The ciliates are a group of
alveolate
The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and Biological classification, superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with ...
s characterized by the presence of hair-like organelles called
cilia
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
, which are identical in structure to
eukaryotic flagella, but are in general 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
Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialized ...
only have them for part of their
life cycle
Life cycle, life-cycle, or lifecycle may refer to:
Science and academia
*Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring
*Life-cycle hypothesis, ...
) and are variously used in swimming, crawling, attachment, feeding, and sensation.
Ciliates are an important group of
protist
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s, common almost anywhere there is water—in lakes, ponds, oceans, rivers, and soils. About 4,500 unique free-living species have been described, and the potential number of extant species is estimated at 27,000–40,000. Included in this number are many
ectosymbiotic and
endosymbiotic
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...
species, as well as some
obligate {{wiktionary, obligate
As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as:
* Obligate aerobe, an organism that cannot survive without oxygen
* Obligate anaerobe, an organism that ...
and
opportunistic
Opportunism is the practice of taking advantage of circumstances – with little regard for principles or with what the consequences are for others. Opportunist actions are expedient actions guided primarily by self-interested motives. The term ...
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
s. Ciliate species range in size from as little as 10
µm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
in some
colpodea
The Colpodea are a class of ciliates, of about 200 species common in freshwater and soil habitats. The body cilia are typically uniform, and are supported by dikinetids of characteristic structure, with cilia on both kinetosomes. The mouth may ...
ns to as much as 4 mm in length in some
geleiids, and include some of the most
morphologically complex protozoans.
In most systems of
taxonomy
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
, "Ciliophora" is ranked as a
phylum
In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
under any of several
kingdoms
Kingdom commonly refers to:
* A monarchy ruled by a king or queen
* Kingdom (biology), a category in biological taxonomy
Kingdom may also refer to:
Arts and media Television
* ''Kingdom'' (British TV series), a 2007 British television drama s ...
, including
Chromista
Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', such ...
,
Protista
A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...
or
Protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
.
In some older systems of classification, such as the influential taxonomic works of
Alfred Kahl
Alfred Detlef Fritz Kahl (18 February 1877 – November, 1946) was a German schoolteacher who took up microscopy in mid-life and became a leading authority on ciliated protozoa. In a burst of scientific productivity that lasted just nine years, h ...
, ciliated protozoa are placed within the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
"Ciliata" (a term which can also refer to a
genus of fish). In the taxonomic scheme endorsed by the
International Society of Protistologists
International is an adjective (also used as a noun) meaning "between nations".
International may also refer to:
Music Albums
* ''International'' (Kevin Michael album), 2011
* ''International'' (New Order album), 2002
* ''International'' (The T ...
, which eliminates formal
rank
Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as:
Level or position in a hierarchical organization
* Academic rank
* Diplomatic rank
* Hierarchy
* H ...
designations such as "phylum" and "class", "Ciliophora" is an unranked
taxon
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
within
Alveolata.
Cell structure
Nuclei
Unlike most other
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, ciliates have two different sorts of
nuclei: a tiny,
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
micronucleus
Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
(the "generative nucleus", which carries the
germline
In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...
of the cell), and a large,
ampliploid macronucleus
A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugation, t ...
(the "vegetative nucleus", which takes care of general cell regulation, expressing the
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
of the organism).
The latter is generated from the micronucleus by amplification of the
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the
small nuclear RNA
Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribe ...
for vegetative growth.
Division of the macronucleus occurs in most ciliate species, apart from those in class Karyorelictea, whose macronuclei are replaced every time the cell divides.
Macronuclear division is accomplished by
amitosis
Amitosis ('' a-'' + ''mitosis''), also called karyostenosis or direct cell division or binary fission, is cell proliferation that does not occur by mitosis, the mechanism usually identified as essential for cell division in eukaryotes. The polyploi ...
, and the segregation of the
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s occurs by a process whose mechanism is unknown.
