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Bs have no known genes, although they do have some genetic organization, which facilitates or modulates their drive. The population equilibrium frequency of B numbers is a balance between the forces of drive and negative effects on reproductive fitness, and this may vary from one population to another.

The drive process is centromere-based and takes place in both male and female gametophytes, although rye is exceptional in this respect, and in other grasses there is drive only through the male side. At the first pollen mitosis the rye Bs do not separate at anaphase, and as the A chromosome chromatids reach the poles the Bs remain undivided at the equator of the spindle and their separation is delayed long enough for them to them be included in an unreduced number in the generative nucleus. The process is described at the cytological level, and may depend on the fact that the spindle is asymmetrical, placing undivided Bs closer to the generative rather than the tube nucleus. This directed nondisjunction is highly effective and delivers the unreduced number of Bs into the generative nucleus 86 % of the time, regardless of the genotype or population concerned.

Rye, Secale cereale, 2n=2x=14+2B (standard B), metaphase I pairing with 5II ring + 2II rod + 1B rod, FEULGEN staining

The second pollen mitosis is normal, so the sperm come to carry the unreduced B number. As this happens through both male and female tracks, the build-up of B chromosomes in the population is inevitable. Selection for high seed set eliminates Bs in modern cultivars as they adversely affect fertility.

The mechanism of nondisjunction at pollen mitosis shows sticking sites on either side of the centromere of the B, probably in the pericentromeric domains, and it is this region of delayed separation that holds the B-chromatids together longer than the As. Furthermore, it is known that there is a genetic element in the distal part of the long arm of the rye B that produces a trans-acting product (protein), which is essential for the sticking action of the sensitive receptors on either side of the centromere region. This is known because, in Bs that are deficient for the distal part of the long arm of the B, nondisjunction fails unless there is another B in the same cell which carries this distal segment. Sequence organization of the distal end of the long arm of the B demonstrates two B-specific sequences that have a complex organization, but it has not been possible thus far to identify genes which could code for proteins to interact with the pericentric receptors. The rest of the rye B has repetitive DNA similar to that of the A chromosomes. It seems to be composed from chromosome 3R and 7R, while chromosome 7R carries a big piece of chrosome 5RL, considering the evolutionary synteny. In addition, it also has a highly conserved structure and, at the cytological level, it has a similar form in all the many different populations where it occurs. It seems that it is a highly optimized selfish element for the function for which it has evolved – its own replication.

The Giemsa banding-positive heterochromatin subterminal domain of rye standard Bs undergoes decondensation during interphase. Contrary to the heterochromatic regions of A chromosomes, this domain is simultaneously marked by trimethylated  H3K4 and H3K27 histons. Both types of high-copy-repeat families of the subterminal domain are transcriptionally active (395).

   Copyright  R. Schlegel   &   V. Korzun    2004 2005 2006 2007 2008  2009  2010 2011 2012 2013 2014 2015  2016