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SUMMARY
Meiotic pairing of the B chromosomes of rye in the genetic environment of wheat was studied in order to investigate the effect of some structurally altered derivatives of B chromosomes on the pairing of the standard B chromosomes in the plant series containing 42 wheat chromosomes plus 2stBs + 1defB, 3stBs + 1defB, 3stB+ lsB, 3stBs+ 1siB, 3stBs+11iB, 3stBs + 2mBs, 4stBs + 1siB, 4stBs+11iB, 5stBs + 1siB, and 6stBs + 1siB chromosomes, and a comparison was made with control plants bearing only an equal number of standard B chromosomes. In the presence of deficient B, small B and small iso B chromosomes in the above plant series together with standard Bs, frequencies of pairing of the standard B chromosomes were markedly lower than the control plants containing only the corresponding number of standard Bs whereas the frequencies of standard B univalents were remarkably higher in the presence of the aforesaid derivatives of B chromosomes. This indicates that the dificient B, small B and small iso B chromosomes exert a suppressing effect on the pairing of the standard Bs; it may be due to the deficiency of a distal segment of the long arm of the standard B chromosome. The large iso B and medium B chromosomes have no such effect on pairing.