The modification of crossing over in maize by extraneous chromosomal elements.

Five regions of the maize genome were tested for their response to endogenous factors influencing recombination. These included heterochromatic B chromosomes and abnormal chromosome 10 as well as the sex in which recombination occurred.The frequency of recombination in the proximal A 2-Bt and Bt-Pr segments of chromosome 5 was increased in the presence of B chromosomes, with the male meiocytes showing a greater response than the female meiocytes. In addition, experiments involving 0, 1, 2 and 4 B's revealed a dosage effect of B chromosomes on crossing over in chromosome 5. Recombination in the proximal Wx-Gl 15 interval of chromosome 9 was found to be slightly higher than normal in male flowers when two B chromosomes were present. This increase was accompanied by a decrease in the adjacent Sh-Wx segment. Crossing over in the distal C-Sh segment and in the C-Sh-Wx-Gl 15 regions of female flowers was unaffected by B's.Comparisons of plants heterozygous for abnormal chromosome 10 (K10 k10) and homozygous for the standard chromosome 10 (k10 k10) showed that abnormal 10 greatly enhances crossing over in the A 2-Bt and Bt-Pr segments of chromosome 5. In contrast to the finding with B's, the effect is greater in female than in male sporocytes. K10 showed no significant effect on recombination in the C-Sh-Wx-Gl 15 region of chromosome 9 except in male sporocytes, where there was a slight increase in the Sh-Wx region of 0 B K10 k10 plants and a possible interaction with B chromosomes to raise the level of recombination between Wx and Gl 15. The fact that the regions adjacent to the centromere of chromosome 9 show little or no response to the presence of K10 indicates that the proximal heterochromatin of this chromosome differs qualitatively from that of other maize chromosomes. This conclusion is supported by a comparison of the effects of B chromosomes, K10 and sex on crossing over in chromosomes 5 and 9.