group+11

**Group name**: B chromosomes **Secretary**: Jade Lam (bt09213@qmul.ac.uk) **Members**: Elisa Brann (bt09414@qmul.ac.uk) Dominique McKenzie-Spooner (bt09414@qmul.ac.uk) Siobhan Banful (ef08008@qmul.ac.uk)


 * What are B chromosomes and what is interesting about their modes of inheritance? **

__B Chromosomes__
 * [[image:a_rye-B-chromsome.jpg align="center"]] ||
 * Fig 1. Rye, //Secale cereale//, with FEULGEN staining at metaphase. The B chromosome is represented by the arrow. B chromosomes remains on the spindle and does not divide like the A chromosomes shown either side of the B chromosome. Note: how do you know this is a B, it looks like a bivlaent withaa single cross over to me. ||

Many species contain chromosomes called B chromosomes (Bs). Such entities are known as supernumerary or accessory chromosomes as they are an addition to the normal karyotype. Similar to As, Bs are constructed from heterochromatin- a tightly packed form of DNA, how ever they vary in structure based on the differing degrees of constitutive and facultative heterochromatin within the chromosome. Bs are unnecessary for growth as they do not contain any genes that are transcribed or translated in the cell and therefore no information held on the chromosome is expressed. Note: I already told you in the previous edit that this is not correct.....they can have genes that are transcribed and translated, but they are not essential. Furthermore they distribute differently in individual species Note: in different individuals of the same species..... through their accumulation via abnormal segregation and successive divisions. This excessive accumulation, however, results in a reduction of overall fitness of the individual, leading to a common belief that such chromosomes are parasitic to the organism. Increased numbers of Bs can have numerous effects on the inheritance of the entire DNA within a cell. Such examples include: an increase in the asymmetry of chiasma distribution, an increase in recombination frequencies and variation, and also an increase in the number of unpaired chromosomes leading to a possibility of infertility. (Jones and Rees 1982) Note: always put the reference inside the sentence at the end. The evolution of these B chromosomes is likely to be a product of genetic polymorphism.

B chromosomes have three distinctive features, which enable them to be recognized. Firstly, they are dispensable elements, meaning that within one population they can be absent or present within each individual, no they are absent or present in the individual, cannot be both! . Second to this, they do not recombine with any members of the standard diploid/polyploidy set of A chromosomes during meiosis. And lastly the inheritance of B chromosomes is completely irregular, and cannot therefore be predicted or estimated using Mendelian theory. Thus the B chromosome is defined as a ‘dispensable supernumerary chromosome that does not recombine with the A chromosomes, which follows its own ‘evolutionary pathway’.’

__Notable Examples of B Chromosomes in Fungi and Plants__

Figure 2- Gilled fungi, one of many examples of fungi containing B chromosomes Perhaps one of the most notable examples of the commonality of Bs, are in Fungi. Within one particular species, several isolates can have varying chromosome numbers, characteristic to B behaviour; often some of the additional chromosomes are completely unnecessary for growth of the isolates. However, although they do not carry genes required for fungal growth, they often do have functional significance. One example being the Pda6 gene in //Nectria haematococca// this gene is involved in host range, and other genes crucial to the disease-causing ability of the fungus. Supernumerary B chromosomes are also highly prevalent in flowering plants and Gymnosperms, and also in Ferns and Bryophytes. Several findings have derived from research within rye and maize, two species that have been used extensively in genetic analysis. In plant species, the ability of Bs to act as diploidizing agents for chromosome pairing in particular hybrids has been observed. The only species found to show exception to the theory that Bs have little or no adaptive significance, have been //Nectria haematococca// (a fungal pathogen) and //Allium schoenoprasum// (chives). Another key finding has been the discovery of B chromosomes behaving as parasites within a host-parasite relationship with A chromosomes. This parasitic behaviour is a mechanism adopted by B chromosomes to maintain their polymorphism and is commonly known as drive. It ensures their survival in populations, causing conflict between their host’s nuclear genome and leading to the anti-B chromosomes depriving their hosts of fundamental genetic resources.

__The Behaviour of B Chromosomes at Meiosis__ B chromosomes vary greatly between species in regards to their behaviour during meiosis. Consequently the reality is that very little is known in regards to generalizations that can be made about their fate. What is known, as previously mentioned, is that they behave in a non-Mendelian fashion (Banaszek & Jadwiszczak 2006). There can be numerous numbers of B chromosomes in a genome. Regardless of this they appear as univalents (unpaired chromosomes) and segregate in a random fashion. As a result of this they then become either over represented, ‘Meiotic drive’, or under represented, ‘Meiotic drag’, in gametes formed during meiosis. Meiotic drive, in animals occurs only in oogenesis (gametogenesis of ovum), where Bs accumulate as a result of non-symmetrical division. In contrast spermatogenesis (gametogenesis of sperm) is symmetrical, and therefore is not the immediate form by which Bs accumulate. One of the well-known characteristics of Bs is the absence of homology with the chromosomes of the normal complement (Jones, 1995), thereby suggesting that there would be no traits of them deriving from the A chromosomes.

