Chromosomal inversion

Did You Know?

• The insect Coelopa frigida shows variation in chromosomal inversions that produce physical differences: larger individuals do not undergo an inversion, while smaller ones do.
• Inversions can occur through ectopic recombination between repetitive sequences or through chromosomal breakage followed by non-homologous end joining.
• The breakpoints of inversions often lie in regions of repetitive nucleotides and may be reused in other inversions.

Detection

Cytogenetic techniques such as karyotype analysis, G banding, and fluorescence in situ hybridization can detect inversions, though small inversions often go undetected in most species. Comparative genomics and population genomics, using areas of high linkage disequilibrium as indicators, are also employed. Human families that may carry inversions may be offered genetic counseling and genetic testing.

History

Alfred Sturtevant found the first evidence of a chromosomal inversion in 1921 in Drosophila melanogaster, and inversions have since been identified in all eukaryotes. Initially noted in polytene chromosomes of heterozygous Drosophila larvae, they were regarded as areas of recombination suppression. Theodosius Dobzhansky observed in 1970 that genes within inversions had higher fitness than those outside, though this area requires further study. The Kirkpatrick and Barton Model (2006) proposes that inversions are selectively advantageous by linking adaptive alleles together.

Effects on recombination

In inversion heterozygotes, pairing with a non-inverted homologous chromosome during meiosis leads to inversion loops. Crossing over within the loop produces unbalanced gametes. For paracentric inversions, recombination yields one dicentric and one acentric chromatid, causing problems during anaphase. Pericentric inversions produce recombinant chromosomes with deletions and duplications, and the resulting offspring are mostly inviable, indirectly suppressing recombination within inverted regions.

Evolutionary consequences

Suppressed recombination between inversion heterozygotes allows independent evolution of ancestral and inverted arrangements. Initially, the inverted arrangement lacks variation, but if not lost through drift, variation can increase over time as more homozygotes appear. Inversions have drawn evolutionary interest for their potential role in local adaptation and speciation, as non-recombining haplotypes may harbor multiple co-adapted gene variants, facilitating adaptation to different environments. However, empirically demonstrating such co-adapted variants is difficult because inversion haplotypes do not recombine. This positive effect assumes adaptive variants are indeed co-adapted, a condition likely violated under spatially or temporally varying selection, where the absence of recombination may constrain rather than aid adaptation. The importance of inversions in adaptation remains an open empirical problem. Inversion polymorphism can arise through genetic drift, selection, or gene flow, and balancing selection via frequency dependence or overdominance can also maintain it, with fitness differences between inverted and ancestral chromosomes potentially leading to stable polymorphism or fixation.