POLARIZATION AND PROGRESSION IN PAIRING: I. INTERLOCKING OF BIVALENTS IN TRILLIUM ERECTUM L.

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Abstract
A total of 346 interlocks, occurring in five plants, were classified as to type of interlocking and type of bivalent concerned. Two main types of interlocking, true and false, occurred in equal numbers, the interlocking involving different bivalents and also the two arms of one bivalent. An interlocked chiasma, the first reported case in which the chromatids could be traced, and a clear example of chromatid interlocking were observed. No correlation was found between the frequency of interlocking and the temperature at which meiosis occurred.The ratio of the frequencies with which the individual bivalents were involved in interlocking is proportional to the relative lengths of the bivalent arms minus a minimum length. Interlocking of bivalents in the 10 possible combinations in pairs occurred with the frequencies expected from the frequencies with which the individual bivalents were involved.All loops of the bivalents were involved in interlocking, odd and even loops being involved in the ratio of 3:1. That adjacent to the centromere and the most distal loops were most frequently concerned and centric interlocking occurred with a frequency only one-third that expected on a random basis. The loops involved were larger than corresponding loops free from interlocking.Interlocking was found to reduce the chiasma frequency. The chiasma frequency of encircled bivalents was higher than that of bivalents in cells devoid of interlocks, which, in turn, had a higher frequency than the encircling bivalents of false interlocks. The factor causing the reduction was found to reduce the chiasma frequency of non-interlocked bivalents accompanying interlocked bivalents. The reduction in chiasma frequency is insufficient to have resulted from loss of chiasmata. It is proportional to the increase in size of the interlocking loops, which is the result, not of increased repulsion, but of simple interference with the positions in which chiasmata are formed at pachytene.It is concluded that interlocking is a result of the derangement of chromosomes that are normally polarized and have their proximal parts in an orderly spatial arrangement prior to pairing.

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