Changes in the structure of B (A) chromosomes in maize.

A class of mosaic endospersm involving the marker Su I was observed among the progeny of individuals hyperploid for the chromosome B (4) and genetically analyzed. The exceptional individuals showing mosaic endosperm were found when the hyperploid material was used as pollen source. While in some cases mosaicism was limited to the endosperm tissue, with no apparent consequences in the embryo, in others the mosaicism was transmitted to the progeny, which showed changes in the structure of the B (4) chromosome, with the formation of unstable chromosomes whose genetic behaviour was similar to that of ring chromosomes. This interpretation was cytologically confirmed. In other cases the B (4) chromosome analyzed in mosaic endosperm individuals underwent altered transmission frequencies or loss, suggesting that its original structure had been modified by breakage-fusion-bridge cycles. The changes in this chromosome revealed by the mosaic phenotype are discussed in relation to the original structure of the B chromosome and the B (4) hyperploid condition.