After a certain number of generations (200–350, in ''Paramecium aurelia'', and as many as 1,500 in ''Tetrahymena
'') the cell shows signs of aging, and the macronuclei must be regenerated from the micronuclei. Usually, this occurs following ''
conjugation
Conjugation or conjugate may refer to:
Linguistics
* Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
* Complex conjugation, the chang ...
'', after which a new macronucleus is generated from the post-conjugal micronucleus.
Cytoplasm
Food vacuole
The food vacuole, or digestive vacuole, is an organelle found in simple eukaryotes such as protists. This organelle is essentially a lysosome. During the stage of the symbiont parasites' lifecycle where it resides within a human (or other mammalia ...
s are formed through
phagocytosis
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
and typically follow a particular path through the cell as their contents are digested and broken down by
lysosomes
A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane prote ...
so the substances the
vacuole
A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...
contains are then small enough to
diffuse
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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
The anal pore or cytoproct is a structure in various single-celled eukaryotes where waste is ejected after the nutrients from food have been absorbed into the cytoplasm.
In ciliates, the anal pore (cytopyge) and cytostome are the only regions of ...
) is discharged by
exocytosis
Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use o ...
. Most ciliates also have one or more prominent
contractile vacuole
A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists and in unicellular algae. It was previously known as pulsatile or pulsating vacuole.
Overview
The contractile va ...
s, which collect water and expel it from the cell to maintain
osmotic pressure
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure ...
, or in some function to maintain ionic balance. In some genera, such as ''
Paramecium
''
''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'', these have a distinctive star shape, with each point being a collecting tube.
Specialized structures in ciliates
Mostly, body cilia are arranged in ''mono-'' and ''
dikinetids'', which respectively include one and two
kinetosome
A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor W ...
s (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
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
s involved in coordinating the cilia. In some forms there are also body polykinetids, for instance, among the
spirotrich
The spirotrichs are a large and diverse group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down t ...
s where they generally form bristles called
cirri
Giovanni Battista Cirri (1 October 1724 – 11 June 1808) was an Italian cellist and composer in the 18th century.
Biography
Cirri was born in Forlì in the Emilia-Romagna Region of Italy. He had his first musical training with his brother ...
.
The infraciliature is one of the main components of the
cell cortex
The cell cortex, also known as the actin cortex, cortical cytoskeleton or actomyosin cortex, is a specialized layer of cytoplasmic proteins on the inner face of the cell membrane. It functions as a modulator of membrane behavior and cell surface p ...
. 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
A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
and
extrusome Extrusomes are membrane-bound structures in some eukaryotes which, under certain conditions, discharge their contents outside the cell. There are a variety of different types, probably not homologous, and serving various functions.
Notable extru ...
s 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
The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...
, and
dinoflagellate
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...
s. These superficially dissimilar groups make up the
alveolate
The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and Biological classification, superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with ...
s.
Feeding
Most ciliates are
heterotroph
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
s, feeding on smaller organisms, such as
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
and
alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...
e, and detritus swept into the oral groove (mouth) by modified oral cilia. This usually includes a series of
membranelle Membranelles (also membranellae) are structures found around the mouth, or cytostome, in ciliates. They are typically arranged in series, to form an "adoral zone of membranelles," or AZM, on the left side of the buccal cavity (peristome). The membra ...
s 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.
Feeding techniques vary considerably, however. Some ciliates are mouthless and feed by absorption (
osmotrophy
Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. ...
), while others are predatory and feed on other protozoa and in particular on other ciliates. Some ciliates parasitize
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s, although only one species, ''
Balantidium coli
''Balantidium coli'' is a parasitic species of ciliate alveolates that causes the disease balantidiasis. It is the only member of the ciliate phylum known to be pathogenic to humans.
Morphology
''Balantidium coli'' has two developmental stage ...
'', is known to cause disease in humans.
Reproduction and sexual phenomena
Reproduction
Ciliates reproduce
asexually, by various kinds of
fission.
