The interaction of knobs and B chromosomes of maize in determining the level of recombination.

Enhancement of recombination by B chromosomes is influenced by the kind of heterochromatic knob present in or near the tested region of the A chromosomes. In homomorphic chromosome 9 bivalents of K(s)/K(s) constitution, double exchanges were increased at the expense of singles, but in the K*/K(s) heteromorphs there was a gain in both single and double exchanges at the expense of no-exchange tetrads. Modification of the B chromosome enhancement in different knob compounds was observed only in the megasporocytes.-Different frequencies of recombination are found in plants with odd and even numbers of B chromosomes; this effect is especially striking in the megasporocytes. The modification in recombination produced by an odd or even number of B chromosomes is a function of the interaction of a particular region and the knob constitution. Odd numbers of B chromosomes were more effective than even numbers in causing increased recombination.-It is concluded that heterochromatic knobs and the essentially heterochromatic supernumeraries may interact in the process of crossing over, with the level of recombination determined in part by knob constitution.