They are also known as accessory chromosomes or supernumerary chromosomes. These chromosomes exhibit lineage specificity. Not all the individuals in a given population possess B chromosomes. For example, only 10-15% of the plant and animal species possess these chromosomes. These chromosomes possess tiny structures and present themselves in multiple copies. Thus, they occur in a few species and may not be so essential. Examples include certain species of wild animals, plants, and fungi. They show the presence of complete heterochromatin. Maize contains euchromatin in addition to the heterochromatin. Some of the species show the presence of metacentric and submetacentric B chromosomes. These chromosomes are also known as conditionally dispensable chromosomes. They mainly become visible in the warm or dry climatic conditions. They get accumulated in the meiotic cell products. The B chromosomes lead to an increased asymmetry in chiasma distribution and suppress the homologous chromosome pairing. They help in reducing multiple pairing in the homologous chromosomes in the allopolyploidy. They impose various effects on the A chromosomes. For example, they increase the crossing over and recombination frequencies. They also lead to an increase in infertility due to an increased number of unpaired chromosomes. The dispensable genetic elements occur in the rye and maize. These B chromosomes lack homology with any member of the basic set of chromosome. These B chromosomes follow the non-Mendelian pattern of inheritance. The number of B chromosomes remains constant in the somatic tissue. It involves increased intraspecific variation in nuclear DNA. The species with large genomes tolerate them. Thus, they exhibit very high rates of polymorphisms.
Synonyms of B chromosomes include supernumerary chromosomes, accessory chromosomes, and lineage-specific chromosomes respectively. They are also known as non-essential chromosomes since they may not be essential for life. Maize is a common example of plants having B-chromosomes. Mostly B-chromosomes consist of maximum regions with non-coding segments. Thus, they involve a very high amount of heterochromatin. However, maize B-chromosomes contain heterochromatin and euchromatin. Among insects, the British grasshopper shows a good number of B-chromosomes. The structure of these chromosomes either involves metacentric or submetacentric structures. Fungal cells also show the presence of supernumerary chromosomes. Most of them, though derived from the same culture, possess a different number of chromosomes. The difference occurs due to the presence of the B chromosomes. These chromosomes do not possess any specific function in fungal growth. However, in rare environments, they show some functionality. An example includes the disease-causing action of the fungal supernumerary chromosomes in pea plants. They encode for certain enzymes and toxins.
Most of the B chromosomes also act like parasites. They invade cells and prove to be detrimental if found excessively.
Image: Mammalian B-chromosomes
Segregation behavior of B chromosomes:
· Mitotic behavior:
B chromosomes exhibit a disjunction type of mitotic behavior. The number of B chromosomes in the cells remains constant. However, variations may occur in the testis follicles. Examples include grasshoppers. Nondisjunction also occurs in the B chromosomes. It mainly targets the sister chromatids in the anaphase of the mitosis. The nondisjunction in the B chromosomes during anaphase leads to the absence of B chromosomes in the daughter cells and accumulation in the other cells. It also leads to the loss of B chromosomes and complete exclusion of the same.
· Meiotic behavior:
These chromosomes do not pair with A chromosomes. Instead, they pair and form chiasma among themselves. In some species, they do not pair and form chiasma among themselves. They either involve no homologous segments or may be too short for the above process. The B chromosomes form univalent, bivalents, or multitalented structures during the metaphase-I.
B chromosomes in Rye:
The sequencing studies of the Rye B chromosomes revealed their origin to be chromosome 3RS and 7R. The other origin included A chromosome. The supernumerary chromosomes also include plastid and mitochondrial DNA regions. Total 10% of the supernumerary chromosome consists of haploid DNA. Its consequence includes an alteration in the host phenotype via nucleotypic effects. However, these phenotypic changes are rare since the B chromosomes are non-essential ones. Amongst them the pericentromeric regions are common. A distinct finding in the Rye B chromosome includes the subtelomeric region with specific sequences. This region shows the presence of heterochromatic regions of the standard Rye chromosomes. Few studies also report the presence of pseudogenes on the B chromosomes of Rye. The pseudogenes do not possess intronic sequences. Neither do they contain essential sequences such as the functional genes. They also possess numerous mutations. In Rye, these pseudogenes possess a capacity to regulate genes present on the standard chromosomes.
B-chromosome and evolution:
Research points out the origin of B-chromosomes from the heterochromatic segments of normal chromosomes. Scientists call these chromosomes as chromosomes with special genetic polymorphisms. Cytogenetic investigation of plants such as rye revealed the origin of B chromosomes or the supernumerary chromosomes from the A chromosomes. In these plants, the process known as non-disjunction maintains the B chromosomes in the egg and the sperm cells.
Allopolyploids arise from genetically distinct sets of chromosomes. Thus, they possess a huge chance of multiple homologous chromosome pairing. The study suggests the role of B chromosomes on suppressing the multiple pairing. The chiasma in A chromosomes distributes asymmetrically due to the B chromosome effect. The major portion of the variations also arises due to a high number of recombinants. Thus, an increase in recombinant frequencies leads to variations. These events accompanying the crossing over arise due to the B-chromosome effect on the A chromosomes.
B chromosomes show a typical behavior of getting accumulated in the germline. Hence, they get transmitted very frequently. Other important evolutionary changes in these chromosomes involve very high rates of repetitive DNA and transposons. In rare cases, B chromosomes also contribute important genetic information to the standard chromosomes.
References:
[1] Chromosomes Today, Volume 12, edited by N. Henriquez-Gil, J.S. Parker, M. Puertas
[2] B chromosome evolution, NCBI
[3] B Chromosome-Wikipedia
[4] B chromosome, ScienceDirect Topics
[5] Chromosome Structure and Aberrations, edited by Tariq Ahmad Bhat, Aijaz Ahmad Wani
[6] De Novo Evolution of Satellite DNA on the Rye B Chromosome, Tim Langdon
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