During fission, the micronucleus undergoes
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
and the macronucleus elongates and undergoes
amitosis
Amitosis ('' a-'' + ''mitosis''), also called karyostenosis or direct cell division or binary fission, is cell proliferation that does not occur by mitosis, the mechanism usually identified as essential for cell division in eukaryotes. The polyploi ...
(except among the
Karyorelictea
Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Loxodes'', is foun ...
n 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.
![Unk](https://upload.wikimedia.org/wikipedia/commons/5/55/Unk.cilliate.jpg)
Typically, the cell is divided transversally, with the
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
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
Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
'' (the emergence of small ciliated offspring, or "swarmers", from the body of a mature parent); ''
strobilation Strobilisation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths. This mode of reproduction is characterized by high offspr ...
'' (multiple divisions along the cell body, producing a chain of new organisms); and ''palintomy'' (multiple fissions, usually within a
cyst
A cyst is a closed sac, having a distinct envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac (like the manner in which ...
).
Fission may occur spontaneously, as part of the vegetative
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
. Alternatively, it may proceed as a result of self-fertilization (
autogamy
Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
), or it may follow
conjugation
Conjugation or conjugate may refer to:
Linguistics
* Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
* Complex conjugation, the chang ...
, a sexual phenomenon in which ciliates of compatible
mating type
Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct Sex, sexes. They also occur in macro-organisms such as fungi.
Definition
Mating types are the microorganism equivalent ...
s 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.
Conjugation
;Overview
Ciliate conjugation is a sexual phenomenon that results in
genetic recombination
Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryo ...
and nuclear reorganization within the cell.
During conjugation, two ciliates of a compatible mating type form a bridge between their
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
s. The micronuclei undergo
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
, the macronuclei disappear, and
haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
micronuclei are exchanged over the bridge. In some ciliates (peritrichs,
chonotrich
Chonotrichia is a subclass of phyllopharyngeid ciliates. These single-celled organisms are sessile at maturity and usually live on crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as deca ...
s and some
suctoria
Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialized ...
ns), conjugating cells become permanently fused, and one conjugant is absorbed by the other.
In most ciliate groups, however, the cells separate after conjugation, and both form new macronuclei from their micronuclei.
Conjugation and autogamy are always followed by fission.
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
Sessility, or sessile, may refer to:
* Sessility (motility), organisms which are not able to move about
* Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant
* Sessility (medicine), tumors and polyps that ...
.
;Stages of conjugation
![Stages of ciliate conjugation](https://upload.wikimedia.org/wikipedia/commons/9/96/Stages_of_ciliate_conjugation.svg)
In ''
Paramecium caudatum
''Paramecium caudatum'' is a species of unicellular protist in the phylum Ciliophora. They can reach 0.33 mm in length and are covered with minute hair-like organelles called cilia. The cilia are used in locomotion and feeding. The species i ...
'', the stages of conjugation are as follows (see diagram at right):
# Compatible mating strains meet and partly fuse
# The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
# Three of these micronuclei disintegrate. The fourth undergoes mitosis.
# The two cells exchange a micronucleus.
# The cells then separate.
# The micronuclei in each cell fuse, forming a diploid micronucleus.
# Mitosis occurs three times, giving rise to eight micronuclei.
# Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.
# Binary fission occurs twice, yielding four identical daughter cells.
DNA rearrangements (gene scrambling)
Ciliates contain two types of nuclei:
somatic
Somatic may refer to:
* Somatic (biology), referring to the cells of the body in contrast to the germ line cells
** Somatic cell, a non-gametic cell in a multicellular organism
* Somatic nervous system, the portion of the vertebrate nervous sys ...
"
macronucleus
A macronucleus (formerly also meganucleus) is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, such as metabolism. During conjugation, t ...
" and the
germline
In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...
"
micronucleus
Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
". 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.
![Development of the Oxytricha macronuclear genome](https://upload.wikimedia.org/wikipedia/commons/9/9f/Development_of_the_Oxytricha_macronuclear_genome.jpg)
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 biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. In ''
Tetrahymena
''Tetrahymena'', a Unicellular organism, unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrah ...
