B chromosomes of maize (Zea mays L.) are positioned nonrandomly within sperm nuclei

We used fluorescence in situ hybridization to identify and map the position of B chromosomes (supernumerary chromosomes) within maize sperm cells. Obs ervations on over 1,000 sperm cells from several genotypes show that, on av erage, the B chromosomes are positioned in the tip one-fourth of the sperm nucleus two-thirds of the time. In contrast, the centromeres and knobs of t he A chromosomes (the normal set) are not restricted to the tip portion of the nucleus. To our knowledge, this is the first example of specific chromo some positioning within a plant gamete. Studies on nuclear architecture of somatic cells in both plants and animals suggest that chromosome behavior a nd gene expression may correlate with chromosome position within the nucleu s. The functional significance of nonrandom positioning of the B chromosome s within maize sperm is as yet unclear.