[12] .

As of June 2018, the Opalinidae family is distinguished into the monotypic order of Opalinida and the class Opalinea (opaline), the latter is included in the superclass Opalinata, which in addition contains the classes Blastocystea and Proteromonadea . The superclass Opalinata itself is part of the Opalozoa subtype of the type st from the group of stranopilus ^[13] .

• Houseman K., Hülsman N., Radek R. Protistology / Per. from English S. A. Karpova. Ed. S. A. Korsuna. - M .: Partnership of scientific publications of KMK, 2010. - 495 p. - ISBN 978-5-87317-662-5 .
• Handbook of the Protists / Archibald, John M., Simpson, Alastair GB, Slamovits, Claudio H. - Springer International Publishing, 2017 .-- ISBN 978-3-319-28147-6 .