'', the micronucleus has 10 chromosomes (five 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 6,000 IESs and about 15% of micronuclear DNA is eliminated during this process. The process is guided by
small RNA
Small RNA (sRNA) are polymeric RNA molecules that are less than 200 nucleotides in length, and are usually non-coding
Non-coding DNA (ncDNA) sequences are components of an organism's DNA that do not encode protein sequences. Some non-coding DNA ...
s and
epigenetic
In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
marks.
In
spirotrich
The spirotrichs are a large and diverse group of ciliate protozoa. They typically have prominent oral cilia in the form of a series of polykinetids, called the adoral zone of membranelles, beginning anterior to the oral cavity and running down t ...
ciliates (such as ''
Oxytricha
''Oxytricha'' is a genus of ciliates in the family Oxytrichidae.
Genomics
The draft macronuclear genome of ''Oxytricha trifallax'' was published in 2013.
Species
References
Spirotrichea
Ciliate genera
{{Ciliate-stub ...
''), 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
Inversion or inversions may refer to:
Arts
* , a French gay magazine (1924/1925)
* ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas
* Inversion (music), a term with various meanings in music theory and musical set theory
* ...
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.
Aging
ln clonal populations of ''Paramecium'', aging occurs over successive generations leading to a gradual loss of vitality, unless the cell line is revitalized by conjugation or
autogamy
Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species o ...
. In ''Paramecium tetraurelia'', the clonally aging line loses vitality and expires after about 200 fissions, if the cell line is not rejuvenated by conjugation or self-fertilization. The basis for clonal aging was clarified by the
transplantation experiments of Aufderheide in 1986 who demonstrated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Additional experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn demonstrated that, during clonal aging,
DNA damage
DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA da ...
increases dramatically. Thus, DNA damage appears to be the cause of aging in ''P. tetraurelia''.
Fossil record
Until recently, the oldest ciliate fossils known were
tintinnid
Tintinnids are ciliates of the choreotrich order Tintinnida, distinguished by vase-shaped shells, the name deriving from a Latin source meaning a small tinkling bell, that are called'' loricae'', which are mostly protein but may incorporate min ...
s from the
Ordovician period
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
The ...
. In 2007, Li ''et al.'' published a description of fossil ciliates from the
Doushantuo Formation
The Doushantuo Formation (formerly transcribed as Toushantuo or Toushantou, from ) is a geological formation in western Hubei, eastern Guizhou, southern Shaanxi, central Jiangxi, and other localities in China. It is known for the fossil Lagerst ...
, about 580 million years ago, in the
Ediacaran period
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
. These included two types of tintinnids and a possible ancestral suctorian. A fossil ''
Vorticella
''Vorticella'' is a genus of bell-shaped ciliates that have stalks to attach themselves to substrates. The stalks have contractile myonemes, allowing them to pull the cell body against substrates. The formation of the stalk happens after the fr ...
'' has been discovered inside a leech cocoon from the
Triassic period
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, about 200 million years ago.
Phylogeny
According to the 2016 phylogenetic analysis,
[ Mesodiniea is consistently found as the sister group to all other ciliates. Additionally, two big sub-groups are distinguished inside subphylum ]Intramacronucleata
Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during binary fission of the cell. In ciliates of this subphylum, division of the macronucleus is achiev ...
: SAL ( Spirotrichea+Armophorea
Armophorea is a class of ciliates in the subphylum Intramacronucleata.
. It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs becau ...
+Litostomatea
The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as ''Didinium'', a species of which preys primarily on the ciliate ''P ...
) and CONthreeP
Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Conthreep ...
or Ventrata
Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Conthreep r ...
(Colpodea
The Colpodea are a class of ciliates, of about 200 species common in freshwater and soil habitats. The body cilia are typically uniform, and are supported by dikinetids of characteristic structure, with cilia on both kinetosomes. The mouth may ...
+Oligohymenophorea
The Oligohymenophorea are a large class of ciliates. There is typically a ventral groove containing the mouth and distinct oral cilia, separate from those of the body. These include a paroral membrane to the right of the mouth and membranelles, ...
