An inversion is a
chromosome rearrangement
In genetics, a chromosomal rearrangement is a mutation that is a type of chromosome abnormality involving a change in the structure of the native chromosome. Such changes may involve several different classes of events, like Deletion (genetics), de ...
in which a segment of a chromosome becomes inverted within its original position. An inversion occurs when a chromosome undergoes a two breaks within the chromosomal arm, and the segment between the two breaks inserts itself in the opposite direction in the same chromosome arm. The breakpoints of inversions often happen in regions of repetitive nucleotides, and the regions may be reused in other inversions. Chromosomal segments in inversions can be as small as 100 kilobases or as large as 100 megabases. The number of genes captured by an inversion can range from a handful of genes to hundreds of genes.
Inversions can happen either through
ectopic recombination, chromosomal breakage and repair, or
non-homologous end joining
Non-homologous end joining (NHEJ) is a pathway that repairs double-strand breaks in DNA. NHEJ is referred to as "non-homologous" because the break ends are directly ligated without the need for a homologous template, in contrast to homology direct ...
.
Inversions are of two types:
paracentric and
pericentric. Paracentric inversions do not include the
centromere
The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
, and both breakpoints occur in one arm of the chromosome. Pericentric inversions span the centromere, and there is a breakpoint in each arm
.
Inversions usually do not cause any abnormalities in carriers, as long as the rearrangement is balanced, with no extra or missing DNA. However, in individuals which are
heterozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
for an inversion, there is an increased production of abnormal
chromatids
A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...
(this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility, due to production of unbalanced gametes. Inversions do not involve either loss or gain of genetic information; they simply rearrange the linear DNA sequence.
Detection
Cytogenetic techniques may be able to detect inversions, or inversions may be inferred from
genetic analysis
Genetic analysis is the overall process of studying and researching in fields of science that involve genetics and molecular biology. There are a number of applications that are developed from this research, and these are also considered parts of ...
. Nevertheless, in most species, small inversions go undetected. More recently, comparative genomics has been used to detect chromosomal inversions, by mapping the genome.
Population genomics may also be used to detect inversions, using areas of high
linkage disequilibrium as indicators for possible inversion sites. Human families that may be carriers of inversions may be offered
genetic counseling
Genetic counseling is the process of investigating individuals and families affected by or at risk of genetic disorders to help them understand and adapt to the medical, psychological and familial implications of genetic contributions to disease; t ...
and
genetic testing
Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...
.
History
The first evidence of a chromosomal inversion was found in 1921 by
Alfred Sturtevant in
''Drosophila melanogaster''.
Since then, inversions have been found in a all
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 ...
.
When discovered by Sturtevant, inversions were regarded as areas of recombination suppression.
Originally, these inversions were noted in
polytene chromosomes
Polytene chromosomes are large chromosomes which have thousands of DNA strands. They provide a high level of function in certain tissues such as salivary glands of insects.
Polytene chromosomes were first reported by E.G.Balbiani in 1881. Pol ...
within the salivary glands of heterozygous ''Drosophila melanogaster'' larvae.
In 1970,
Theodosius Dobzhansky
Theodosius Grigorievich Dobzhansky (russian: Феодо́сий Григо́рьевич Добржа́нский; uk, Теодо́сій Григо́рович Добржа́нський; January 25, 1900 – December 18, 1975) was a prominent ...
noted that genes within an inversion had higher fitness than those that are found outside of the inversions, although this is an area that needs further study.
One of the more recent models of inversions is the Kirkpatrick and Barton Model (2006), which states that inversions are selectively advantageous by linking together adaptive alleles. This is in contrast to non-inverted regions, which may allow adaptive and maladaptive alleles to be carried.
Also, inversions may be used as detectors for global climate change. In ''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'' , a study done in 2015 showed that a specific inversion (3R) may contribute to adaptions to climate change. In the species ''
Drosophila subobscura
''Drosophila subobscura'' is a species of fruit fly in the family Drosophilidae. Originally found around the Mediterranean, it has spread to most of Europe and the Near East. It has been introduced into the west coasts of Canada, the United Stat ...
'', researchers have been able to track global climate change by measuring the magnitude and directional shift in chromosome inversion frequencies, relative to temperatures at specific global sites.
Effects on recombination
When an inversion carrying chromosome is paired with a non-inverted homologous chromosome (Inversion heterozygotes) during
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 ...
, they fail to synapse properly and inversion loops are formed. A
crossing-over within the loop can produce unbalanced
gametes. In a paracentric inversion,
recombination results in one dicentric
chromatid
A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...
and one acentric chromatid. During
Anaphase, both recombinants are faced with problems. The acentric chromatid is pulled to one pole or the other, and the dicentric recombinant generates dicentric bridges as it is pulled in two directions.
