D6 Chromosome change in evolution | 37 | v4

ABSTRACT

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Karyotypes

Karyotype describes the number and shape (physical morphology) of chromosomes. Diploids have 2n chromosomes, where n is the haploid chromosome number found in gametes. Those chromosomes found equally in both sexes are called autosomes, and there is also a specific number of sex chromosomes. Each species has a characteristic karyotype, the set of chromosome pairs with particular shape and size. Chromosome shape is described by centromere position, from metacentric (centromere near center), through submetacentric (centromere offset), acrocentric (centromere near end), to telocentric (centromere adjacent to telomere). There may be variation in chromosome size and shape (polymorphism) within a species, and it may change over evolutionary time scales. Changes are dramatic in some organisms, and very slight in others.

Polymorphism and fertility

Heterozygosity for chromosomal rearrangements can prevent correct segregation of genes in meiosis. Recombination (chiasmata, crossovers) between the normal and rearranged segments tends to produce unbalanced gametes with two copies or no copies (duplications or deletions) of some chromosome segments. These reduce fertility by failing to produce viable zygotes. This makes it difficult for new rearrangements to become established, and can act as a postmating isolation mechanism between species by causing hybrid sterility. When chromosome polymorphisms persist a special mechanism is usually found that reduces the deleterious effects and maintains the polymorphism.

Mechanisms of rearrangement

Rearrangements require at least two breaks and one rejoin in DNA/chromosomes, but almost anything is possible. The probable causes are illegitimate recombination between dispersed repeated sequences, particularly transposable elements, and incorrect repair of double-strand breaks in DNA caused by radiation or other mutagens. Rearrangements may occur within a chromosome, or involve exchange or transfer between chromosomes.

Deletions

Deletions involve loss of genetic material, which is more or less deleterious, depending on the number and role of the genes which are lost. Acentric chromosome segments (that is, detached from a centromere) are lost.

Duplications

Duplications give an extra copy of a block of genes. They are generally less harmful than deletions, and one copy of the duplicated genes can become mutated and evolve a new role. This is the origin of all new genes throughout most of evolution. Strong selection for multiple copies of a gene (e.g. multidrug resistance) usually detects cells with the necessary multiple duplications (gene amplification), often visible as a heterogeneous staining region on the chromosome.

Centromeric fusions and fissions

Two telocentric or acrocentric chromosomes (with negligible short arms) may fuse at the centromere to produce a single metacentric chromosome (centromere near the middle) or submetacentric (two arms clearly unequal). This is called a centric fusion or Robertsonian translocation. The reverse is centric fission, which is splitting of the centromere in a metacentric to give two telocentrics. In this way chromosome number can change. Correct meiotic segregation in a heterozygous cell requires that the two telocentrics segregate to one pole, and the metacentric (with both arms) goes to the other pole.

Translocations

Translocations involve exchange of distal regions of nonhomologous (genetically different) chromosomes. It is necessary that either both rearranged or both original chromosomes occur in the same gamete to ensure genetic balance. In meiosis, normal chromosomes and chromosomes with translocations pair up alternately to form a ring or chain of several chromosomes. If two adjacent chromosomes segregate to the same pole, genetically unbalanced gametes will be produced, but if alternate chromosomes go together the gametes will contain completely balanced genomes. Some species are heterozygous for many translocations, and form long chains or rings of chromosomes in meiosis.

Inversions

Inversions turn part of the chromosome around, reversing its polarity. A pericentric inversion includes the centromere, a paracentric inversion is contained in one arm of the chromosome and does not move the centromere. A chromosome with an inversion can pair with a normal chromosome in meiosis in a heterozygote if they form an inversion loop. Meiotic recombination within an inversion loop duplicates one end of each chromatid and deletes the other end in a reciprocal manner. These recombined chromatids are therefore not viable, and do not reach the next generation. Inversions therefore act as recombination suppressors, preventing recombination between the inverted segment and its normally oriented counterpart. Some species have polymorphic chromosomal inversions and have mechanisms to prevent loss of fertility (see below).

Paracentric inversions

These do not include the centromere. Meiotic recombination in a paracentric inversion loop generates a dicentric chromatid (two centromeres) and an acentric (no centromere) fragment. Most dipterans (true two-winged flies) can tolerate this because they have no recombination in males so sperm all contain complete chromosomes, and female dipterans have mechanisms to ensure that only unrecombined chromatids reach the egg cell, so zygotes are viable.

Pericentric inversions

These invert the centromeric region, and the effects of meiotic recombination in the loop duplicating and deleting the ends of the chromatid cannot be avoided because all chromatids have one centromere. Many zygotes will have unbalanced genomes. Pericentric inversions only persist as polymorphisms in species where they do not recombine at meiosis.

Changes in sex chromosomes

Y chromosomes have few genes and are lost in some species (XX females, XO males). Fusion of an X chromosome with an autosome causes a copy of the free autosome to become a neo-Y chromosome, only found in males which have another copy attached to their single X. Fusion of a Y chromosome with an autosome causes the unattached autosome to become a neo-X chromosome, one free copy being in males (to balance the copy attached to the Y), two copies in females as before. When two or more different X or Y chromosomes exist they are numbered X₁, X₂, Y₁, Y₂, Y₃, etc.

Evolutionary effects

Polymorphisms for paracentric inversions are common in dipterans where they may contain sets of alleles coadapted to control sex, or adapted to particular ecological environments. The suppression of recombination protects the cluster of genes and facilitates their co-evolution. Chromosomal change is common in evolution, and can cause hybrid sterility between closely related species, forming a postmating isolation mechanism.