ANALYSIS OF THE DELAYED KNOB REPLICATION HYPOTHESIS INDUCED BY B CHROMOSOMES IN MAIZE

The "delayed knob replication" hypothesis induced by B chromosomes in maize was analyzed through cytological studies of the kinds of structural rearrangements arising in "high-loss" plants when the microspores contained two heterologous knobbed chromosomes. Strong support for this hypothesis was obtained since the predictable kinds of chromosome aberrations were found. Depending on whether the two bridges rupture on the same or on opposite sides of the spindle plate, fusion of freshly broken ends produced two kinds of structural rearrangements: (1) non-reciprocal translocations, in which only one translocated chromosome was recovered and the zygote after fertilization was partially trisomic for one knobbed segment and deficient for the other; and (2) dicentric chromosomes, in which both knobbed segments were lost. Reciprocal translocations were not expected since the two translocated chromosomes move to opposite poles.