B chromosome

B chromosomessupernumerary chromosomesB-chromosomeB-chromosomes
In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes).wikipedia
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Karyotype

karyotypingkaryogramFN
In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes).
The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.

Selfish genetic element

selfish DNAselfish genetic elementsselfish
In some cases, B chromosomes act as selfish genetic elements.
Discussing B chromosomes in plants he wrote:

Species

specificspecific epithetspecific name
In addition to the normal karyotype, wild populations of many animal, plant, and fungi species contain B chromosomes (also known as supernumerary, accessory, (conditionally-)dispensable, or lineage-specific chromosomes).

Chromosome

chromosomeschromosomalChromosomal number
By definition, these chromosomes are not essential for the life of a species, and are lacking in some (usually most) of the individuals.

Marker chromosome

B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies.

Trisomy

trisomiestrisomicAutosomal trisomy
B chromosomes are distinct from marker chromosomes or additional copies of normal chromosomes as they occur in trisomies.

Michael J. D. White

Michael James Denham WhiteM. J. D. WhiteMichael JD White
(White 1973 p173)

Rye

Secale cerealerye flourwinter rye
Next generation sequencing has shown that the B chromosomes from rye are amalgamations of the rye A chromosomes.

Haplochromis latifasciatus

Astatotilapia latifasciataH. latifasciatusZebra Obliquidens
Similarly, B chromosomes of the cichlid fish Haplochromis latifasciatus also have been shown to arise from rearrangements of normal A chromosomes.

Heterochromatin

heterochromaticfacultative heterochromatin30nm structure of chromatin
Most B chromosomes are mainly or entirely heterochromatic (i.e. largely non-coding), but some contain sizeable euchromatic segments (E.g. such as the B chromosomes of maize).

Non-coding DNA

junk DNAnoncoding DNAnon-coding
Most B chromosomes are mainly or entirely heterochromatic (i.e. largely non-coding), but some contain sizeable euchromatic segments (E.g. such as the B chromosomes of maize).

Euchromatin

euchromatic
Most B chromosomes are mainly or entirely heterochromatic (i.e. largely non-coding), but some contain sizeable euchromatic segments (E.g. such as the B chromosomes of maize).

Grasshopper

grasshoppersshort-horned grasshopperCaelifera
For instance, the British grasshopper Myrmeleotettix maculatus has two structural types of B chromosomes: metacentrics and submetacentrics.

Myrmeleotettix maculatus

Mottled Grasshopper
For instance, the British grasshopper Myrmeleotettix maculatus has two structural types of B chromosomes: metacentrics and submetacentrics.

Satellite DNA

dna, satellitesatellite sequencesDNA satellite
The supernumeraries, which have a satellite DNA, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.

Polyploidy

tetraploidpolyploidtriploid
In wheat, an allopolyploid, the B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes.

Fungus

Fungifungalnecrotrophic
Chromosome polymorphisms are very common among fungi.

Fusarium solani

Nectria haematococcaHaematonectria haematococcaFusarium eumartii
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.

Phytoalexin

phytoalexinsphytoallexin
This supernumerary DNA was found to code for a group of enzymes that metabolize toxins, known as phytoalexins, that are secreted by the plant's immune system.

Horizontal gene transfer

lateral gene transferhorizontal transfergene transfer
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.

Mycosphaerella graminicola

Septoria triticiZymoseptoria triticiSeptoria leaf blotch
The wheat-infecting fungal pathogen Zymoseptoria tritici contains 8 dispensable B-chromosomes - the largest number of dispensable chromosomes observed in fungi.

List of organisms by chromosome count

List of number of chromosomes of various organismschromosome countchromosome count in comparison to other species

Brachyscome

Brachycome
In this species n=2, though some plants have 1, 2 or 3 additional large B chromosomes.

Parasitic chromosome

Parasitic Chromosomes are often B chromosomes, such that they are not necessarily present in the majority of the species population and are not needed for basic life functions, in contrast to A chromosomes.

Harlan Lewis

F.H.LewisFrank Harlan Lewis
It was not until 1951 that he published his next paper in Clarkia, which was concerned with the widespread occurrence of supernumerary chromosomes in Clarkia elegans, now known as Clarkia unguiculata.