Accessory Chromosomes:
In many species, one too many extra chromosomes in addition to the normal somatic complement are found; these extra chromosomes are called accessory chromosomes, Bchromosomes or supernumerary chromosomes. About 600 plant species and more than 100 animal species are reported to possess B- chromosomes. B-chromosomes are generally smaller in size than the chromosomes of the normal somatic complement but in some species they may be larger (e.g., in Sciara). One of- the most important features of these chromosomes is that their numbers may vary considerably among individuals of the same species; in maize as many as 25- 30 B-chromosomes may become accumulated in some individuals without any marked effect on their phenotype. These chromosomes are generally gained by and lost from the individuals of a species without any apparent adverse or beneficial effect.
However, the presence of several B-chromosomes often leads to some reduction in vigour and fertility in maize. In most cases, they are largely heterochromatic, while in some species (e.g. maize) they are partly heterochromatic, and in some other (e.g., Tradescantia) they are entirely euchromatic. They are believed to be generally inactive genetically, but they may not be completely devoid of genes.
The origin of B-chromosomes in most species is unknown. In some animals they may arise due to fragmentation of the heterochromatic Y chromosome. In maize, morphological features and pairing behaviour of B-chromosome clearly shows that they do not have any segment which is homologous to a segment of any chromosome of the normal somatic complement. Bchromosomes are relatively unstable; in many species they tend to be eliminated from somatic tissues due to lagging and non-disjunction and they frequently change in morphology through fragmentation. Further, they may also show irregular distribution during meiosis, but they are invariably maintained in the reproductive tissue.
Metaphase spread showing B-chromosomes. Note the unusual difference in the size of the chromosomes.