Chromosomal inversion

An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end-to-end. An inversion occurs when a single chromosome undergoes a break within the chromosomal arm, and rearranges itself opposite to before the break. The breakpoints of inversions often happen in regions of repetitive nucleotides, and the regions may be reused in other inversion morphs . Inversions can be larger segments of the chromosomes (100 kilobases) or as small as 100 mega-bases. This difference in size can lead to only a few genes captured within the inversion, to hundreds of genes. Inversions can happen either through ectopic recombination, chromosomal breakage and repair, or non-homologous end joining.

Inversions are of two types: paracentric and pericentric. Paracentric inversions do not include the centromere, and both breakpoints occur in one arm of the chromosome. Pericentric inversions include the centromere, and there is a breakpoint in each arm.

A clay model showing why heterozygous inversion loops are visible in polytene chromosome preparations

An inversion loop in the A arm of a chromosome from an Axarus species midge

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 for an inversion, there is an increased production of abnormal chromatids (this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility, due to production of unbalanced gametes. An inversion does not involve a loss of genetic information, but simply rearranges the linear gene sequence.

Detection

Cytogenetic techniques may be able to detect inversions, or inversions may be inferred from genetic analysis. 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 and genetic testing.

History

The first evidence of a chromosomal inversion was in 1921 by Sturtevant. Since then, inversions have been found in a multitude of organisms, ranging from plants such as maize, to birds and humans. When discovered by Sturtevant, inversions were seen as areas of recombination suppression.

Originally, these inversions were noted in polytene chromosomes. These large chromosomes were found in the salivary glands of larval Drosophila, when the larvae is heterozygous. In 1970, Dobzhansky 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 , a study done in 2015 showed that a specific inversion (3R) may contribute to adaptions to climate change. In the species Drosophila subobscura, 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, 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 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 is not lost (eg. due to drift), 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 between this population and a population without the inversion. Balancing selection can also result in inversion polymorphism by frequency dependence or overdominance. 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 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 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 in multiple ways. Since heterozygote inversions can be underdominant, they can cause hybrid fitness loss resulting in post-zygotic isolation. 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, 18 genes controlling colors are linked together by inversions as together they confer higher fitness.

Nomenclature

Three chromosomal abnormalities with ISCN nomenclature, with increasing complexity: (A) A tumour karyotype in a male with loss of the Y chromosome, (B) Prader–Willi Syndrome i.e. deletion in the 15q11-q12 region and (C) an arbitrary karyotype that involves a variety of autosomal and allosomal abnormalities including inversions (abbreviated as inv).

Human karyotype with annotated bands and sub-bands as used for the nomenclature of chromosome abnormalities. It shows dark and white regions as seen on G banding. Each row is vertically aligned at centromere level. It shows 22 homologous autosomal chromosome pairs as well as both the female (XX) and male (XY) versions of the two sex chromosomes.

Further information: Karyotype

The International System for Human Cytogenomic Nomenclature (ISCN) is an international standard for human chromosome nomenclature, 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 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. 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.

Further reading

• Lehtonen S, Myllys L, Huttunen S (2009). "Phylogenetic analysis of non-coding plastid DNAthtjtdjj in the presence of short inversions" (PDF-preview). Phytotaxa. 1: 3–20. doi:10.11646/phytotaxa.1.1.2.