![Metaphase spread of the Siberian Roe deer (Capreolus pygargus)](https://upload.wikimedia.org/wikipedia/commons/9/9a/Metaphase_spread_of_the_Siberian_Roe_deer_%28Capreolus_pygargus%29.jpg)
In addition to the normal
karyotype
A karyotype is the general appearance of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is disce ...
, wild populations of many animal, plant, and fungi
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes). By definition, these
chromosomes
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
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.) B chromosomes. B chromosomes are distinct from
marker chromosome
A marker chromosome (mar) is a small fragment of a chromosome which generally cannot be identified without specialized genomic analysis due to the size of the fragment.Thompson & Thompson Genetics in Medicine, Chapter 5, 57-74 https://www.clinicalk ...
s or additional copies of normal chromosomes as they occur in
trisomies.
Origin
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
A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...
which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelian laws of inheritance." (
White
White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White on ...
1973 p173)
Next generation sequencing has shown that the B chromosomes from
rye are amalgamations of the rye A chromosomes.
Similarly, B chromosomes of the cichlid fish ''
Haplochromis latifasciatus
''Formally Haplochromis latifasciatus, Now Astatotilapia Latifasciata'' is a species of cichlid that is endemic to Uganda where restricted to the Lake Kyoga system, including Lake Bisina (a medium-sized lake to the east of Lake Kyoga itself) and ...
'' also have been shown to arise from rearrangements of normal A chromosomes.
Function
Most B chromosomes are mainly or entirely
heterochromatic
Heterochromia is a variation in coloration. The term is most often used to describe color differences of the iris, but can also be applied to color variation of hair or skin. Heterochromia is determined by the production, delivery, and concentra ...
(i.e. largely
non-coding
Non-coding DNA (ncDNA) sequences are components of an organism's DNA that do not encode protein sequences. Some non-coding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, microRNA, piRNA, ribosomal RNA, and regula ...
), but some contain sizeable
euchromatic
Euchromatin (also called "open chromatin") is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active Transcription (genetics), transcription. Euchromatin stands in contrast ...
segments
[Trifonov, Vladimir A; Dementieva, Polina V; Larkin, Denis M; O'Brien, Patricia CM; Perelman, Polina L; Yang, Fengtang; Ferguson-Smith, Malcolm A; Graphodatsky, Alexander S. 6 August 2013]
Transcription of a protein-coding gene on B chromosomes of the Siberian roe deer (''Capreolus pygargus'')
(e.g. such as the B chromosomes of maize). In some cases, B chromosomes act as
selfish genetic element
Selfish genetic elements (historically also referred to as selfish genes, ultra-selfish genes, selfish DNA, parasitic DNA and genomic outlaws) are genetic segments that can enhance their own transmission at the expense of other genes in the genome, ...
s. In other cases, B chromosomes provide some positive adaptive advantage. For instance, the British
grasshopper
Grasshoppers are a group of insects belonging to the suborder Caelifera. They are among what is possibly the most ancient living group of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago.
Grasshopp ...
''
Myrmeleotettix maculatus'' has two structural types of B chromosomes: metacentrics and submetacentric. The supernumeraries, which have a
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.
The name "satellite DNA" refers to the p ...
, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.
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.
B chromosomes have a tendency to accumulate in meiotic cell products resulting in an increase of B number over generations, thereby acting as
selfish genetic elements
Selfish genetic elements (historically also referred to as selfish genes, ultra-selfish genes, selfish DNA, parasitic DNA and genomic outlaws) are genetic segments that can enhance their own transmission at the expense of other genes in the genome, ...
. However, this effect is counterbalanced for selection against infertility.
In fungi
Chromosome polymorphisms are very common among
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
. 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
''Fusarium solani'' is a species complex of at least 26 closely related filamentous fungi in the division Ascomycota, family Nectriaceae. It is the anamorph of ''Nectria haematococca''. It is a common soil fungus and colonist of plant materia ...
'' 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
Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined, not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized ''de novo'' by plants t ...
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 (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
events because sequence analysis often indicates that they have a different evolutionary history from essential chromosomal DNA.
The wheat-infecting fungal pathogen ''
Zymoseptoria tritici
''Zymoseptoria tritici'', synonyms ''Septoria tritici'', ''Mycosphaerella graminicola'', is a species of filamentous fungus, an ascomycete in the family ''Mycosphaerellaceae''. It is a wheat plant pathogen causing septoria leaf blotch that is d ...
'' contains 8 dispensable B-chromosomes - the largest number of dispensable chromosomes observed in fungi.
References
Further reading
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External links
B ChromosomesB chromosomes in wood mice, genus Apodemus
{{Chromosome genetics
Chromosomes