+Nassophorea
The Nassophorea are a class of ciliates. Members are free-living, usually in freshwater but also in marine and soil environments. The mouth is anterior ventral and leads to a curved cytopharynx supported by a prominent palisade of rods or ''ne ...
+Phyllopharyngea
The Phyllopharyngea are a class of ciliates, some of which are extremely specialized. Motile cells typically have cilia restricted to the ventral surface, or some part thereof, arising from monokinetids with a characteristic ultrastructure. In ...
+Plagiopylea
The plagiopylids are a small order of ciliates, including a few forms common in anaerobic habitats.
The body cilia are dense, and arise from monokinetids with an entirely unique ultrastructure; one or two rows of dikinetids run into the oral cav ...
+Prostomatea
Prostomatea is a class of ciliates. It includes the genera ''Coleps
''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate.
Description
Sp ...
).[ The class ]Protocruziea
Protocruziea is a class of ciliates in the subphylum Intramacronucleata
Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during binary fission of the ...
is found as the sister group to Ventrata
Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Conthreep r ...
/CONthreeP
Ventrata is an infraphylum of ciliates inside the subphylum Intramacronucleata that unites the classes Phyllopharyngea, Colpodea, Nassophorea, Prostomatea, Plagiopylea and Oligohymenophorea. It is equivalent to the clade CONthreeP or Conthreep ...
. The class Cariacotrichea was excluded from the analysis, but it was originally established as part of Intramacronucleata
Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during binary fission of the cell. In ciliates of this subphylum, division of the macronucleus is achiev ...
[.]
The odontostomatids were identified in 2018 as its own class Odontostomatea, related to Armophorea
Armophorea is a class of ciliates in the subphylum Intramacronucleata.
. It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs becau ...
.
Classification
Several different classification schemes have been proposed for the ciliates. The following scheme is based on a molecular phylogenetic analysis
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
of up to four genes from 152 species representing 110 families:[
* Class Mesodiniea (e.g. '']Mesodinium
Mesodinium is a genus of ciliates that are widely distributed and are abundant in marine and brackish
waters. Currently, six marine species of ''Mesodinium'' have been described and grouped by nutritional mode: plastidic (''M. chamaeleon'', ''M ...
'')
Subphylum Postciliodesmatophora
* Class Heterotrich
The heterotrichs are a class of ciliates. They typically have a prominent adoral zone of membranelles circling the mouth, used in locomotion and feeding, and shorter cilia on the rest of the body. Many species are highly contractile, and are ...
ea (e.g. ''Stentor
In Greek mythology, Stentor (Ancient Greek: Στέντωρ; ''gen''.: Στέντορος) was a herald of the Greek forces during the Trojan War.
Mythology
Stentor is mentioned briefly in Homer's ''Iliad'' in which Hera in the guise of Stent ...
'')
* Class Karyorelictea
Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''Loxodes'', is foun ...
Subphylum Intramacronucleata
* Class Armophorea
Armophorea is a class of ciliates in the subphylum Intramacronucleata.
. It was first resolved in 2004 and comprises three orders: Metopida, Clevelandellida, and Armophorida. Previously members of this class were thought to be heterotrichs becau ...
* Class Odontostomatea[ (e.g. '' Discomorphella'', '' Saprodinium'')
* Class Cariacotrichea (only one species, '']Cariacothrix caudata
''Cariacothrix'' is a genus of ciliates in the subphylum Intramacronucleata. It contains only one species, ''Cariacothrix caudata'', and is the only genus in the monotypic family Cariacotrichidae, order Cariacotrichida, and class Cariacotrichea.
...
'')
* Class Muranotrichea
Muranotrichea is a class of free-living marine anaerobic ciliates, that, together with the classes Parablepharismea and Armophorea, form a major clade of obligate anaerobes within the SAL group ( Spirotrichea, Armophorea, and Litostomatea).
Mur ...
