The most significant effect of B chromosomes is on seed and pollen fertility. Flowering time is generally delayed by B-chromosomes and several characters (plant height, plant weight and tiller number) are adversely affected. A very important effect of B chromosomes relates with their effect on pairing of A chromosomes in (i) plants carrying these B-chromosomes and in (ii) species hybrids involving a parent carrying B-chromosomes. In rye, although 0-6 B chromosomes did not influence the chiasmata frequencies, 8 B chromosomes drastically influenced the range of chiasma frequencies among pollen mother cells (PMCs) and among bivalents within PMCs. In maize, the use of various A/B interchange stocks demonstrated that the genetic elements affecting meiotic pairing of A-chromosomes appeared to be widely distributed throughout the length of B-chromosomes. The effect of B-chromosomes in suppressing or promoting meiotic pairing has been shown in a number of intergeneric or interspecific hybrids (wheat x Aegilops sp.; wheat x rye, Lolium x Festuca; interspecific hybrids within the genus Lolium, etc.). For more details, the readers should consult the book B-chromosomes written by Jones and Rees (1982).
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