''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 Plan_(drawing), plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a mea ...

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, the fir ...

and gene expression, and

micronuclei 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 ...

, 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'' ( , ; 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 ...

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

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 ...

size of about 50 Mbp. 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 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. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...

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 use ...

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 Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

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 Species described in 1859 Spirotrichea