B chromosome
Encyclopedia
In addition to the normal karyotype
, wild populations of many animal, plant, and fungi species
contain B chromosomes (also known as supernumerary or accessory chromosomes). By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals. Thus a population would consist of individuals with 0, 1, 2, 3 (etc) supernumeraries.
Most B chromosomes are mainly or entirely heterochromatic
, (and so would be largely non-coding) but some, such as the B chromosomes of maize, contain sizeable euchromatic
segments. In general it seems unlikely that supernumeraries would persist in a species unless there was some positive adaptive advantage, which in a few cases has been identified. For instance, the British grasshopper
Myrmeleotettix maculatus has two structural types of B chromosomes: metacentrics and submetacentrics. The supernumeraries, which have a satellite DNA
, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.
In plants there is a tendency for B chromosomes to be present in the germ-line, but to be lost from other tissues such as root tips and leaves. There is evidence of deleterious effects of supernumeraries on pollen fertility, and favourable effects or associations with particular habitats are also known in a number of species.
The evolutionary origin of supernumerary chromosomes is obscure, but presumably they must have been derived from heterochromatic segments of normal chromosomes in the remote past. In general "we may regard supernumeraries as a very special category of genetic polymorphism which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelian laws of inheritance." (White
1973 p173)
B chromosomes may play a positive role on normal A chromosomes in some circumstances. The B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes in allopolyploids. Bivalent pairing is ensured by a gene on chromosome 5 of the B genome Phlocus. The B chromosomes also have the following effects on A chromosomes:
B chromosomes have tendency to accumulate in meiotic cell products resulting in an increase of B number over generations. However this effect is counterbalanced for selection against infertility.
B chromosomes are not to be confused with marker chromosome
s or additional copies of normal chromosomes as they occur in Trisomies
.
Supernumerary chromosomes do not carry genes that are necessary for basic fungal growth, but may have some functional significance. For example, it has been discovered that the supernumerary chromosome of the pea pathogen Haematonectria haematococca
carries genes that are important to the disease-causing capacity of the fungus. This supernumerary DNA was found to code for a group of enzymes that metabolize toxins, known as phytoalexin
s, that are secreted by the plant's immune system. It is possible that these supernumerary elements originated in horizontal gene transfer
events because sequence analysis often indicates that they have a different evolutionary history from essential chromosomal DNA.
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
, wild populations of many animal, plant, and fungi species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
contain B chromosomes (also known as supernumerary or accessory chromosomes). By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals. Thus a population would consist of individuals with 0, 1, 2, 3 (etc) supernumeraries.
Most B chromosomes are mainly or entirely heterochromatic
Heterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
, (and so would be largely non-coding) but some, such as the B chromosomes of maize, contain sizeable euchromatic
Euchromatin
Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often under active transcription. Unlike heterochromatin, it is found in both cells with nuclei and cells without nuclei...
segments. In general it seems unlikely that supernumeraries would persist in a species unless there was some positive adaptive advantage, which in a few cases has been identified. For instance, the British grasshopper
Grasshopper
The grasshopper is an insect of the suborder Caelifera in the order Orthoptera. To distinguish it from bush crickets or katydids, it is sometimes referred to as the short-horned grasshopper...
Myrmeleotettix maculatus has two structural types of B chromosomes: metacentrics and submetacentrics. The supernumeraries, which have a satellite DNA
Satellite DNA
Satellite DNA consists of very large arrays of tandemly repeating, non-coding DNA. Satellite DNA is the main component of functional centromeres, and form the main structural constituent of heterochromatin....
, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.
In plants there is a tendency for B chromosomes to be present in the germ-line, but to be lost from other tissues such as root tips and leaves. There is evidence of deleterious effects of supernumeraries on pollen fertility, and favourable effects or associations with particular habitats are also known in a number of species.
The evolutionary origin of supernumerary chromosomes is obscure, but presumably they must have been derived from heterochromatic segments of normal chromosomes in the remote past. In general "we may regard supernumeraries as a very special category of genetic polymorphism which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelian laws of inheritance." (White
Michael JD White
*White M.J.D. and Webb G.C. Blattodea, Mantodea, Isoptera, Grylloblattodea, Phasmatodea, Dermaptera, and Embioptera. Borntraeger. ISBN 3443260055...
1973 p173)
B chromosomes may play a positive role on normal A chromosomes in some circumstances. The B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes in allopolyploids. Bivalent pairing is ensured by a gene on chromosome 5 of the B genome Phlocus. The B chromosomes also have the following effects on A chromosomes:
- increases asymmetry chiasma distribution
- increases crossing over and recombination frequencies: increases variation
- cause increased unpaired chromosomes: infertility
B chromosomes have tendency to accumulate in meiotic cell products resulting in an increase of B number over generations. However this effect is counterbalanced for selection against infertility.
B chromosomes are not to be confused with marker chromosome
Marker chromosome
A marker chromosome is a structurally abnormal chromosome in which no part can be identified. The significance of a marker is very variable as it depends on what material is contained within the marker....
s or additional copies of normal chromosomes as they occur in Trisomies
Trisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
.
Supernumerary chromosomes in fungi
Chromosome polymorphisms are very common among fungi. Different isolates of the same species often have a different chromosome number, with some of these additional chromosomes being unnecessary for normal growth in culture. The extra chromosomes are known as conditionally dispensable, or supernumerary, because they are dispensable for certain situations, but may confer a selective advantage under different environments.Supernumerary chromosomes do not carry genes that are necessary for basic fungal growth, but may have some functional significance. For example, it has been discovered that the supernumerary chromosome of the pea pathogen Haematonectria haematococca
Haematonectria haematococca
Haematonectria haematococca is a plant pathogen and is the teleomorph of Fusarium solani.- External links :* *...
carries genes that are important to the disease-causing capacity of the fungus. This supernumerary DNA was found to code for a group of enzymes that metabolize toxins, known as phytoalexin
Phytoalexin
Phytoalexins are antimicrobial substances synthesized de novo by plants that accumulate rapidly at areas of incompatible pathogen infection. They are broad spectrum inhibitors and are chemically diverse with different types characteristic of particular plant species...
s, that are secreted by the plant's immune system. It is possible that these supernumerary elements originated in horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
events because sequence analysis often indicates that they have a different evolutionary history from essential chromosomal DNA.