Karyorelictea is a
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 differently ...
of
ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a differen ...
s in the subphylum
Postciliodesmatophora
Postciliodesmatophora is a subphylum of ciliates.
Members of this subphylum share stacks of postciliary microtubular ribbons associated with somatic kinetosome
A basal body (synonymous with basal granule, kinetosome, and in older cytologic ...
. Most species are members of the
microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, ''
Loxodes'', is found in freshwater.
The majority of karyorelict taxa have not been cultivated in the laboratory, although clonal lines of ''
Loxodes'' have been developed.
Systematics
According to Lynn (2008), the Karyorelictea class is divided into three orders:
*
Loxodida
Loxodida is an order of karyorelict ciliates.
The term ''Loxodida'' derives from the ancient greek (), meaning "oblique, tilted".
References
External links
*
{{Taxonbar, from=Q18573102
Karyorelictea
Ciliate orders ...
, containing the families
Cryptopharyngidae and
Loxodidae;
*
Protoheterotrichida
Protoheterotrichida is an order of karyorelict ciliates. It contains the family Geleiidae.
The term ''Protoheterotrichida'' derives from the three ancient Greek words (), meaning "first, earliest, most prominent", (), meaning "another, diffe ...
, containing the families
Aveliidae and
Geleiidae
Geleiidae is a family of karyorelict ciliates. It is sometimes synonymized with family Aveliidae.
Geleiidae are very large ciliates (200–5000 µm), with a cylindrical shape, and a thinner, often beak-shaped, apical region. They are charac ...
;
*
Protostomatida, containing the families
Kentrophoridae
''Kentrophoros'' is a genus of ciliates in the class Karyorelictea. Ciliates in this genus lack a distinct oral apparatus and depend primarily on symbiotic bacteria for their nutrition.
Systematics
''Kentrophoros'' is the sole genus in the ...
and
Trachelocercidae
Trachelocercidae is a family of ciliates in the 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 ...
.
These three
orders
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
were defined
morphologically, and have been confirmed with
molecular phylogenetics
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
.
An additional family,
Wilbertomorphidae
Wilbertomorphidae is a family of karyorelictean ciliates. The family is monotypic, because it contains a single genus ''Wilbertomorpha'' with a single known species, ''Wilbertomorpha colpoda''.
Like most karyorelicteans, ''Wilbertomorpha'' lives ...
, is of uncertain affiliation and has not been assigned to an order.
Nuclear dimorphism
All ciliates, including karyorelicteans, possess two different kinds of
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
, which separate the functions of gene expression and sexual recombination. The
macronuclei, or
somatic nuclei, are the site of
transcription, while the smaller
micronuclei, or
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 ...
nuclei, are only active during sexual reproduction, where they first undergo meiosis to form
gametic nuclei, which are exchanged when two mating cells conjugate. Two gametic nuclei fuse to form a
zygotic nucleus, which divides by mitosis into two daughter nuclei, one of which develops into a new micronucleus and the other into a macronucleus; the old macronucleus typically disintegrates (see
main article).
In most ciliates, a macronucleus can divide during asexual reproduction to form new daughter macronuclei, through a process called
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 ...
. However, in karyorelicteans, the macronuclei are unable to divide. Instead, they must be produced by division and differentiation of a micronucleus every time, even during asexual reproduction.
Because of their non-dividing somatic macronuclei, the karyorelicteans were thought to represent an intermediate evolutionary stage between the hypothetical ancestor of ciliates that did not have nuclear dualism, and the other more "advanced" ciliates which had both nuclear dualism and macronuclei that could divide by amitosis. The name of the group therefore makes reference to their supposedly "primitive" nuclei. This theory has since been superseded, as molecular phylogenies have shown that the karyorelicteans are not the most "primitive" or basally-branching group of ciliates.
Ecology
Almost all karyorelictean species, except ''Loxodes'', have been described from the marine
interstitial habitat
An interstitial space or interstice is a space between structures or objects.
In particular, interstitial may refer to:
Biology
* Interstitial cell tumor
* Interstitial cell, any cell that lies between other cells
* Interstitial collagenase, ...
, where they live in the pore-water spaces between sediment grains. Animals from such habitats are known as
meiofauna
Meiobenthos, also called meiofauna, are small benthic invertebrates that live in both marine and fresh water environments. The term ''meiofauna'' loosely defines a group of organisms by their size, larger than microfauna but smaller than macrof ...
, and karyorelicteans have many morphological similarities to meiofaunal animals despite being protists: most karyorelicteans are relatively large (1 mm or more in length), have a worm-like (vermiform) body shape with an elongated tail, and exhibit thigmotactic behavior. Most karyorelicteans feed on bacteria or algae, and prefer
microaerobic
A microaerophile is a microorganism that requires environments containing lower levels of dioxygen than that are present in the atmosphere (i.e. < 21% O2; typically 2–10% O2) for optimal growth.[ A more r ...]
conditions. However, one genus, ''
Kentrophoros'', lacks an oral apparatus and feeds instead on symbiotic
sulfur-oxidizing bacteria that are attached to one side of the cell.
Etymology
The class name ''Karyorelictea'' derives from the
ancient greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
(), meaning "hard-shelled seed, or nucleus", and from the Latin , meaning 'abandoned'.
Alternative genetic code
An alternative
genetic code
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
is used by the
nuclear genome
Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. I ...
of some karyorelictid ciliates (e.g. ''
Parduczia
''Parduczia'' is a genus of karyorelict ciliates in the family Geleiidae.
''Parduczia'' species are filiform to serpentiform ciliates characterized by their giant size (1200 to 2500 µm on average) and their very long buccal split.
The genus n ...
'' sp.).
This code corresponds to
translation table 27 and involves the reassignment of three codons:
*
UAA
into
Gln (Q)
;
*
UAG
into
Gln (Q)
;
*
UGA
into
Trp (W)
or
Termination (*)
.
References
{{Taxonbar, from=Q117501
Ciliate classes