In a pericentric inversion, similar imbalanced chromosomes are produced. The recombinant chromosomes resulting from these crosses include
deletions and
duplications. The offspring produced by such gametes are mostly inviable, and therefore, recombination is indirectly suppressed within inverted regions.
Evolutionary consequences
The suppressed recombination between inversion heterozygotes provides an opportunity for the independent evolution of the ancestral and inverted arrangements. At the beginning, the inverted arrangement lacks variation, while the ancestral one does not. If the inverted
haplotype
A haplotype ( haploid genotype) is a group of alleles in an organism that are inherited together from a single parent.
Many organisms contain genetic material ( DNA) which is inherited from two parents. Normally these organisms have their DNA or ...
is not lost (eg. due to
drift
Drift or Drifts may refer to:
Geography
* Drift or ford (crossing) of a river
* Drift, Kentucky, unincorporated community in the United States
* In Cornwall, England:
** Drift, Cornwall, village
** Drift Reservoir, associated with the village
...
), the variation in the inverted arrangement is increased over time, and recombination rate in the inverted region is somewhat restored as more homozygotes are introduced.
Inversion polymorphism can be established in two ways. Genetic drift or selection can result in
fixation of an inversion in a local population. Inversion polymorphism can result from
gene flow
In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent a ...
between this population and a population without the inversion.
Balancing selection can also result in inversion polymorphism by frequency dependence or
overdominance
Introduction
Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage regulated by a singl ...
.
The
fitness differences between the inverted and the ancestral chromosome can either produce a stable polymorphism or can result in the fixation of one or the other chromosome.
Inversions have been essential to
sex chromosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
evolution. In mammals, The Y chromosome is unable to recombine with the X chromosome, almost along its entire length. This non-recombining portion results from a series of inversions that overlap. Decreased recombination rate between sex determining
loci and sex-anatagonistic genes is favored by selection. This causes linkage disequilibrium between the male determining locus and an
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
at another locus that is beneficial to males. This can happen through inversions resulting in a non-recombining block including both loci, as is the case in the mammalian Y chromosome.
Inversions can also be essential in the origination of new sex chromosomes. They can cause linkage disequilibrium between a sex-determining mutation and sex-antagonistic loci and create a new sex chromosome from an autosome.
Inversions can be involved in
speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
in multiple ways. Since heterozygote inversions can be
underdominant, they can cause hybrid fitness loss resulting in
post-zygotic isolation
The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensu ...
. They can also accumulate selected differences between species, causing both pre- and post-zygotic isolation.
Inversions often form geographical clines in frequency which can hint to their role in local adaptation. A prominent instance of such a cline is inversion 3RP in ''Drosophila melanogaster'' that can be observed in three different continents.
When an inversion contains two or more locally adaptive alleles, it can be selected and spread. For example; in the butterfly ''
Heliconius numata
''Heliconius numata'', the Numata longwing, is a brush-footed butterfly species belonging to the family Nymphalidae, subfamily Heliconiinae.
Distribution and habitat
This species is native to most of South America, from Venezuela to southern B ...
,'' 18 genes controlling colors are linked together by inversions as together they confer higher fitness.
Nomenclature
![Three chromosomal abnormalities with ISCN nomenclature](https://upload.wikimedia.org/wikipedia/commons/6/6b/Three_chromosomal_abnormalities_with_ISCN_nomenclature.png)
The
International System for Human Cytogenomic Nomenclature The International System for Human Cytogenomic Nomenclature (previously International System for Human Cytogenetic Nomenclature), ISCN in short, is an international standard for human chromosome nomenclature, which includes band names, symbols and a ...
(ISCN) is an international standard for
human chromosome nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
, which includes band names, symbols and abbreviated terms used in the description of human chromosome and chromosome abnormalities. Abbreviations include ''inv'' for inversions.
Notable cases
* Brenden Adams: former holder of the
Guinness World Record
''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a reference book published annually, listing world ...
for tallest teenager. His height is caused by an inversion of
chromosome 12.
* An example of chromosomal Inversion in organisms is demonstrated in the insect, ''
Coelopa frigida
''Coelopa frigida'' is a species of seaweed fly or kelp fly. It is the most widely distributed species of seaweed fly. It can be found on most shorelines in the temperate Northern Hemisphere. Other species of seaweed flies include ''Coelopa nebula ...
''. This particular species of ''Coelopa'' have a variation of chromosomal inversions that allow the species to create a series of physical differences. Individual ''C. frigida'' that are larger do not undergo a chromosomal inversion, whereas individuals that are smaller do undergo a chromosomal inversion.
References
Further reading
*
{{Mutation
Chromosomes
Chromosomal abnormalities
Modification of genetic information