Ciliate

This article is about the protozoan phylum Ciliophora. For the type of ciliate cells in general, see Ciliate cells. For the type of leaf margin, see Leaf § Margins (edge).

Ciliates Temporal range:
"Ciliata" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Domain:	Eukarya
Kingdom:	Chromalveolata
Superphylum:	Alveolata
Phylum:	Ciliophora Doflein, 1901 emend.
Classes
Karyorelictea Heterotrichea Spirotrichea Litostomatea Phyrea Plagiopylea See text for subclasses.

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.

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 are large single cells, a few reaching 2 mm in length, and are some of the most complex protozoans in structure.

The term "Ciliophora" is used in classification as a phylum. Ciliophora can be classified under Protista or Protozoa. The term "Ciliata" is also used, as a class. (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.

Cell structure

Cell structure of an ciliophora: 1-contractile vacuole, 2-digestive vacuole, 3-macronucleous, 4-micronucleous, 5-cytoprocto, 6-cytopharynx, 7-cytostome, 8-cilium.

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.

Feeding

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.

Reproduction

Cilliate undergoing the last processes of binary fission, a form of asexual reproduction

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. The cell then divides in two. Both new cells each obtain a copy of the micronucleus and macronucleus. By contrast, sexual reproduction involves the exchange of genetic material (or conjugation) between two organisms of different mating type (akin to sex, but more than two mating types can exist). This often occurs when food is rare. The two cells form a bridge between their cytoplasms, the micronuclei undergo meiosis, the macronuclei disappear, and the haploid micronuclei are exchanged over the bridge. The cells separate and form new macronuclei from their micronuclei. The two cells then divide, forming four new cells.

Specialized structures in ciliates

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.

Fossil record

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.

DNA Rearrangements (gene scrambling)

Ciliates contain two types of nuclei: the somatic “macronucleus” and the germline “micronucleus”. Only the DNA in the micronucleus is passed on during sexual reproduction (conjugation). On the other hand, only the DNA in the macronucleus is actively expressed and results in the phenotype of the organism. Macronuclear DNA is derived from micronuclear DNA by amazingly extensive DNA rearrangement and amplification.

The macronucleus begins as a copy of the micronucleus. The micronuclear chromosomes are fragmented into many smaller pieces and amplified to give many copies. The resulting macronuclear chromosomes often contain only a single gene. In Tetrahymena, the micronucleus has 10 chromosomes (5 per haploid genome), while the macronucleus has over 20,000 chromosomes.

In addition, the micronuclear genes are interrupted by numerous "Internal Eliminated Sequences" (IESs). During development of the macronucleus, IESs are deleted and the remaining gene segments, Macronuclear Destined Sequences (MDSs), are spliced together to give the operational gene. Tetrahymena has about 6000 IESs and about 15% of micronuclear DNA is eliminated during this process. The process is guided by small RNAs and epigenetic chromatin marks.

In spirotrich ciliates (such as Oxytricha), the process is even more complex due to "gene scrambling": the MDSs in the micronucleus are often in different order and orientation from that in the macronuclear gene, and so in addition to deletion, DNA inversion and translocation are required for "unscrambling". This process is guided by long RNAs derived from the parental macronucleus. More than 95% of micronuclear DNA is eliminated during spirotrich macronuclear development.

Classification

Stentor roeseli

Subphylum Postciliodesmatophora

• Class Karyorelictea
• Class Heterotrichea (e.g. Stentor)

Subphylum Intramacronucleata

Oxytricha trifallax

• Class Spirotrichea
  • Subclass Choreotrichia (e.g. Tintinnidium)
  • Subclass Oligotrichia (e.g. Halteria)
  • Subclass Stichotrichia (e.g. Stylonychia)
  • Subclass Hypotrichia (e.g. Euplotes)

A trophozoite of Balantidium coli

• Class Litostomatea
  • Subclass Haptoria (e.g. Didinium)
  • Subclass Trichostomatia (e.g. Balantidium)
• Class Phyllopharyngea
  • Subclass Phyllopharyngia (e.g Chilodonella uncinata
  • Subclass Rhynchodia
  • Subclass Chonotrichia
  • Subclass Suctoria (e.g. Podophrya)
• Class Nassophorea
• Class Colpodea (e.g. Colpoda)
• Class Prostomatea (e.g. Coleps)
• Class Oligohymenophorea
  • Subclass Peniculia (e.g. Paramecium)
  • Subclass Hymenostomatia (e.g. Tetrahymena)
  • Subclass Scuticociliatia
  • Subclass Peritrichia (e.g. Vorticella)
  • Subclass Astromatia
  • Subclass Apostomatia
• Class Plagiopylea

Other

• Genus Cryptocaryon