__The Evolution of B Chromosomes__ However, it remains widely accepted that B chromosomes have in fact arisen from A chromosomes, but follow their own independent pathway (Jones & Rees 1982) Note: what do you mean by pathway.... . Early experiments researching the origin of these chromosomes demonstrate that they contain DNA similar to that of As. It was later found that some repetitive DNA is shared with As while others are specific to Bs, and that these are a by-product of karyotype evolution (Camacho //et al//. 2000) Note: itis karyotype evolution? . In conclusion we can see that the origin of Bs is not clearly understood, however the idea that they are derived from heterochromatic segments of A is most commonly acknowledged (Charlesworth //et al.// 1994). Other views include; sex chromosomes as ancestors, reproductive mechanisms based on chromosome elimination, and intraspecific hybridization origin. Bs are composed of repeated DNA sequences which may form a significant part of the genome. It has been found that they contain more repeated DNA than the genome from which they were derived, demonstrating a large amount of amplification within a short period of time (Camacho //et al//. 2000). This repeated DNA has been implicated in some species' sex chromosome evolution as they have independently accumulated on these chromosomes. This mechanism can explain synapsis inhibition, as this repeat sequence may be the initiator of early heteromorphic sex chromosome differentiation (Nanda //et al.// 1993). It has been suggested that Bs accumulate DNA from different sources of transposable DNA, a mechanism used to explain how the variability in mammalian Y chromosomes has arisen (Steinemann //et al.// 1993). Therefore any active genes inherited by B chromosomes could potentially become silenced by insertions of transposable elements. In conclusion, the behaviour and evolution of B chromosomes remains elusive. Whilst there are clear examples of the effects of B chromosomes, particularly in plants and fungi, these do not necessarily translate to animals. This only leads us to ask more questions about the differences in behaviour and accumulation of B chromosomes across differing kingdoms. However, whilst there appears to be scope for further research into the area of accessory chromosomes, the little effect they appear to have on the organisms’ means that perhaps further research is in fact limited. We can simply speculate at this moment in time, about their real significance, if there is indeed any.

__References__ Banaszek, A., & Jadwiszczak, K.A.,( 2006//). B-chromosomes behaviour during meiosis of yellow-necked mouse, Apodemus flavicollis//. Folia Zool. 55, 113-122 Jones, R N, and Rees, H. (1982). //B Chromosomes.// Academic Press, London. Jones, R.N., (1995). //B chromosomes in plants.// New Phytologist, 131, 411-434 Jones, R.N. and Houben, A. (2003//) B chromosomes in plants: escapes from the A chromosome genome?// Trends in Plant Science 8, 417-421 Zolan, M.E., Heyler, N.K. and Stassen, N.Y. (1994) //Inheritance of Chromosome-Length Polymorphisms in Coprinus cinereus¸//Genetics 137, 87 Camacho, J.P.M., Sharbel, T.F. and Beukeboom, L.W. (2000) //B-chromosome evolution,// Phil. Trans. R. Soc. Lond.355, 163 Charlesworth, B., Sniegowski, P. & Stephan,W. 1994 //The evolutionary dynamics of repetitive DNA in e// //ukaryotes//. Nature 371, 215 - 20. Nanda, I., Schartl, M., Epplen, J. T., Feichtinger,W. & Schmid, M. 1993 //Primitive sex chromosomes in poeciliid fishes harbor simple repetitive DNA sequences.// // J. //Exp. Zool. 265, 301- 8. Stearns S.C & Hoekstra R.F (2005) //Evolution: Introduction// second edition Oxford University Press. New York 362-363 Steinemann, M., Steinemann, S. & Lottspeich, F. (1993) //How Y chromosomes become genetically inert.// Proc. Natl Acad. Sci. USA 90, 5737-41.


 * __Contributing Group members__**
 * Dominique Mackenzie-Spooner bt09404@qmul.ac.uk**
 * Jade Lam bt09213@qmul.ac.uk**
 * Siobhan Banful ef08008@qmul.ac.uk**
 * Elisa Brann bt09414@qmul.ac.uk**


 * Note: Much better **