| Ciliates Temporal range: Ediacaran - Recent |
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| "Ciliata" from Ernst Haeckel's Kunstformen der Natur, 1904 | |
| Scientific classification | |
| Domain: | Eukarya |
| Kingdom: | Chromalveolata |
| Superphylum: | Alveolata |
| Phylum: | Ciliophora Doflein, 1901 emend. |
| Classes | |
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Karyorelictea |
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The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but typically shorter and present in much larger numbers with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of the life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.
The term "Ciliophora" is used in classification as a phylum.[1] Ciliophora can be classified under Protista[2] or Protozoa.[3] The term "Ciliata" is also used,[4] as a class.[5] (However, this latter term can also refer to a type of fish.) Protista classification is rapidly evolving, and it is not rare to encounter these terms used to describe other hierarchical levels.
Ciliates are one of the most important groups of protists, common almost everywhere there is water — in lakes, ponds, oceans, rivers, and soils. Ciliates have many ectosymbiotic and endosymbiotic members, as well as some obligate and opportunistic parasites. Ciliates tend to be large protozoa, a few reach 2 mm in length, and are some of the most complex protozoans in structure.
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Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a small, diploid micronucleus (reproduction), and a large, polyploid macronucleus (general cell regulation). The latter is generated from the micronucleus by amplification of the genome and heavy editing. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes is by a process whose mechanism is unknown. This process is by no means perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and then fuse to form new micronuclei and macronuclei.
Food vacuoles are formed through phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down via lysosomes so the substances the vacuole contains are then small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct (anus) is discharged via exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. These often have a distinctive star-shape, with each point being a collecting tube.
Most ciliates feed on smaller organisms (heterotrophic), such as bacteria and algae, and detritus swept into the oral groove (mouth) by modified oral cilia. This usually includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet, which forms food vacuoles.
This varies considerably, however. Some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates. This includes the suctoria, which feed through several specialized tentacles.
Ciliates can undergo both asexual and sexual reproduction. Asexual reproduction occurs by binary fission. The micronucleus undergoes mitosis and the macronucleus elongates and splits in half. Both new cells each obtain a copy of the micronucleus and macronucleus. Sexual reproduction involves conjugation, which involve two cells. After conjugation, the two cells divide, forming four new cells.
In some forms there are also body polykinetids, for instance, among the spirotrichs where they generally form bristles called cirri. More often body cilia are arranged in mono- and dikinetids, which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called kineties, which run from the anterior to posterior of the cell. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia.
The infraciliature is one of the main component of the cell cortex. Another are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates, Apicomplexa, and dinoflagellates. These superficially dissimilar groups make up the alveolates.
Until recently, the oldest ciliate fossils known were tintinnids from the Ordovician Period. In 2007, Li et al. published a description of fossil ciliates from the Doushantuo Formation, about 580 million years ago, in the Ediacaran Period. These included two types of tintinnids and a possible ancestral suctorian.[6]
Ciliates are unicellular eukaryotes with two types of nuclei: the somatic “micronucleus” and the germline “macronucleus”. In ciliates, segments of the parental micronuclear gene rearrange in a random order. This concept, known as gene scrambling, occurs when the germline micronuclear genome develops into the somatic macronuclear genome. Segments arrange themselves through various mutations in the gene, such as deletions, duplications, inversions, and translocations.
During the gene scrambling process, genes in the micronuclear genome of ciliates are interrupted by sequences called internal eliminated sequences (IESs). As the micronucleus develops into a somatic nucleus, IESs are excised from the germline gene. The remaining gene segments, called Macronuclear Destined Segments (MDS), are spliced and together and make an operational gene. In this process, the MDSs are often scrambled in an order and orientation relative to the macronuclear copy of the gene for reasons unknown. Each macronuclear segment can be recognized and traced in this gene scrambling process as they are tagged with distinct key sequences.
The study of gene scrambling in ciliate micronucleus genes ties back to the phylogenetic tree.
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