Chromosome Fusion

Chromosomes are classified based on where the centromere is located. When the centromere is in the middle of a chromosome and the two arms are thus of equal length, it is called a metacentric chromosome. If the centromere is located at or near one end of the chromosome, it is called an acrocentric chromosome.

Rearrangement of the chromosomes can include Robertsonian fusion, tandem fusion, translocation, inversions, and drastic rearrangements. Let’s take a look at the ways chromosome rearrangement can occur:

Robertsonian Fusion

One such rearrangement is known as Robertsonian rearrangement and is the result of either the fusion of two centromeres into one, or the splitting of the centromere in to two.

Robertsonian fusion changes the chromosome number, but not the arm number. When chromosomes line up during cell division, a metacentric chromosome lines up with two acrocentric chromosomes. An example of this is the house mouse Mus musculis, which has 40 chromosomes. Another population of mice from the Italian Alps was found to have only 22 chromosomes. This population differs slightly from the normal house mouse as well, and is classified as a different species Mus poschiavanus.

Other populations have been discovered with chromosome numbers varying between 22 and 40. The number of chromosome arms is the same and banding studies reveal the genes to have the same structural features and pattern of genes. Obviously, in terms of their relationship, these different species are all one group.

Tandem Fusion

Tandem fusion, on the other hand, is a fusion of two chromosomes in which one end of a chromosome fuses with the end or the centromere of another chromosome. Tandem fusion changes arm number and chromosome number.

Tandem fusions have been found in some antelope species where a sex chromosome fused with another chomosome that is not a sex chromosome. This is rare, and we can assume that the organisms probably had a common forerunner.

The antelope displays this fusion range in size from the eland (the largest of all the antelope) to smaller species such as the sitatunga and the bushbuck. They all share common features, however, such as similar shapes of the horns and stripes on the body which may be prominent, as in the case of the bongo, or less prominent, as in the case of the eland. Species with this type of fusion are the eland, bongo, lesser and greater kudu, bushbuck, sitatunga, and nilgai (Indian antelope) where the y-chromosome is fused to an autosome.

Tandem fusions are found also in Malaysian swamp buffalo and Asian river buffalo.

A further interesting example of this type of fusion is the Asian deer. In the species Muntiacus muntjac, the females have only six chromosomes while the males have seven chromosomes. However, in a different species of the group, Muntiacus reevesi, both the males and the females have 46 chromosomes.

Banding studies show that the same genetic material is present in both species; the chromosomes in M. muntjac are just fused together to form very long chromosomes. Once again, no new information is added, it is just reshuffled. This provides different expressions and increased variety, just like many tunes can be played on the same piano.

Translocation

Translocations can lead to reduced fertility, or, in some human cases, Down's syndrome. This can occur when part of chromosome 21 gets translocated to another autosome. In some insects and plants, viable offspring can still be produced.

Pericentric Inversion

This type of inversion provides changes in arm number but not chromosome number. The number of arms depends on the position of the centromere. If it is located at the end then there is one arm, but if the centromere is in the middle there are two arms. The inversion can change acrocentric chromosomes to metacentric chromosomes. The rodents Neotoma (pack rat) and Peromyscus (wood mice) differ by this inversion.

Paracentric Inversion

In this type of inversion the centromere is not included. This inversion is relatively uncommon, but has been proposed for some bats, hares, and apes.

Drastic Rearrangements

Under certain circumstances of severe environmental stress, drastic rearrangements can produce greater varieties, which could enhance survival. These changes can be rapid when new adaptive zones are entered.

Chromosome banding studies show that the information is still the same—it is just rearranged. In addition, the types of rearrangements that occur in different animals are quite group-specific, meaning that one type of rearrangement doesn’t necessarily occur in another group.

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