* Class Parablepharismea
Parablepharismea is a class of free-living marine and brackish anaerobic ciliates that form a major clade of obligate anaerobes within the SAL group ( Spirotrichea, Armophorea, and Litostomatea), together with the classes Muranotrichea and Armo ...
* Class Colpodea
The Colpodea are a class of ciliates, of about 200 species common in freshwater and soil habitats. The body cilia are typically uniform, and are supported by dikinetids of characteristic structure, with cilia on both kinetosomes. The mouth may ...
(e.g. ''Colpoda
Colpoda is a genus of ciliates in the class Colpodea, order Colpodida, and family Colpodidae.
Description
''Colpoda'' are distinctly reniform (kidney-shaped) and are strongly convex on one side, concave on the other. The concave side often ...
'')
* Class Litostomatea
The Litostomatea are a class of ciliates. The group consists of three subclasses: Haptoria, Trichostomatia and Rhynchostomatia. Haptoria includes mostly carnivorous forms such as ''Didinium'', a species of which preys primarily on the ciliate ''P ...
** Subclass Haptoria (e.g. ''Didinium
''Didinium'' is a genus of unicellular ciliates with at least ten accepted species. All are free-living carnivores. Most are found in fresh and brackish water, but three marine species are known. Their diet consists largely of ''Paramecium'', ...
'')
** Subclass Rhynchostomatia
** Subclass Trichostomatia (e.g. ''Balantidium
''Balantidium'' is a genus of ciliates. It contains the parasitic species ''Balantidium coli'', the only known cause of balantidiasis.
History
The first-described species of ''Balantidium'', ''B. entozoon'', was described in 1838 by Ehrenberg as ...
'')
* Class Nassophorea
The Nassophorea are a class of ciliates. Members are free-living, usually in freshwater but also in marine and soil environments. The mouth is anterior ventral and leads to a curved cytopharynx supported by a prominent palisade of rods or ''ne ...
* Class Phyllopharyngea
The Phyllopharyngea are a class of ciliates, some of which are extremely specialized. Motile cells typically have cilia restricted to the ventral surface, or some part thereof, arising from monokinetids with a characteristic ultrastructure. In ...
** Subclass Chonotrichia
Chonotrichia is a subclass of phyllopharyngeid ciliates. These single-celled organisms are sessile at maturity and usually live on crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as deca ...
** Subclass Cyrtophoria
** Subclass Rhynchodia
The Rhynchodia are a subclass of ciliates in the class Phyllopharyngea
The Phyllopharyngea are a class of ciliates, some of which are extremely specialized. Motile cells typically have cilia restricted to the ventral surface, or some part th ...
** Subclass Suctoria
Suctoria are ciliates that become sessile in their developed stage and then lose their redundant cilia. They feed by extracellular digestion. They were originally thought to feed by suction – hence their name. In fact, they use specialized ...
(e.g. '' Podophyra'')
** Subclass Synhymenia
* Class Oligohymenophorea
The Oligohymenophorea are a large class of ciliates. There is typically a ventral groove containing the mouth and distinct oral cilia, separate from those of the body. These include a paroral membrane to the right of the mouth and membranelles, ...
** Subclass Apostomatia
** Subclass Astomatia
Astomes (order Astomatida) are a group of ciliate eukaryotes commonly found in the guts of annelid worms, especially oligochaetes, and other invertebrates. As their name implies, these parasites are characterized by an absence of mouth. The cell i ...
** Subclass Hymenostomatia
The hymenostomes are an order of ciliate protozoa. Most are free-living in freshwater, such as the commonly studied genus ''Tetrahymena'', but some are parasitic on fish or aquatic invertebrates. Among these is the important species ''Ichthyoph ...
(e.g. ''Tetrahymena
''Tetrahymena'', a Unicellular organism, unicellular eukaryote, is a genus of free-living ciliates. The genus Tetrahymena is the most widely studied member of its phylum. It can produce, store and react with different types of hormones. Tetrah ...
