The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize II.

The problem of meiotic homologue pairing is approached by comparing chiasma frequencies in rearranged chromosome segments that differ substantially in relative length and intrachromosomal location. Results are consistent with affirmative answers to some questions previously raised: (1) whether there may be an underlying direct relationship between frequency of pairing and length of segment, (2) whether pairing commonly can be initiated independently in intercalary regions, and (3) whether there also can be a role for extension of pairing in adjoining regions for the establishment of pairing in intercalary regions, which requires pairing partner change. In addition, results here suggest that there may be: (1) greater capacity for establishment of pairing of more distal compared to proximal regions in a way that may also be dependent on their lengths, at least when these are relatively short, and additionally in a way which cannot be attributed to special properties of telomeres, (2) nearly random distribution of pairing of any two genetically long intercalary region representatives where three are present, without regard to the matching of the remainder of the chromosomes involved, and (3) a strong tendency for change of pairing partner in long distal segments when these are present in triplicate. Although sharp heterogeneities of pairing capacity were not found, it is suggested that they may exist with spacing too close for easy detection with the resolving power available.