''Sterkiella histriomuscorum'', formerly ''Oxytricha trifallax'', is a

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

species in the genus '' Sterkiella'', known for its highly fragmented genomes which have been used as a

model A model is an informative representation of an object, person, or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin , . Models can be divided in ...

for ciliate genetics.

Genetics

Like all

s, ''O. trifallax'' has two different types of nuclei:

macronuclei 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 ...

, which are the site of

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, often th ...

and gene expression, and

micronuclei A micronucleus is a small cell nucleus, 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 inst ...

, which are only active during sexual reproduction but are otherwise transcriptionally inactive. Macronuclei are formed by the differentiation of micronuclei, which usually involves some degree of RNA-mediated DNA editing. ''O. trifallax'' is the first species sequenced with an unusually high degree of fragmentation in its macronuclear genome. Up to 96% of the micronuclear genome is eliminated during the differentiation into a macronucleus; in comparison, in other ciliates like ''

Paramecium ''Paramecium'' ( , , plural "paramecia" only when used as a Common name, vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and Ocean, marine environments. Paramecia are often abundant in stagna ...

'' only about 30% is eliminated. The macronuclear genome has a

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

size of about 50 Mbp. The

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s of the macronucleus are also unusually short. The macronuclear genome encodes about 18,500 genes, but these are distributed on 16,000 chromosomes, which are called nanochromosomes due to their length. Because of their unusually short nanochromosomes, most of which contain only a single gene, they have been used as a model organism to study

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes (see #Sequences, Sequences). Telomeres are a widespread genetic feature most commonly found in eukaryotes. In ...

s and to screen for non-coding RNA genes. The micronuclear genome has also been sequenced, and contains about 3,500 scrambled genes. Scrambled genes are genes whose individual segments are located in different parts of the micronuclear genome, and therefore have to be "unscrambled" during the DNA editing step into a conventional gene in the macronuclear genome. More than 225,000 individual DNA segments have to be unscrambled during the development of a macronuclear genome from its micronuclear precursor. The

mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

genome has also been sequenced, and is the largest known ciliate mitochondrial genome, with a length of about 70 kbp. Like other ciliate mitochondrial genomes, those of ''O. trifallax'' are linear molecules and contain a number of split genes. Their mitochondria also possess a separate plasmid, which may have been involved in

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

during the evolution of the mitochondrial genome.

Taxonomy and systematics

The taxonomy of ''O. trifallax'' has been contested. It was reclassified as ''Sterkiella histriomuscorum'' on the basis of morphological characteristics, but a

molecular phylogeny 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 ...

supports the original classification as a species of ''Oxytricha''.

References

Other resources

* * {{Taxonbar, from1=Q15378940, from2=Q62003487 Protists described in 1859 Spirotrichea Ciliate species