What Are Accessory Chromosomes?

What are chromosomes?

In the nucleus of each cell there are DNA molecules. These DNA molecules are packaged into thread-like structures called chromosomes.

Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.
These chromosomes are not visible under normal condition even under microscope, however, they can be viewed during the time of cell division.
In short, the package of DNA and histone proteins is called chromosome.
Kinds of chromosomes:
Lampbrush chromosomes
Polytene chromosomes
Supernumerary chromosomes/ Accessory chromosomes

Now what are these Accessory chromosomes?
• They are also called as B chromosomes or Supernumerary chromosomes or extra chromosomes.
•         They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses.
•         They are present in wild species of many animal, plant and fungi species in addition to the normal karyotype.
•         Wilson first discovered these in 1905 in hemipteran insect Metapodius.
•         They are not fundamental for the life process.
•         A population would contain only a few individuals with these kind of chromosomes.
•         More commonly their ancestry is entirely unknown. Generally they are of smaller size and appear to be genetically inert.
•         Their presence produces little detectable phenotypic expression in the organism. This suggests that structurally they are largely heterochromatic, whereas in some species like maize, they are euchromatic in segments.
•         It is studied that when they are present in large numbers in the same nucleus, they reduce the vigour and fertility. They therefore, cannot be considered genetically inert as had been previously supposed.
•         Supernumerary chromosomes, as a group, are relatively unstable members of a chromosome complement.
•         Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.
Moreover, they can have the following effects on A chromosomes or normal chromosomes:
1.       They increase asymmetry chiasma distribution
2.       They increase crossing over and recombination frequencies: increases variation
3.       They cause increased unpaired chromosomes: infertility
• The fungal pathogen Mycosphaerella graminicola contains 8 dispensable B chromosomes.

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