'')
** Subclass Peniculia (e.g. ''Paramecium
''
''Paramecium'' ( , ; also spelled ''Paramoecium'') is a genus of eukaryotic, unicellular ciliates, commonly studied as a representative of the ciliate group. ''Paramecia'' are widespread in freshwater, brackish, and marine environments and a ...
'')
** Subclass Peritrich
The peritrichs (Latin: Peritrichia) are a large and distinctive group of ciliates.
The peritrichs were first defined by Friedrich von Stein in 1859. Initially they were considered spirotrichs, then treated as a separate category, before receiving ...
ia (e.g. ''Vorticella
''Vorticella'' is a genus of bell-shaped ciliates that have stalks to attach themselves to substrates. The stalks have contractile myonemes, allowing them to pull the cell body against substrates. The formation of the stalk happens after the fr ...
'')
** Subclass Scuticociliatia
Scuticociliatia is a subclass of ciliates in the class Oligohymenophorea. Its members are called scuticociliates. These unicellular eukaryotes are marine microorganisms that are usually free-living and widely distributed in the world's oceans. Ar ...
* Class Plagiopylea
The plagiopylids are a small order of ciliates, including a few forms common in anaerobic habitats.
The body cilia are dense, and arise from monokinetids with an entirely unique ultrastructure; one or two rows of dikinetids run into the oral cav ...
* Class Prostomatea
Prostomatea is a class of ciliates. It includes the genera ''Coleps
''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate.
Description
Sp ...
(e.g. ''Coleps
''Coleps'' is a genus of ciliates in the class Prostomatea with barrel-shaped bodies surrounded by regularly arranged plates composed of calcium carbonate.
Description
Species of ''Coleps'' can grow up to 250 µm in length, but are usua ...
'')
* Class Protocruziea
Protocruziea is a class of ciliates in the subphylum Intramacronucleata
Intramacronucleata is a subphylum of ciliates. The group is characterized by the manner in which division of the macronucleus is accomplished during binary fission of the ...
* Class Spirotrichea
**Subclass Choreotrichia
**Subclass Euplotia
**Subclass Hypotrichia
The hypotrichs are a group of ciliated protozoa, common in fresh water, salt water, soil and moss. Hypotrichs possess compound ciliary organelles called " cirri," which are made up of thick tufts of cilia, sparsely distributed on the ventral ...
**Subclass Licnophoria
**Subclass Oligotrichia
**Subclass Phacodiniidea
**Subclass Protohypotrichia
Other
Some old classifications included Opalinidae
The opalines are a small group of peculiar heterokonts, currently assigned to the family Opalinidae, in the order Slopalinida. Their name is derived from the opalescent appearance of these microscopic organisms when illuminated with full sunli ...
in the ciliates. The fundamental difference between multiciliate flagellates
A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and their ...
(e.g., hemimastigid
The Apusozoa are an Obazoa phylum comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the pre ...
s, ''Stephanopogon
''Stephanopogon'' is a genus of flagellated marine protist that superficially resembles a ciliate.
Characteristics
''Stephanopogon'' closely resembles certain ciliates and was originally classified with them (, but is now considered related to h ...
'', ''Multicilia
''Multicilia'' is a flagellated genus of Amoebozoa.
It includes the species ''Multicilia marina''.NCBI
The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the ...
'', opalines) and ciliates is the presence of macronuclei in ciliates alone.
Pathogenicity
The only member of the ciliate phylum known to be pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic to humans is ''Balantidium coli
''Balantidium coli'' is a parasitic species of ciliate alveolates that causes the disease balantidiasis. It is the only member of the ciliate phylum known to be pathogenic to humans.
Morphology
''Balantidium coli'' has two developmental stage ...
'', which causes the disease balantidiasis
Balantidiasis is a protozoan infection caused by infection with ''Balantidium coli''.
Symptoms and signs
Usually asymptomatic in immunocompetent individuals, but the symptoms of balantidiasis include:
* Intermittent diarrhea
* Constipation
* Vomi ...
. It is not pathogenic to the domestic pig, the primary reservoir of this pathogen.
References
Further reading
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External links
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