Ciliate

Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus (the "generative nucleus," which carries the germline of the cell), and a large, polyploid macronucleus (the "vegetative nucleus," which takes care of general cell regulation, expressing the phenotype of the organism). The latter is generated from the micronucleus by amplification of the genome and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the small nuclear RNA for vegetative growth.

Division of the macronucleus occurs by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. This process is not 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 by 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 (anal pore) is discharged by 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. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube.

Cilia are arranged in rows called kineties. 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 components of the cell cortex. Others 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.

Ciliates reproduce asexually, by various kinds of fission.[14] During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus.

Ciliate undergoing the last processes of binary fission

Typically, the cell is divided transversally, with the anterior half of the ciliate (the proter) forming one new organism, and the posterior half (the opisthe) forming another. However, other types of fission occur in some ciliate groups. These include budding (the emergence of small ciliated offspring, or "swarmers", from the body of a mature parent); strobilation (multiple divisions along the cell body, producing a chain of new organisms); and palintomy (multiple fissions, usually within a cyst).[15]

Fission may occur spontaneously, as part of the vegetative cell cycle. Alternatively, it may proceed as a result of self-fertilization (autogamy),[16] or it may follow conjugation, a sexual phenomenon in which ciliates of compatible mating types exchange genetic material. While conjugation is sometimes described as a form of reproduction, it is not directly connected with reproductive processes, and does not directly result in an increase in the number of individual ciliates or their progeny.[17]

Overview

Ciliate conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell.[17][15] During conjugation, two ciliates of a compatible mating type form a bridge between their cytoplasms. The micronuclei undergo meiosis, the macronuclei disappear, and haploid micronuclei are exchanged over the bridge. In some ciliates (peritrichs, chonotrichs and some suctorians), conjugating cells become permanently fused, and one conjugant is absorbed by the other.[13][18] In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei.[19] Conjugation and autogamy are always followed by fission.[15]

In many ciliates, such as Paramecium, conjugating partners (gamonts) are similar or indistinguishable in size and shape. This is referred to as "isogamontic" conjugation. In some groups, partners are different in size and shape. This is referred to as "anisogamontic" conjugation. In sessile peritrichs, for instance, one sexual partner (the microconjugant) is small and mobile, while the other (macroconjugant) is large and sessile.[17]

Stages of conjugation

Stages of conjugation in Paramecium caudatum

In Paramecium caudatum, the stages of conjugation are as follows (see diagram at right):

1. Compatible mating strains meet and partly fuse
2. The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
3. Three of these micronuclei disintegrate. The fourth undergoes mitosis.
4. The two cells exchange a micronucleus.
5. The cells then separate.
6. The micronuclei in each cell fuse, forming a diploid micronucleus.
7. Mitosis occurs three times, giving rise to eight micronuclei.
8. Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.
9. Binary fission occurs twice, yielding four identical daughter cells.

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

Oxytricha trifallax

• Class Armophorea
• Class Cariacotrichea (only one species, Cariacothrix caudata)
• Class Muranotrichea
• Class Parablepharismea
• Class Colpodea (e.g. Colpoda)
• Class Litostomatea
  • Subclass Haptoria (e.g. Didinium)
  • Subclass Rhynchostomatia
  • Subclass Trichostomatia (e.g. Balantidium)
• Class Nassophorea
• Class Phyllopharyngea
  • Subclass Chonotrichia
  • Subclass Cyrtophoria
  • Subclass Rhynchodia
  • Subclass Suctoria (e.g. Podophyra)
  • Subclass Synhymenia
• Class Oligohymenophorea
  • Subclass Apostomatia
  • Subclass Astomatia
  • Subclass Hymenostomatia (e.g. Tetrahymena)
  • Subclass Peniculia (e.g. Paramecium)
  • Subclass Peritrichia (e.g. Vorticella)
  • Subclass Scuticociliatia
• Class Plagiopylea
• Class Prostomatea (e.g. Coleps)
• Class Protocruziea
• Class Spirotrichea
  • Subclass Choreotrichia
  • Subclass Euplotia
  • Subclass Hypotrichia
  • Subclass Licnophoria
  • Subclass Oligotrichia
  • Subclass Phacodiniidea
  • Subclass Protohypotrichia

Some old classifications included Opalinidae in the ciliates. The fundamental difference between multiciliate flagellates (e.g., hemimastigids, Stephanopogon, Multicilia, opalines) and ciliates is the presence of macronuclei in ciliates alone.[23]